Chapter 3: Gases Notes
Notes for chapter 3 Gases of chemistry book 1 for Fsc pre medical and Pre engineering and for entry test preparation
GASES
States of Matter
Matter exists in four states solid , liquid , gas and plasma.
The gases are the simplest form of matter.
Properties of Gases
The general properties of gases are the following:
- Gases have no definite volume: They occupy all the available space. The volume of the gas is the volume of the container.
- Gasses have no definite shape. They take the shape of the container.
- Gasses have low densities. Gases have low densities as compared with liquids and solids. The gases bubble through liquids and tend to rise up.
- Gasses con diffuse and effuse. They can diffuse and effuse rapidly through each other in all directions. The odour spreads in this way. The property of diffusions operates in liquids as well but is negligible in solids.
- Gases are compressible. They can easily be compressed by applying pressure because there are large empty spaces between the molecules.
- Gases are expendables. They can expand on heating or increasing the available volume . They expand to fill the entire volume of their containers.
Liquids and solids do not show an appreciable increase in volume when they are heated. On sudden expansion of gases, cooling effect is observed. this phenomenon is called Joule-Thomson effect.
- Gases exert pressure. They exert pressure on the walls of the container.
- Gases are miscible. They can be mixed in all proportions forming a homogeneous mixture.
- weak intermolecular forces. The intermolecular forces in gases are very weak.
- Gases are liquefiable. They may be converted to the liquid state with sufficiently low temperature and high pressure.
Properties of Liquids
All liquids show the following general properties.
1. Liquids have definite volume but indefinite shape. They adopt the shape of the container in which they are placed but their volume does not change.
2. Liquids molecules are in constant random. The evaporation and diffusion of liquid molecules are due to this motion.
3. Liquids are denser than gases. The densities of liquid are much greater than those of gases but are close to those of solids.
4. Liquid molecules are very close together. Molecules of liquid lie close together with very little space between them so they cannot be compressed.
5. Intermolecular attractive forces. The intermolecular attractive forces in liquids are intermediate between gases and solids. The melting and boiling points of gases, liquid and solids depend upon the strength of such forces. The strength of these forces is different in different liquids.
6. Liquid molecules possess kinetic energy. Molecules of liquids possess kinetic energy due to their motion. Liquid can be converted into solids on cooling, e I , by decreasing their kinetic energy. Molecules of liquid collide among them-selves and exchange energy but those of solid cannot do so.
Properties of Solids
1. Solid particles are very close together.
The particles of solid substances are very close to each other and they are tightly packed. Due to tight packing of particles solids are non-compressible and non-diffusible.
2. Strong attractive force.
There are strong attractive forces in solids which hold the particles together firmly. Due to this reason solids have definite shape and volume.
3. The solid particles possess only vibration motion.
Units of Pressure
“The pressure of air (Earth’s atmosphere) at sea level that will support a column of mercury 760 mm in height is called one atmosphere.”
It is the force exerted by 76 cm long column of mercury on an area of 1 cm2 at O0C .It is the average pressure of atmosphere at sea level. A second common pressure unit is the torr. One torr is the pressure exerted by a column of mercury 1 mm in height.
So,
1 atm=760 torr=760mm Hg
The SI unit for pressure is the Pascal (pa). Remember that pressure is force per unit area, so Pascal can by expressed using the SI units for force and area. The SI unit of force is the Newton (N), and are area is measured in square (m2).Thus, I pa =1Nm-2 , and IN =1 kg ms-2,
So,
1 pa =1 kg m-1 S-2
Hence, 1 atm =760 torr =760mm Hg=101325 pa =101.325 kpa (kilopascal).
The most commonly unit of pressure used in engineering work is pounds per square inch (psi).
1 atm -760=760torr =14.7 pounds inch 2 .
The unit of pressure mill bar is commonly used by meteorologists.
Gas Laws
“The relationships between the volume of a given amount of gas and the prevailing conditions of temperature and pressure are called the gas laws.”
When the external conditions of temperature and pressure are changed, the volume of a given quantity of all gases in affected. This effect is nearly the same irrespective of the nature of the gas. Thus gases show a uniform behaviour towards the external conditions. The gas laws describe this uniform behaviour of gases.
Boyle,s Law
“At constant temperature, the volume of a given mass of a gas is inversely proportional to the pressure applied a the gas.”
Mathematically:
PV=K ----------(1)
Where”K”is a proportionality constant. The value of k is different for different amounts of the same gas .
According to Eq (1), boyle’s law can be stated as follows:
“At constant temperature, the product of pressure and volume of a fixed amount of a gas remains constant.”
So, P1V1 =k and P2V2=k
Hence, P1V1 =P2 V2
Where P1 and V1 are the initial pressure and volume while P2 and V2 are the final pressure and volume.
Experimental Verification of Boyles,s Law
Consider a fixed amount of a gas in a cylinder at constant temperature say at 25oC. The cylinder is fitted with a moveable piston and a manometer to read the pressure of the gas directly .Let the initial volume of the gas is 1 dm3 and its pressure is 2 atm when the piston has one weight on it. When the piston is loaded with a weight two times greater with the help of two equal weights, the pressure becomes 4 atm and the volume is reduced to dm 3. Similarly when the piston is loaded with a weight three times greater.
Then the pressure becomes 6 atm and volume is reduced to dm3 .Thus
P1 V1 =2atmx1dm3 =2 atm dm3 =k
P2 V2 =4atm x dm3 = 2atm dm3 =k
P2 V2 =4atm x dm3 = 2atm dm3 =k
Hence the Bye’s law is verified.
The value of k will remain the same for the same quantity of a gas at the same temperature.
Example 1 : A gas having a volume of 10 dm3 is enclosed in a vessel at 0oC and the pressure is 2.5 atmospheres . This gas is allowed to expand until the new pressure is 2 atmospheres. what will be the new volume of this gas, if the temperature is maintained at 273 k.
Solution: Given: P1 =2.5atm ; P2 =2atm
V1 =10dm3 ; V2 =?
Since temperature is constant ( 0o C =273k )
Formula Used: V2 =P1 V1
V2 =
V2 =
V2 =12.5 dm3 Answer
Graphical Explanation of Boyles,s Law
Take a given amount of gas and enclose it in a cylinder having a piston in it. The Boyle,s law can be represented graphically in three different ways as described below:
(i) Keeping the temperature constant at 0o C, when the pressure of the gas is varied, its volume changes. On plotting a graph between pressure on the x-axis and volume on the y-axis, a curve is obtained. Fig (a). This graph shows that volume is inversely proportional to pressure. Increase in pressure decreases the volume.
“The pressure –volume curve obtained at constant temperature is called isotherm”
Now increase the temperature of the same gas to 25o C .keeping the temperature constant graph between pressures of the gas is varied, its volume changes. Again on plotting a obtained .this curve goes away from both the axes, fig (b). The reason is that, at higher temperature, the volume of the gas has increased. Again, this graph shows that volume is inversely proportional to pressure. The increase in pressure decreases the volume.
Similarly, if we increase the temperature further, keeping it constant, and again plot another isotherm, It further goes away from the axes and produces the same result.
Fig (a) . Isotherm of a gas at 0oC Fig (b). Isotherm of a gas at different temperatures
(ii) Keeping the temperature constant at T1, when the volume of the gas is increased, the gas spreads over a larger region of space. Consequently, it exerts less pressure on the container. It means increase in column decreases the pressure of the gas.
On plotting a graph between the reciprocal of volume ( )n x-axis and pressure (P) on the y-axis. It shows that p is directly proportional ( )to . It indicates that the pressure goes down as the gas expands. This straight line will meet at the origin where both p and ()are zero. Thus we can say that p goes to zero as V goes to infinity ( =0). Now increase the temperature of the same gas from T1 to T2 . Keeping the temperature constant at T2 . When volume is varied, its pressure changes. Again plot the graph between ( )on x-axis, and P on y-axis, again a straight line is obtained which meet at the origin . This straight line will be away from the x-axis, Fig(a) . Again the straight line straight line indicates that P is directly proportional to ( ).
(iii) Keeping the temperature constant at T1, when the pressure of the gas is varied, its volume changes. On plotting a graph between P on x-axis and the product PV on y-axis, a straight line parallel to x-axis is obtained, Fig (b). This straight line shows that PV remains constant even if we change pressure.
Now increase the temperature of the same gas form T1 to T2. Keeping the temperature constant at T2, when the pressure of the agars is varied, its volume change. On plotting graph between P on x-axis and the product PV on y-axis, again a straight line parallel to x-axis is obtained, Fig (b) . This straight line shows that PV remains constant even if we change pressure. However, the value of PV increases with increase in temperature.
Charles’s Law
“At constant pressure, the volume of a given mass of a gas is directly proportional to the absolute temperature.”
Mathematically: V = T (P and N are constant)
V = kT
= k
Where k is a proportionality constant.
If the temperature is changed from T1 to T2 and the volume is changed from V1to V2 then we have
= k
and = k
so, = = k
=
Experimental Verification of Charles, s Law
Consider a fixed amount of a gas enclosed in a cylinder fitted with a moveable piston. Let the volume of the gas is V1 and its temperature is T1. If the gas in the cylinder is heated, the temperature and the volume of the gas will increase . The new temperature be T2 and volume will be V-2 . It can be shown that the ratio.
=
Hence Charles’ s law is verified.
Example 2 : 250cm3 of hydrogen is cooled from 127o C to -27o C by maintaining the pressure constant. Calculate the new volume of the gas at the low temperature.
Solution: Given: V1 =250cm3 ; V2 =?
T1 =273+127=400k ; T2 =273-27=246k
Formula Used: =
V2 = xT2
V2 =
V2 = 153.75 cm3 Answer
Derivation of Absolute Zero
In order to derive absolute zero of temperature considers the following quantitative statement of Charles’s law.
Statement: “At constant pressure , the volume of a given mass of gas increases or decreases by of its original volume at 0 oC for every 1 oC rise or fall in temperature respectively.”
In order to understand the above statement .suppose V o is the volume of a given
Mass of gas at 0 oC and V1 is the volume at T oC .Then from the statement of Charles,s law.
For 1 oC rise in temperature, the increase in volume =Vo x=
And, for to C rise in temperature ,the increase in volume =xt
=
The volume , Vt of the gas at to C becomes ,
Vt =vol of gas at 0oC =increase in vol.at toC
Vt=Vo +
Vt=Vo (1+)
This general equation can be used to know the volume of the gas at various temperatures.
Suppose at 0oC , the original volume , Vo of the gas is 546 cm 3 .Thus
Vo =546 cm3
At 10o C, V10 =546(1+dm3
At 100 oC V100 =
cm3
Thus , the increase in temperature from 10o C to 100 oC, increases the volume from 566 cm3 to 746cm3 .
Applying the Charles’s Law,
=
The two ratios are not equal. So , Charles’s law is not being obeyed when temperature is measured on the Celsius scale. For this reason a new temperature scale has been developed.
The new temperature scale starts from-273oC (more correctly-273.16 oC) which is called zero Kelvin (0k) or zero absolute. The advantage of this scale is that all the temperatures on this scale are in positive figures .In order to develop the new temperature scale, the best way is to plot a graph between the variables of Charles,s law . That is, V and T.
Table: volume –Temperature data for a given amount of a gas at constant pressure
Volume (cm3)
|
Celsius Temperature(oC)
|
Temperature (K)
|
=k=cm3 k-1
|
1092
|
273
|
546
|
2
|
846
|
150
|
423
|
2
|
746
|
100
|
373
|
2
|
646
|
50
|
323
|
2
|
566
|
10
|
283
|
2
|
548
|
1
|
274
|
2
|
546
|
0
|
273
|
2
|
544
|
-1
|
272
|
2
|
526
|
-10
|
263
|
2
|
400
|
-73
|
200
|
2
|
346
|
-100
|
173
|
2
|
146
|
-200
|
73
|
2
|
0
|
-273
|
0
|
2
|
Graphical Explanation of Charles,s Law
Keeping the pressure of a given mass (say 1 mole) of gas constant, when the temperature of a gas is varied, its volume changes. The volumes at different temperatures are measured, Now if a graph is plotted between temperature (oC) on x-axis and volume on y-axis , a straight line is obtained ,this straight line cuts the temperature-axis and (x-axis) at -273. 16oCif it is extrapolated. The volume of a gas becomes zero at -273.16oC. This temperature is the lowest temperature which is attainable if the substance remains in the gaseous stat. Actually, all real gases first converted into liquid and then into solids before reaching this temperature.
If many plots of this type are examined. It is found that a given gas follows different straight lines for different masses of gas and for different pressures. Greater the mass of gas taken, greater will be the slope of the straight line. The reason is that greater the number of moles greater the volume occupied .All these straight lines when extrapolated meet at a common point of -273.16oC (0k). It is clear that this temperature of -273.16oC will be attained when the volume becomes zero. It is true for an ideal gas. But for a real gas. Thus-273.16oC represents the coldest temperature. This is the zero point (0K) for an absolute scale of temperature.
Charles’s law is obeyed when the temperature is taken on the Kelvin scale. For example, at 283K (10o C) the volume is 566 cm3 and at 373 k (100o C) the volume is 746 cm3 . According to Charlie’s law.
==k
Hence , Charles’s law is obeyed .
Scales of Thermometry
For measuring temperature, three scales of thermometry are used.
(a) Celsius or Centigrade Scale
One this scale, the temperature of ice at 1 atmospheric pressure is 0oC, and the temperature of boiling water at 1 atmospheric pressure is 100oC. The space between these temperature marks is divided into 100 equal parts and each part is 1 oC
(b) Fahrenheit Scale
On this scale, the temperature of ice at 1 atmospheric pressure is 32Fand the temperature of boiling water is 212o F .The space between these temperature marks is divided into 180equal parts and each part is 1oF
(c) Absolute or Kelvin scale
One the absolute or Kelvin scale, the temperature of ice at 1 atmospheric pressure is 273 K and that of boiling water is 373 K. On the Kelvin scale, absolute zero (0K) is the temperature at which the volume of a gas becomes zero. It is the lowest possible temperature that can be achieved. Thus, temperatures on the Kelvin scale are not divided into degrees .Temperatures on this scale are reported in units of Kelvin , not in degrees temperature on Kelvin Scale=273.16oC or 0K=273.16oC
Temperature on Kelvin scale=273+temperature oC
Or
T (k)=273+ oC
The following relationship helps us to understand the introversions of various scales of temperatures.
T (k)=273+ oC
oC=
of=(oC)+32
Remember that the Fahrenheit and Celsius scales are both relative temperature scales. They define two reference points (such as 0oC and 100oC). The Kelvin scale, however, is an absolute scale. Zero on this scale is the lowest temperature that can achieved.
Standard Temperature and Pressure (STP)
The temperature 273.16 k (0o C) and pressure 1 atmosphere (760 mm Hg) have been chosen as standard temperature and standard pressure.
STP 273.16K (0o C) and 1 atm (760 mm Hg)
General Gas Equation [Ideal Gas Equation]
“ A gas equation which is obtained by combining Boyle ‘s law , Charles ,s law and Avogadro ,s law is called general gas equation.”
It relates pressure, volume temperature and number of moles of gas.
According to Boyle’s Law V (at constant T and n)
According to Charles’s Law V T (at constant P and n)
According to Avogadro’s Law VT (at constant P and T)
If the variables P,T and n are not Kept constant then all the above three relationships can be combined together.
V
V=R
Where ‘R’ is a general gas constant.
pV=nRT(First Form)
The above equation is called ideal or general gas equation.
The general gas equation shows that if we have any quantity of an ideal gas then the product of its pressure and volume is equal to the product of number of moles,
Variation on the Gas Equation
During chemical and physical processes, any of the four variables (P,V,n ,T) in the ideal gas equation ,PV =nRT may be fixed and any of them may change.
General Gas Equation For one mole of gas
For one mole of gas , the general gas equation is
PV==RT [n=1]
For two different pressure, volumes and absolute temperatures, the above equation can be written as,
Hence, we can write,
= (Second Form)
Numerical Value of Ideal Gas Constant , R
The numerical value of R depends upon the units chosen for P,V and T.
(i) When pressure is expressed in atmosphere and volume in dm3.
For 1 mole of a gas at STP, we have
N= 1 mole, P=1 atm, T =273.16k, V=22.414dm3
Putting these value in general gas equation,
R=
R=
R=0.0821dm3 atm k -1 mol-1
(ii) when pressure is expressed in mm of mercury or torr and volume of gas in cm3 .
R= 0.0821dm3 atm k-1 mol-1
Since 1 dm3 =1000cm3 : 1 atm =760mm of Hg
R=0.0821x1000 cm3 x760mm Hg K-1 mol-1
R=62400 cm3 mm Hg K-1 mol-1
Since 1 mm=1 torr
R=62400 cm3 torr K-1 mol-1
(iii) When pressure is expressed in Nm-2 and volume in m3 (SI Units)
For 1 mole of gas at STP,
N= 1 mol, P=101325 Nm-2, T=273.16 K, V=0.022414m3
Substituting the values in general gas equation,
R=
R=
R=8.3143 NmK-1 mol-1
Since 1 Nm=1 j
R=8.3143jk -1 mol-1
Remember that, whenever the pressure is given in Nm-2 and the volume in m3 ,then the value of used must be 8.3143 jk-1 mol-1 .
Remember that when the pressure is given in Nm-2 and the volume in m3, then the value of R used must be 8.3143JK-1 mol-1 .joule (j) is unit of work and energy in the system.
(iv) When energy is expressed in ergs.
R=8.3143jk -1 mol-1
Since 1 J =107 erg
R=8.3143x107 erg K-1 mol -1
(v) When energyis expressed in calories.
R=8.3143jk -1 mol-1
Since 1 cal=4.184j
Or 1 j=
R=
R = 1.987 cal K-1 mol-1
(vi) R can be expressed in units of work or energy per Kelvin per mole .
Form ideal gas equation, we can write
R =
If the pressure is written as force per units area and volume as area x length,
R====
Hence R can be expressed in units of work or energy per Kelvin per mole.
Physical Meaning of Value of R
The physical meaning of the value 0.0821 dm3 atm K-1 mol-1 of R is that if we have one mole of an ideal gas at 273.16 K and one atmospheric pressure and its temperature is increased by 1 K then it will absorb 0.0821 dm3 atm of energy , dm3 atm being the unit of energy .Hence the value of R is a universal parameter for all the gases . It tells us that the Avogadro’s number of molecules of all the ideal gases have the same demand of energy.
Density of an Ideal Gas
We know that, n=
Where a is the number of moles of gas .m is the mass of the substance in grams and M is the molar mass of the substance.
On substituting the above expression into the ideal gas equation,
We obtained
PV=RT (Third From)-------(6)
PM==RT
PM=dRT (d=)
d= (Fourth From)---------(7)
Hence the density of an ideal gas is directly proportional to its molar mass.
Greater the pressure on the gas , the closer will be the molecules and greater the density temperature of the gas ,the lower will be the density of the gas.
With the help of equation (7) ,we can calculate the relative molar mass, M of an ideal gas if its pressure ,temperature and density are know.
Example3:
A sample of nitrogen gas is enclosed in a vessel of volume 380 cm3 at 120o C and pressure of 101325 Nm-2 .This gas is transferred to a 10dm3.
Flask and cooled to 27o C.Calculate the pressure in Nm-2 exerted by the gas at27o C.
Solution:
V1 =380cm3 =0.38 dm3 ; V2 =10dm3
P1 =101325 Nm-2 ; P2 =?
T1 =273+120=393k ; T2 =273+27=300k
Formula used:
=
P2 =
P2 =
P2 =0.028atm
P2 =0.029x101325=2938.4 Nm-2 Answer
Example 4:
Calculate the density of CH4 at 0o C and 1 atmospheric pressure, what will happen to the density if
(a) temperature is increase to 27o C.
(b) the pressure is increased to 2 atmospheres at 0o C.
Solution:
MCH4=12+4=16g mol-1
P=1 atm
R= 0.0821dm3 atm K-1 mol-1
T = 273+0=273K
Formula Used:
d =
d =
d=0.7138 g dm3 Answer
(a) Density at 27o C
MCH4=16g mol -1
P= 1 atm
R=0.0821 dm3 atm K-1 mol-1
T=273+27=300K
Formula Used:d=
d =
d = 0.6496g dm3 Answer
Thus by increasing the temperature from 0o C , the density of gas has decreased form 0.7143 to 0.6496 g dm3 .
(b) Density at 2 atm pressure and 0o C.
MCH4=16g mol-1
P = 2 atm
R =0.0821 dm3 atm K-1 mol-1
T = 273+0=273k
Formula Used: d =
d =
d=1.428 g dm-3 Answer
Thus the increasing of pressure has increased the density . the density becomes double by doubling the pressure .
Example 5:
Calculate the mass of 1 dm3 of NH3 gas at 30o C and 1000 mm Hg pressure, considering that NH4 is behaving ideally.
Solution: Given: P=1000 mm Hg = 1.316 atm ; V=1 dm3
MNH4 =14+3=17g mol-1 : R=0.0821dm3 atm K-1 mol-1
T=273+30-303l ; m=?
Formula Used: PV =
M=
M =
M =0.0908 g Answer
Avogadro ‘s Law
The law may be stated in a number of ways as follows;
1. “Equal volumes of ideal gases at the same temperature and pressure contain equal number of molecules.”
2. “Equal number of molecules of ideal gases at the same temperature and pressure occupy equal volumes.”
3. “At constant temperature and pressure, the volume of an ideal gas is directly proportional to the number of moles or molecules of gas.”
Mathematically: Vn (at P and T constant)
V=kn
Explanation:
Since one mole of an ideal gas at STP has a volume of 22.414 dm3 ,So 22.414dm3 of an ideal gas at STP will have Avogadro’s number of molecules , i.e, 6.02x1023 .
Mathematically , it can be written as,
22.414 dm3 of an ideal gas at STP has number of molecules =6.02x1023
1 dm3 of an ideal gas at STP has number of molecules =molecules
=2.68x1022 molecules
In other words if we have one dm3 of each of H2 ,O2 ,N2 and CH4 in separate vessels at STP, then the number of molecules in each will be 2.68x 1022 .
Similarly when the temperature or pressure are equally changed for these four gases, then the new equal volumes will have the same number of molecules , 2.68x1022 .
One dm3 of H2 at STP weight approximately 0.0899 grams and one dm3 of O2at STP weight 1.4384 grams, but their number of molecules are the same .Although , oxygen molecule is 16 times heavier than hydrogen but this does not disturb the volume occupied by the molecules because molecules , of the gases are widely separated from each other at STP .One molecule of gas is approximately at a distance of 300 times its own diameter from its neighbors at room temperature .
Daltan’s Law of Partial Pressure
“The total pressure exerted by a mixture of non-reacting gases is equal to the sum of their individual partial pressure.”
Mathematically: Pt =P1 +P2 +P3
Where Pt is the total pressure of the mixture of gases and P1 ,P2 and P3 are the partial pressure of gas1 . gas2 and gas 3 respectively in the mixture.
The partial pressure of a gas in a mixture of gases in the pressure a gas would exert if it were the only gas in the container. Partial pressures are commonly represented by small p‘s.
Explanation: Suppose we have four 10 dm3 cylinders. The first one contains H2 gas at a pressure of 400 torr; the second one contains CH4 gas at a pressure of 100 torr at the same temperature. Now transfer these gases to the fourth cylinder of capacity 10dm3 at the same temperature. According to Dalton‘s law, the pressure in the fourth cylinder is found to be equal to the sum of the pressures that each gas exerted by itself.
Pt =PH2 +PCH 4 +Po2
Pt=400+500+100=1000 torr
There three non-reacting gases are behaving independently under the normal conditions. The rapidly moving molecules of each gas in a mixture have equal opportunities to collide with the walls of the container. Hence, each gas in the container exerts a pressure independent of the pressure of other gases in the container. The total pressure is the result of total number of collisions per unit area in a given time. Since the molecules of each gas move independently, so the general gas equation can be applied to the individual gases in the gaseous mixture.
(i)PH2 V=n H2 RT PH2 =nH2 PH2 nH2
(ii)PH2 V=n H4RTPH2 =nH2 Pch2 nCh4
(iii)PO2 V=no2 RT Po2 =no2 Po2 no2
is a constant factor
H2 , CH4 and O2 have their own partial pressures . Since volume and temperature are the same so their number of moles will be different and will be directly proportional to their partial pressures .
Adding Equations (i),(ii) and (iii),we get ,
(PH2 +PCH4 +Po2 )V= (nH2 + nCH4 +no2 )RT
Pt = nt RT
Where Pt =PH2 +PCH4 +Po2 , nt =nH2 +nCH4 + no2
Hence, the total pressure of the mixture of gases depends upon the total number of .moles of the gases.
Example 6: There is a mixture of H2, He and CH4 occupying a vessel of volume 13 dm3 at 37 oC and pressure of 1 atmosphere. The masses of H2 and He are 0.8g and 0.12g respectively. Calculate the partial pressure in torr Hg of each gas in the mixture.
Applications of Dalton
The four important applications of Dalton
1. Determination of the pressure of dry gas: The Daltons
Remember that :
While solving the numerical, the aqueous tension is subtracted from the total pressure (P moist gas )
2. Process of Respiration: In living beings, the process of respiration depends upon differences in partial pressures. When animals inhale air then oxygen moves into lungs as the partial pressure of oxygen in the air is 159 torr while the partial pressure of oxygen in the lungs is 116 torr . On the other hand, CO2 produced during respiration moves out in the opposite direction as its partial pressure is low in the air relative to its partial pressure in the lungs .
3 Pilots feel uncomfortable breathing at higher altitudes: At higher altitudes ,the pilot feel uncomfortable breathing because the partial pressure of oxygen in the unperssurized cabin is low as compared to torr in air where one feels comfortable breathing.
4 Deep sea divers feel uncomfortable breathing: deep Sea divers take oxygen mixed with an inert gas (He) and adjust the partial pressure of oxygen according to the requirement. In sea after every 100feet depth, the diver experiences about 3 atm pressure .Therefore ,normal air cannot be breathed in depth of sea . Moreover, the pressure of N2 increases in depth of sea and it diffuses in the blood.
Diffusion and Effusion
Diffusion
“The spontaneous intermingling (intermixing) of molecules of one gas with another at a given temperature and pressure is called diffusion.”
Explanation of Diffusion from Kinetic Theory
According to the Kinetic molecular theory of gases, the molecules of the gases move haphazardly. They collide among themselves, collide with the walls of the container and change their directions. In other words the molecules of gases are scattered or intermixed due to collisions and random motion to from a homogeneous mixture.
Example: (1) The spreading of fragrance of a rose or a scent is due to diffusion.
(2) Suppose NO2 (a brown gas an d O2 ( a colorless gas )are separated from each other by a partition . When the partition is removed, both diffused into each other due to collisions and random motion. They generate a homogeneous mixture. The partial pressure of
Both are uniform throughout the mixture.
Effusion:
“The escape of gas molecules through an extremely small opening into a region of low pressure is called effusion.”
The spreading of molecules in effusion is not due to collisions but to their escape one by one .Actually the molecules of a gas are habitual in colliding with the walls of the vessel. When a molecule approaches just in front of the opening it enters the other portion of the vessel. This type of escape of gas molecules through a small hole into a region of low pressure or vacuum is called effusion.
Graham’s Law of Diffusion
“The rate of diffusion or effusion of a gas is inversely proportional to the square root of it’ s density at constant temperature and pressure.”
Kinetic Molecular Theory of Gases
“A gas model which explains the physical behavior of gases and the gas laws is called the kinetic molecular theory of gases.
The kinetic molecular theory of gases was first suggested by Bernoulli (1738).R.J .Clausisus (1857) derived the Kinetic gas equation and deduced all the gas laws from it .Maxwell (1859) gave the law of distribution of velocities .Bolt Mann (1870) gave the concept of distribution of energies among the gas molecules. Vander walls (1873) modify the Kinetic gas equation in order to apply for real gases.
Postulates of Kinetic Molecular Theory of Gases
The basic assumptions of the Kinetic molecular theory of an ideal gas are the following:
- All gases consist of a large number of very small particles called molecules. Noble gases (He,Ne, Ar,etc )have mono=atomic molecules .
- The molecules of a gas mve randomly in straight lines in all directions with various velocities. They collide with one another and with the walls of the container and change their direction.
- The pressure exerted by a gas is due to the collections of the gas molecules with the walls fo the container. The collisions among the molecules and with the walls of the container are perfectly elastic. There is no loss energy during collision or mutual friction.
- The molecules of a gas are widely separated from one another .There is large empty spaces between the gas molecules.
- There are no attractive forces between the gas molecules .There is no force of gravity on gas molecules.
- The actual volume of the gas molecules is negligible as compared to the volume of the gas.
- The motion of the gas molecules due to gravity is negligible as compared to the effect of continued collisions between them.
- The average kinetic energy of the gas molecules is directly proportional to absolute temperature of the gas.
Average K.E T
Average K.E=
Note : The average kinetic energy is considered because the gas molecules are moving with different velocities . The velocity of each molecules change with each collision .Some of the molecules has very low velocities, while others move very rapidly.
Kinetic Gas Equation
Keeping in view the basic assumptions of kinetic molecular theory of gases, Clausius derived an expression for the pressure of an ideal gas . This equation relates the pressure of the gas with mass of a molecule of the gas, the number of molecules of the gas in the volume of the gas and the mean square velocities of gas molecules .The kinetic gas equation has the following form.
PV=
Where P=pressure of the gas
V=Volume of the gas
M=mass of one molecule of gas
N=number of gas molecules in volume
=mean square velocity (average of the square of the
Velocities of the gas molecules)
The idea of the mean square velocity is important because all the molecules of a gas at a particular temperature have different velocities .These velocities are distributed among the molecules.
Mean velocity ,Mean Square Velocity ,Root Mean Square Velocity
Mean(average ) velocity,
“The average value of the different velocities of all the molecules in a sample of gas at a particular temperature is called the mean velocit .”
If there are n1 molecules with velocity c1 , n2 molecules with velocity c2 and n3molecules with velocity c3,then ,
Where is known as the mean of average velocity .The bar (-) over the velocity (c ) indicates average or mean value.
In this reference n1 + n2 +n3 =N
mean square velocity ,c2
“The average value of the square of the different velocities of all the molecules in a sample of gas at a particular temperatures is called the square velocity.”
Where is known as the mean square velocity.
Root mean square Velocity ,crms
“The square root of the mean of the squares of the different velocities of all the molecules in a sample of gas at a particular temperature is called the root mean square velocity.”
Crms =
“The square root of the mean square velocity.
The expression for the root mean square velocity deduced from the kinetic gas equation is written as:
Where , Cmns root mean square velocity
M =molecular mass of the gas
T = absolute temperature
This equation is a quantitative relationship between the absolute temperature and the velocities of the gas molecules .According to this equation ,the higher the temperature of a gas ,the greater the velocities .
1. Concept of Absolute Zero of Temperature
According to the equation, T=, absolute temperature of a gas is directly
Proportional to the average translational kinetic energy of the gas molecules. It means that a change in temperature brings about a change in the intensity of molecular motion .Thus, when T=0k (-273.16o C), the translational kinetic theory, absolute zero of temperature, both velocity and kinetic energy of gas molecules become zero. Also at this temperature, the gas exerts no pressure on the walls of the container.
The absolute zero temperature cannot be attained .However, a temperature as low as 10-5 K is obtained.
2. Concept of Average Kinetic Energy of a gas
The average translational kinetic energy of a gas depends only on its absolute temperature and is independent of the pressure, volume, or type of gas. The mean kinetic energy of each gas molecule is the same at the same temperature. The mean kinetic energy of gas molecule does not depend upon its mass. Therefore, a small molecule such as H2 will have , at the same temperature , the same average kinetic energy as a much heavier molecule such as CO2 . Hence, for different gases average kinetic energy of molecules at the same temperature is the same. Thus at the same temperature.
K.E of gas 1 = K.E of gas2
3. Concept of Heat Flow
When heat flows from one body to another, the molecules in the hotter body give up some of their kinetic energy through collisions to molecules in the colder body. This process of flow of heat continues until the average translational kinetic energies of all the molecules become equal. Consequently, the temperature of both the bodies becomes equal.
4. Concept of Temperature of Gases, Liquids and solids
Temperature is the measure of average translational kinetic energies of molecules. In gases and liquids, temperature is the measure of average translational of kinetic energies of their molecules , In solids , temperature is the measure of vibrational kinetic energy of molecules because there is only vibrational motion about mean position .
Liquefaction of Gases
General Principle of Liquefaction of Gases
Under suitable conditions of temperature and pressure all gases may be liquefied. Liquefaction occurs only when the attractive forces between the molecules are greater than the kinetic energy of the molecules.
Two conditions favour this situation.
1. High pressure. High pressure is applied on gases. This brings the molecules of a gas close to each other. Thus the forces between the molecules increase.
2. Low temperature. Low temperature is created in gases .This decreases the kinetic energy of the molecules. Thus the attractive forces between the molecules increase.
In general, the liquefaction of a gas requires high pressure and low temperature.
Liquefaction means the conversion of a gas into a liquid.
For each gas, there is a certain temperature above which the gas cannot be liquefied even if a very high pressure is applied. This temperature is known as critical temperature. The critical temperature of a gas may be defined as, “the highest temperature which a gas con be liquefied by increasing the pressure is called critical temperature.’ The critical temperature is represented by Tc. Below the critical temperature a gas can be liquefied however great the pressure may be. The value of the critical temperature of a gas depends upon its size, shape and intermolecular forces present in it.
“The pressure which is required to liquefy the gas at its critical temperature is called critical pressure.”
The critical pressure is represented by Pc.
The critical temperature and the critical pressure of the gases are very important because they provide us the information about the condition under which gases liquefy.
Examples:
(1) CO2 is a nonpolar gas its critical temperature is 31.1oC. It must be cooled below this temperature before it can be liquefied by applying high pressure. However, if temperature is maintained below 31.1oC. Then lower pressure than critical pressure is required to liquefy it.
(2) NH3 is a polar gas. Its critical temperature is 132.44 oC. It must be cooled below this temperature before it can be liquefy by applying sufficient high pressure.
Table : Critical Temperature and Critical Pressure of Some Gases
Gas
|
Critical Temperature Tc(oC)
|
Critical Pressure Pc9(atm)
|
Carbon dioxide , CO2
|
31.142(304.3k)
|
73.0
|
Freon -12 , CC12F2
|
111.54(384.7k)
|
39.6
|
Ammonia , NH3
|
132.44(405.6k)
|
111.5
|
Water Vapours, H2O
|
374.44(647.6k)
|
217
|
Oxygen , O2
|
-118.75(154.4k)
|
49.7
|
Argon , Ar
|
-112.26(150.9k)
|
48
|
Nitrogen, N2
|
-147.06(126.1k)
|
33.5
|
Joule-Thomson Effect (Joule-Kelvin Effect)
“The cooling effect produced when a compressed gas is allowed to undergo sudden expansion through the nozzle of a jet is called joule-Thomson effect.”
The effect is due to energy used in overcoming the intermolecular attractive forces of the gas. Joule-Thomson effect is used in the liquefaction of gases by cooling.
Explanation: In a compressed gas, the molecules are very close to each other. Therefore, the intermolecular attraction is fairly large. When the compressed gas is allowed to undergo sudden expansion in passing through a small orifice (hole) into a region of low pressure, the molecules move apart. In doing so, the intermolecular attractive forces must be overcome. For this purpose energy is required. This energy is taken from the gas itself which is thereby cooled.
Method from the liquefaction of Gases
Lind’s method of liquefaction of gases
The basic principle of the Linde’s method of liquefaction of gases is the joule-Thosmson effect. The apparatus used for this purpose is shown in the fig.
For liquefaction of air, pure dry air is compression .The compressed air is then passed through a spiral pipe having a jet at the end. The air expands at the jet into the chamber where the pressure of the air falls down from 200 atm to 1atm. As a result of joule-Thomson effect a considerable drop of temperature occurs .The cooled air goes up and cools the incoming compressed air of the spiral tube .the cooled air goes again to the compression pump .The process of compression and expansion is repeated again and again till the air is cooled to such an extent that it liquefies. The liquefied air collects at the bottom of the expansion chamber from where it is drawn off. All gases except H2 and He can be liquefied by this method.
Non-Ideal Behavior of gases
(i) Pressure, and (ii) temperature
A convenient way of showing the deviations of a real gas from ideal behavior is to plot a graph between on x-axis and on y-axis. The ratio is called the compressibility factor. Its value is equal to 1 for 1 mole of an ideal gas for all values of temperature and pressure. For a real gas, the compressibility factor deviate from 1.The extent of deviation from 1 depends upon the pressure and temperature. it may be either greater than 1 or lesser than 1 . A value greater than one shows that thee gas is less compressible than an ideal gas. On the other hand, a value less than one indicate that the gas is more compressible than an ideal gas.
For an ideal gas, since the increase of pressure decreases the volume in such a way that remains constant at constant temperature, so a straight line is obtained parallel to the pressure axis.
In order to understand the deviations of a real gas from ideal behavior, let us study the plot of against p of a few real gases like He, H2, N2 and CO2 first at 0oC and then at 100oc.
(a) Effect of Pressure Variation On Deviations at 0oC
In the fig, the curves between compressibility factor and pressure for the four real gases, He, H2, N2 and CO2 along with the behavior of an ideal gas, are shown.
Fig Non-ideal behavior of gases at 0o C
(i) At low pressures, all the curves for these four gases approach to unity as the pressure approaches zero. Thus all real gases behave as ideal gases at low pressures (upto10 atm).
(ii) At high pressure, the behavior of He and H2 differs from that of N2 and CO2at 0oC.For both he and H2 at 0oC, the compressibility factor is always greater than one. This curves show a continuous increase for both He. It means that these gases are less compressible than expected from ideal behavior. For both N2 and CO2 at 0oC, the curve first decreases and then increases again as the pressure increases. This decrease in the compressibility factor below unity is more pronounced in CO2 than in N2. For CO2, the dip in the curve is greatest.
The extent of deviation of these four gases show that these gases have their own limitations for obeying general gas equation. It depends upon the nature of the gas that at which value of pressure, it will start disobeying .
(b) Effect of Pressure Variation on Deviation at 100oC
In the fig, the curves between compressibility factor and pressure for the four real gases, He, H2, and CO2at 100oC along with the behavior of an ideal gas, are shown. It is clear from the shape of the curves, of these four gases that the deviations from the ideal behavior become less at 100oC than at 0oC. The graphs come closer to the expected straight line and the deviations are shifted towards higher pressure. It means that the increase in temperature makes the gases ideal.
Conclusions:
On the basis of experimental observations, we draw the following two conclusions.
1. Gases are ideal at low pressure and non-ideal at high pressure.
2. Gases are ideal at high temperature and non-ideal at low temperature.
Causes for Deviation From Ideality
In the derivation of ideal gas equation, PV=nRT, it was assumed that all gases behaved exactly alike under all conditions of pressures and temperatures. In 1873, van der Waals pointed out that all real gases deviate from ideal behavior at high pressures and low temperatures. It was found that the following two incorrect postulates in the kinetic theory of gases are the cause of deviation.
1. The molecules exert no forces of attraction on each other in a gas.
When the pressure on a gas is high and the temperature is low then the intermolecular attractive forces become significant. Therefore, the ideal gas equation does not hold good .Actually, under these conditions, the gas does not remain ideal .
2. The actual volume of gas molecules is negligible as compared to the volume of the container .
The actual volume of gas molecules is very small as compared to the volume of the container, so it can be neglected. This volume, however, does not remain negligible when there is high pressure on the gas. This can be understood from the following figures.
Van der Waals equation For Real Gases
Keeping in view the molecular volume and the intermolecular attractive forces,
Van derWallls pointed out that the ideal gas equation does not hold good for real gases
He suggested that both volume and pressure factors in ideal gas equation needed
Correction in order to make it applicable to the real gases.
Volume Correction
The volume of a gas is the free space in the container available for the movement
of molecules . For an ideal gas, the actual volume of the molecules is considered zero. So
the volume of an ideal gas is the same as the volume of the container . It also represents the volume in which the molecules are free to move. For a real gas, the molecules are rigid spherical particles which have a definite volume. Consequently, they take up part of the space in the container. Thus the volume in which the molecules of a real gas are actually free to move is less than the volume of the container. Thus, van der Walls suggested that a suitable volume correction term may be subtracted from the volume of the vessel containing the gas . The volume correction term for one mole of the gas is ‘b’. If v is the volume of the vessel containing the gas, then the volume available to gas molecules for their free movement is written as.
V free = V vessel =b
Where b is constant depending upon the nature of the gas . b is also called the excluded volume or co-volume . It has been found that b is not the actual volume of molecules but it is four times the actual volume of the molecules. B =4 Vm .
Where Vm is the actual volume of one mole of gas molecules.
Pressure Correction
Consider a molecule in the interior of a gas. Since it is completely surrounded by other molecules equally in all directions so the resultant attractive force acting on the molecule is zero .However, when the molecule approaches the wall of the container, the equal distribution of molecules about it is upset. There is now a greater number of molecules away from the wall with the result that these molecules exert net inward pull on the molecule. Thus, when a molecule is about to strike the wall of the container, it experiences a force of attraction towards the other molecules in the gas .So it will strike the wall with a lower velocity and will exert a lower pressure. Hence the observed pressure, P Exerted by the molecules is less than the ideal pressure, Pi by an amount Pi .It is, therefore, necessary to add a pressure correction term in order to obtain the ideal pressure.
Pi =P+ Pi
Pi is the true kinetic pressure, if the forces of attraction would have been absent. Pi is the amount of pressure lessened due to attractive forces.
It is suggested that a part of the pressure P for one mole of a gas used up against intermolecular attractions should decrease as volume increase. Consequently, the value of pi in terms of a constant ‘a’ which accounts for the attractive forces and the volume V of vessel can be written as
Pi =
Plasma Stat (or Plasma)
What is Plasma?
“A high temperature ionized gas mixture consisting of free electrons, positive ions and natural atom sis called plasma.”
It is the fourth state of matter, in addition to solid, liquid and gas. It was first identified by William Crookes in 1879. It is a gaseous state. It is a gaseous state. It is electrically neutral because it consists of almost equal numbers of free electrons and positive ions. It conducts electricity because it contains free electrons and positive ions. It constitutes more than 99% of the visible universe. It is everywhere in our space environment. Naturally occurring plasma is rare on earth. Plasma is used to conduct electricity in neon signs and fluorescent bulbs. It occurs only in lightning discharges and in artificial devices like fluorescent lights, neon signs, etc. Scientists have constructed special chambers to experiment with plasma in laboratories.
How is Plasma formed?
Plasma is formed by the interaction of energy with atoms or molecules. Atoms or molecules may be ionized on heating. An atom on losing an electron changes to a positive ion. In a sufficiently heated gas, ionization occurs many times, creating clouds of free electrons and positive ions. However, all the atoms are not necessarily ionized and some of them may remain neutral atoms. “A high temperature ionized gas mixture consisting of free electrons, positive ions and natural atom sis called plasma.”
Plasma contains a significant number of electrically charged particles which are free electrons and positive ions. It is due to this reason plasma shows electrical properties and behavior.
What plasma can potentially do?
Plasma can generate explosions, carry electrical currents and support magnetic fields within themselves. Space plasmas can contain enough heat to melt the earth thousands of times over. Crystal
Artificial and Natural plasma:
Artificial Plasma can be created by using charges on a gas, e.g,. in neon signs .Plasma at low temperature is not exist. This is because outside a vacuum, low temperature plasma reacts rapidly with any molecule it meets. This aspect makes it both very useful and not easy to use.
Natural plasma exists only at very high temperatures or low temperature vacuums. It does not break down or react rapidly. It is extremely hot having a temperature over 20.000 oC. Plasmas possess very high energy. They vaporize any material they touch.
Characteristics of Plasma:
1. Plasma shows a collective response to electric and magnetic fields. This is because it has sufficient numbers of charged particles. The motions of the particles in the plasma generate fields and electric currents from within plasma density. It refers to the density of the charged particles.
- Although plasma contains free electrons and positive ions and conducts electricity but as whole it is electrically neutral. It contains equal number of free electrons and positive ions.
Where is plasma found?
Plasma is the most abundant from of matter. The entire universe is almost of plasma. It existed before any other forms of matter came into being .Plasmas are found from the sun to qarks. Quarks are the smallest particles in the universe. Most of the matter in inner-stellar space is plasma. The sun is a 1.5 million kilometer ball of plasma which is heated by nuclear fusion.
On the earth, plasma only occurs in a few limited places such as lightning bolts, flames, auroras and fluorescent lights. On passing electric current through neon gas, both plasma and light are produced.
Applications of plasma:
The important applications of plasma are the following.
(i) A fluorescent light bulbs and tube is not like an ordinary light bulb. Inside the tube is a gas. When light is turned on, electricity flows through the tube. This electricity acts as a source of energy and chares up the gas. This charging and exciting of the atoms creates a glowing plasma inside the tube or bulb.
(ii) Neon signs are glass tubes filled with gas. When they are turned on, electricity flows through the tube. The electricity charges the gas and creates plasma inside the tube. The plasma glows a special colour depending on the nature of the gas inside the tube.
(iii) They are used for processing go semiconductors, sterilization of some medical products, lamps ,lasers, diamond coated films , high power micro wave sources, and pulsed power switches.
(iv) They provide foundation for the generation of electrical energy from fusion pollution control, and removal of hazardous chemicals.
(v) Plasmas are used to light up offices and homes, make computers and electronic equipment to work.
(vi) They drive lasers and particle accelerators, help to clean up the environment, pasteurize foods, and make tools corrosion-resistant.
Future Horizons:
Scientists are working on putting plasma to effective use. Low energy plasma should by able to survive without instantly reacting and degeneration. The application of magnetic fields involves the use of plasma. The magnetic fields create low energy plasma which relate molecules which are in a metastable state. The metastable molecules do not react until they collide with another molecule with just the right energy. This enables the met stable molecules to survive long enough to react with a designated molecule. The met stable molecules are selectivity. It makes a potentially unique solution to problems like radioactive contamination. Scientists are experimenting with mixtures of gases to work as met stable agents on plutonium and uranium.
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