Chapter 3 S Block elements Notes

15:17:00 Unknown 2 Comments

Caustic Soda
6.      It is used in salting out of soap.

Reason:

Due to their small atomic size, charge density of Li⁺ ion and Be⁺⁺ ions are higher than their group members. These high charge densities result strong polarizing effect and high heats of hydration.

    2)      First ionization enthalpies of Alkali metals and Alkaline earth metals are generally low. However, ionization enthalpies of IIA elements are higher than IA group elements.

Reason:

Alkali metal and Alkaline earth metals have one and two electrons in their valence orbits respectively. They loose their valence electrons to have the stable configuration of preceding noble gas. Hence 1^st ionization enthalpies of IA and IIA group elements are generally low.

Since each Alkaline earth metal has one extra proton than corresponding alkali metal. This higher nuclear charge attracts electrons more strongly; hence increase the ionization enthalpies of IIA group.

     3)      Ionization potential decreases Lithium to Cesium.

Reason:

Down the group from Lithium to Cesium, atomic size increases due to increasing number of orbits. Therefore valence electrons, in heavier atoms of IA group, are loosely attracted by nucleus and hence low ionization energy is required to remove valence electrons.

     4)      Alkali metals have larges covalent radii.

Reason:

Alkali metals have smallest nuclear charge in their respective periods and force of attraction of nucleus on valence electrons is weakest. Therefore they have largest covalent radii.

     5)      Alkali metals and Alkaline earth metals easily form cations.

Reason:

Alkali metals and Alkaline earth metal, having large atomic size and low ionization potential, easily lose their valence electrons to attain the electronic configuration of preceding noble gas and form cations of +1 and +2 charges respectively.

            K                      K⁺ + e^-

      [Ar] 4s¹                  [Ar]

            Ca                    Ca⁺ + e^-

      [Ar] 4s²                  [Ar] 4s¹

            Ca⁺                   Ca⁺⁺ + e^-

      [Ar] 4s¹                  [Ar]

     6)      Na⁺ ions are smaller than sodium atom.

Reason:

Sodium like other alkali metals looses its valence electron, forming Na⁺ ion, to attain stable electronic configuration of Neon. After removal of electrons third orbit is empty and in Na⁺ ion number of protons exceeds number of electrons. Therefore strong hold of nucleus over electrons causes decrease in size of sodium ion than sodium atom.

     7)      Alkaline earth metal ions are more strongly hydrated than alkali metal ions.

Reason:

Hydration of ions depends upon charge density and ionic radii. Due to small ionic radii and more positive charge on ions of group IIA, strong electric field is produced around these ions. Therefore Alkaline earth metal ions are more strongly hydrated than Alkali metal ions.

     8)      Li⁺ ions are more readily hydrated than K⁺ ions.

Reason:

Hydration depends upon charge density of the ion Lithium with small ionic radii, has high chare density as compared to K⁺ ion. Therefore Li⁺ ions are more readily hydrated than potassium ion.

     9)      Alkali metals are powerful reducing agent.

Reason:

Substances having tendency to loose electrons are called reducing agent. Alkali metal wit their largest size and low ionization potential values, can easily loose their electrons.

            M                     M⁺ + e^- (M= Li, Na, K, Rb, Cs)

Therefore Alkali metals are powerful reducing agent.

     10)  Li⁺/Li couple has exceptionally high negative electrode potential.

Reason:

Substances, having tendency to loose electrons, have highly negative values of standard electrode potential. Li⁺/Li couple has very high negative electrode potential value because high value of heat of hydration for Lithium eases the oxidation of Li and Li⁺.

    11)  Alkali metal can not be used in voltaic cells.

Reason:

High negative values of standard electrode potential of Alkali metals indicate ease of oxidation. Since in voltaic cells water is used as solvent and Alkali metals are readily oxidized in water. Therefore they can not be used in voltaic cells.

     12)  Alkaline earth metals are harder than Alkali metals.

Reason:

Alkali metals and Alkaline earth metals form positively charged M⁺ and M⁺⁺ ions respectively. Due to the greater charge attraction of metal ions and electron gas of Alkaline earth metal crystal, they are much harder than Alkali metals.

     13)  In manufacturing of Sodium, the two electrodes are separated by Iron-gauze diaphragm.

Reason:

In manufacturing of Sodium, Sodium deposits on cathode in molten state and Chlorine gas is obtained on anode. Iron diaphragm is used to separate cathode from anode to prevent reaction between molten sodium and chlorine gas.

            2Na + Cl₂                     2NaCl

     14)  Alkali metals are highly reactive.

Reason:

Since all Alkali metals have one electron in their valence shell. They have great tendency to loose their electron to attain electronic configuration of preceding noble gas. Therefore Alkali metals are highly reactive.

     15)  Na⁺ ions are discharged at cathode in preference to H⁺ ion in the manufacturing of Sodium hydroxide.

Reason:

In the manufacturing of NaOH, Na⁺ ions are more easily deposited on mercury cathode.

                        Na⁺ + e^-                                    Na

                        Na + Hg                                   Na/Hg

Due to high voltage of H⁺ ion on the surface of mercury, Na⁺ ions are more easily deposited on mercury cathode.

     16)  How the given reaction is avoided during preparation of Sodium hydroxide.

                        6OH^- + Cl₂                    ClO₃^- + Cl^- + 3H₂O

Reason:

In the preparation of NaOH, NaOH obtained on cathode and Chlorine gas on anode. To prevent above reaction floating mercury is used as cathode.

     17)  Zinc hydroxide is soluble in excess of Sodium hydroxide.

Reason:

If NaOH in excess reacts with insoluble Zinc hydroxide as a result complex compound Tetrahydroxozincate(II) ion is formed which is soluble in water.

                        Zn(OH)₂ + 2OH^-                       [Zn(OH)₄]^-

                                                                        Tetrahydroxozincate(II)

     18)  Plaster of Paris is used in making plaster coats and moulds.

Reason:

Plaster of Paris when mixed with water sets with expansion in few minutes. This setting with expansion property helps in use of Plaster of Paris in plaster coats and moulds.

Group Trends

Atomic Radii OR Atomic Size:

      It may be defined as,

            “The distance between the outer electrons and nucleus is called Atomic radii or Atomic size.”

Atomic size is measured by diffraction of x-rays through substance in solid state. It is measured in A^o (Angstrom).

                        1A^o = 10^-10 m = 10^-8 cm

Factors Affecting Atomic Size:

                  There are three factors which may affect the atomic size.

1)      Number of Shell:

Increase in number of shell will increase distance between outer electrons and nucleus (Atomic size). Therefore down in a group atomic size increases due to increase in number of shell in an atom. Greater increase in atomic size occurs due to increase in number of shell.

2)      Nuclear Charge:

Due to increase in nuclear charge the attraction between outer electrons and nucleus is increased, which will cause a decrease in atomic size. Therefore atomic size decreases across in a period with increasing nuclear charge. Small variation in atomic size of elements occurs due to variation in their nuclear charge.

3)      Screening Effect of Shielding Effect:         

Electrons in inner shells will tend to shield electrons in outer most shell from nucleus therefore effective nuclear charge is less than actual nuclear charge; this effect of inner electrons is called Screening or Shielding effect.

Shielding effect of inner electrons will weaken the force of attraction between outer electrons and nucleus therefore increases due to increase in inner electrons therefore atomic size increases down in a group.

Ionization Potential

Definition:

            It may be defined as,

                        "The minimum amount of energy required to remove one mole of electrons from one mole of gaseous atoms of the element to from one mole of gaseous cations is called Ionization Potential."

            Ionization potential is the measure of strength of electrostatic attractions between nucleus and outer electrons. Ionization potential is measured in KJ/mole.

            When first electron is removed from the atom the energy required is called 1^st I.P.

            When electron is removed form singly charged cation then the amount of energy is called 2^nd I.P.

Factors Affecting Ionization Potential:

            There are three factors on which ionization potential of elements.

1)      Atomic Size:

                  Increase in atomic size of elements will cause increase in distance between the nucleus and outer electrons. This wills weak the force of attraction and therefore removal of outer electrons becomes easier and ionization potential decreases. Down in a group ionization potential decreases with increasing atomic size. Hence element at the top of each group has highest value of I.P. and element at the bottom has least value.

2)      Nuclear Charge:

                  Increase in nuclear charge will cause stronger attraction between nucleus and outer electrons therefore ionization potential increases with increasing nuclear charge across in a period. Hence element at the left most in periodic table has least value of I.P. and element at the right most in periodic table has highest value of I.P.

3)      Screening Effect OR Shield Effect:

                  Electrons in inner shells will tend to shield electrons in outer most shell from nucleus therefore effective nuclear charge is less than actual nuclear charge; this effect of inner electrons is called Screening or Shielding Effect.

            Shielding effect of inner electrons will weaken the force of attraction between outer electrons and nucleus therefore increase in shielding effect decreases ionization potential. Down in a group shielding effect increases due to increase in inner electrons therefore ionization potential decreases down in a group.

Electro-Negativity

Definition:

                  "The ability of an atom to attract shared pair of electrons to itself is called Electro-negativity."

            Electro-negativity depends upon the electrostatic attraction between outer electrons and nucleus.

Factors Affecting Electro-Negativity:

            Electro-Negativity depends upon following factors.

1.      Atomic Size:

                  Increase in atomic size of elements will cause increase in distance between the nucleus and outer electrons. This wills weak the force of attraction and as result electro negativity decreases. Down in a group electro negativity decreases with increasing atomic size. Hence element on the top of each group has highest and element at the bottom has lowest electro negativity.

2.      Nuclear Charge:

                  Increase in nuclear charge will cause stronger attraction between nucleus and outer electrons therefore electro negativity increases with increasing nuclear charge across in a period. Hence element at left most of the periodic table has least value of electro negativity in that period and element at the right most has highest value.

Heat Of Hydration

Definition:

      "The process in which water molecules surround other substance is called Hydration."

                        "The amount of heat released during hydration is called Heat of Hydration."

            The heat of hydration depends upon the charge density of the ions present. More the charge density more will be the attraction of ions with water molecules, therefore ions having more charge density has more heat of hydration.

            Charge density of the ion depends upon the size of charge and volume of ions, since alkaline earth metal ions have more charge and less volume than alkali metal ions therefore alkaline earth metal ions more easily hydrated than alkali metal.

Electrode Potential

Definition:

            "The potential difference between metal and its salt solution is called Electrode potential."

            Electrode potential is the measure of the conversion of metal and its ion. Ease of conversion causes high electrode potential.

            Alkali metals and alkaline earth metals are strong reducing agent. The values of electrode potentials are used to predict the ease of conversion of metal into its ion. Electrode potentials of these metals are more negative because these metals undergo oxidation very easily.

            Electrode potential of Lithium couple (Li⁺/Li) has exceptionally high value of electrode potential because Lithium has very value of heat of hydration which promotes the oxidation of Lithium into its ion.

            The electrode potential decreases down the group due to increase in atomic size. The increase in atomic size will decrease ionization potential, due to which metal are easily into their ions.

Density, Melting Point & Boiling Point:

            Density, melting point and boiling point of a substance depends upon inter molecular attractions between their particles. Stronger the attractions more will be the density, melting point and boiling point.

            Density of elements in a group increases down in a group with increasing inter atomic attraction between their atoms in the crystal. Therefore element at the top of each group has least density and element at the bottom has highest density.

            Alkaline earth metals are denser than alkali metals due to the presence of M⁺² ions in their structure which causes greater inter atomic attraction.

            Melting and boiling point also depends upon inter atomic attractions. Down in a group melting and boiling point decreases due to decrease in inter atomic attraction.

            Melting and boiling point of alkaline earth metals are more than alkali metals because alkaline earth metals have more inter atomic attractions than alkali metals.