Ncert Solutions Chemistry Class 12th

7.13

The Gibbs free energy formation ( Δ_fG) of Cu₂S is less than that of H₂S and CS₂. Therefore, H₂ and C cannot reduce Cu₂S to Cu.

On the other hand, the Gibbs free energy formation of Cu₂O is greater than that of CO. hence, C can reduce Cu₂O to Cu.

C (s) + Cu₂O (s) → 2Cu (s) + CO (g)

Hence, the extraction of copper from its pyrite ore is difficult than from its oxide ore through reduction.

In the froth floatation method, the role of depressants is to prevent certain types of particles from forming the froth with the air bubbles. For example, NaCN is used as a depressant to separate lead sulfide ore (PbS) from zinc Sulphide ore (ZnS).NaCN forms a zinc complex Na₂ [Zn (CN)₄] on the surface of ZnS thereby preventing it from the formation of froth.

7.46

Nitrogen owing to its small size has a tendency to form p – p multiple bonds with itself. Nitrogen thus forms a very stable diatomic molecule, N₂.

On moving down, a group, the tendency to form p–p bonds decreases [because of the large size of heavier elements]. With the increase in the size of atoms, the strength p– p bonds decreases.

Thus p– p bonds are weaker for heavier atoms. Therefore, phosphorus [like other heavier metals] exists in the P₄ state, as shown below.

Copper can be extracted by hydrometallurgy but not zinc as per the below-given reason:

The E^o of zinc ( Zn²⁺/Zn = - 0.76V) is lower than that of copper (Cu²⁺/Cu = 0.34V). this means that zinc is a stronger reducing agent and can displace copper from a solution of Cu²⁺ ions.

Zn (s) + Cu²⁺ (aq) → Zn²⁺ + Cu (s)

In order to displace zinc by hydrometallurgy, we need stronger reducing agent like K ( E^o_K+/K = - 2.93V)

2K (s) + 2H₂O (l) → 2KOH (aq) + H₂ (g)

As a result, these metals cannot be used in hydrometallurgy to extract zinc. Hence, copper can be

extracted by hydrometallurgy but not zinc.

Below 1683K, the melting point of silicon, the Δ _fG^o curve for the formation of SiO₂ lies above the Δ _fG^o curve for MgO, so, at temperature below 1683 K, Mg can reduce SiO₂. On the other hand, above 1683 K, the Δ_fG^o curve for MgO lies above Δ_fG^o curve for SiO₂. Hence, at a temperature above 1683 K, Si can reduce MgO to Mg.

The reaction,

Cr₂O₃ + 2 Al → Al₂ O₃ + 2 Cr (ΔG⁰ = – 421 kJ)

Is thermodynamically feasible as is apparent from Gibbs energy value. The change in Gibbs free energy is

related to the equilibrium constant, k as

ΔG = - RT ln K

A certain amount of energy activation is required even for such reactions which are thermodynamically feasible, therefore heating is required.

ΔG = ΔH + S

Increasing the temperature increases the value of TΔS, making the value of ΔG more and more negative.

Therefore, the reaction becomes more and more feasible as the temperature is increased.

7.45

Nitrogen has a smaller size than Bismuth, because on going down the group the atomic size increases

i.e. N

Due to smaller size, there is very high electron density around Nitrogen as compared to Bismuth. Therefore, nitrogen atom can easily release electrons. And, we know higher the electron donating tendency, higher is the basic strength. Due to which NH₃ is more basic than BiH₃.

Note: The basic strength in group 15 is in the following pattern:

NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃

The method of leaching consists of treating the powdered ore with a suitable reagent which can selectively dissolve the ore but not the impurities. The impurities are filtered out and are recovered from the solution. For example, bauxite ore containing SiO₂, iron oxide, and titanium oxides impurities are concentrated by this method. Leaching is significant as it helps in removing the impurities like SiO₂, FeO₂, TiO₂, etc. from the bauxite ore.

The ore can be concentrated by the process of magnetic separation, only if either the ore or the gangue can be attracted in presence of magnetic field. In table 6.1, the ores of iron such as haematite (Fe₂O₃), magnetite (Fe₃O₄), siderite (FeCO₃), and iron pyrite (FeS₂) can be separated by the process of magnetic separation.