Ncert Solutions Chemistry Class 12th

8.17 On moving along the lanthanoid series, the atomic number increases. Also, with the increase in atomic number, the number of electrons in the 4f orbital also increases.

The 4f electrons have a poor shielding effect. Therefore, the effective nuclear charge experienced by the outer electrons increases. Consequently, the force of attraction between the nucleus and valence electrons increases. This results in a decrease in the size of lanthanoids with the increase in the atomic number. This is termed as lanthanoid contraction.

CONSEQUENCES:

1. The properties of second and third transition series are similar in

2. Separation of lanthanoids from its compounds is possible due to lanthanide

3. The variation in the basic strength of lanthanide hydroxides os due to this (Basic strength decreases from La (OH)₃ to Lu (OH)₃)

7.29

Moist chlorine is a good reducing agent because it can accept electrons from other species as it is very electronegative.

(iii) The bleaching action of chlorine is due to oxidation by nascent oxygen produced. This nascent oxygen can be produced by

Chlorine dissolves in water in absence of sunlight and forms hydrochloric acid and hypochlorous acid (HOCl)

Since hypochlorous acid is not much stable it decomposes giving nascent oxygen. Cl₂ + H₂O  HCl + HOCl

HOCl HCl + [O]

This [O] is called as nascent oxygen. This nascent oxygen is responsible for bleaching action.

NOTE: Since chlorine has the ability to kill harmful micro-organism it also acts as a good disinfecting agent.

7.27

The reasons for the anomalous behaviour of fluorine are as follows:

The smallest size of fluorine

The highest electronegativity

Low bond dissociation enthalpy of F-F bond

Non-availability of d-orbitals in its valence shell The anomalous properties are as follows;

Fluorine shows only one oxidation state-1 while all other halogens show variable oxidation states like

-1, +1, +3, +5, +7 (due to small size, high electronegativity)

Fluorine forms strong hydrogen bonding in its hydrides unlike other halogens. (due to high hydration energy than other elements)

The compounds of fluorine have higher ionic character than other halogens. (due to high electronegativity forming polar interactions)

Fluorine has no tendency to form polyhalide ion whereas other halogen form polyhalideion (due to non-availability of d orbitals in valence shell)

7.26

The oxidizing power of an element depends on three factors.

1. Bond dissociation energy

2. Electron gain enthalpy

3. Hydration enthalpy

Fluorine is considered the strongest oxidizing agent compared to all known elements due to its small size (being the first member of 17 group and ionic radii decrease downwards), low bond enthalpy of F-F (due to high electron-electron repulsion).

High negative value of electron gain enthalpy (the highest electronegativity ) and very high hydration energy of F^- ions (hydration energy is the energy released when a compound is dissolved in water .

 In case of fluorine it is more due to the strong interaction between F^- ions and water, as it is electro-negative). The comparison of both the elements makes this clear that F₂ is a strong oxidizing power as compared to Cl_2.

The electron gain enthalpy of chlorine (-349 KJ mol^-1) is more negative than that of fluorine (-333 KJ mol^{- 1}). However, the bond dissociation energy of fluorine (158.8 KJ mol^-1) is much lesser than that of chlorine (242.6 KJ mol^-1).

Also, because of its small size, the hydration energy of fluorine (515 KJ mol^-1) is much higher than that of chlorine (381 KJ mol^-1). Therefore, the latter two factors more than compensate for the less negative electron gain enthalpy of fluorine. Thus, fluorine is a much stronger oxidizing agent than chlorine.

7.24

Contact process consists of three steps. First is the burning of sulphide ores to form SO2 The second step (key step) is SO₂ is oxidised to SO₃ by heating the mixture on the heterogeneous catalyst V₂O₅.

Now, this step is very crucial for maximising the yield of H₂SO₄ It can be done by controlling the temperature and pressure of reaction.

The forward reaction is exothermic and there is a decrease in a number of moles and As Le- Chatelier principle states that when a system experiences a disturbance (concentration, pressure changes) it will respond to nullify the changes produced to bring initial condition.

So, when we decrease temperature (BEING EXOTHERMIC),

it will try to restore original equilibrium, by experiencing a forward reaction where the temperature is high.

Now in case of pressure, moles of product is less than that of reactant and we know that no. of moles is directly proportional to volume (pressure is inversely proportional to volume). So volume of reactant is more than product if we increase pressure (decrease volume) the reaction will shift to forward direction.

NOTE: The optimum conditions to maximise the yield of sulphuric acid is the pressure of 2-3 atm and temperature around 723K.

7.23

 H₂SO₄ is one the most important and widely used chemicals for many purposes. It has following uses:

As an electrolyte sulphuric acid dissociates in aqueous solution to form ions. These ions are responsible for the conduction of electricity. As a result, sulphuric acid is used in the manufacture of the lead storage battery.

As a dehydrating agent (to remove water) because it has a great affinity for water. It doesn't get dissolved in water but absorb it, so it is used as a drying

For removing layers of basic oxides from the metal surfaces like iron, copper, before the metals are galvanized, electroplated. Because when the metals are dissolved in sulphuric acid the basic oxide layer formed (due to reaction with air, water) will be dissolved in it as it is acidic in nature.