This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Thermodynamic Principles of Metallurgy”.
1. Ellingham Diagram does not give any information regarding the kinetics of the reduction process.
Explanation: Ellingham diagrams are based on thermodynamic concepts, therefore, these diagrams simply suggest whether the reduction of a given metal oxide with a particular reducing agent is possible or not. It, however, does not tell us anything about the kinetics of the reduction process.
2. Which of the following is the correct Gibbs equation?
a) ΔG = ΔH + TΔS
b) ΔG = ΔH – TΔS
c) ΔG = ΔH – 2TΔS
d) ΔG = ΔH – 3TΔS
Explanation: For any process, the change in Gibbs free energy (ΔG) is given by the equation,
ΔG = ΔH – TΔS
Where ΔH is the enthalpy change, ΔS is the entropy change and T is the absolute temperature.
3. What is the criterion of the feasibility of a reaction at any temperature?
a) ΔG of the reaction must be positive
b) ΔG of the reaction must be negative
c) ΔG of the reaction must be equal to zero
d) Does not depend on ΔG of the reaction
Explanation: The criterion of the feasibility of a reaction at any temperature is that the ΔG of the reaction must be negative. For a reaction to be spontaneous, the value of the change in Gibbs energy of the reaction must always be negative.
4. How can a reaction with positive ΔG be made to occur?
a) By increasing the temperature
b) By decreasing the temperature
c) By coupling it with another reaction
d) It is not possible for the reaction to occur
Explanation: A reaction with ΔG positive can still be made to occur by coupling it with another reaction having large negative ΔG so that the net ΔG of the two reactions is negative (ΔG = change in the Gibbs energy of the reaction).
5. What does the Ellingham diagram consist of?
a) Plots of ΔfG° vs T
b) Plots of T vs ΔfG°
c) Plots of ΔfG° vs ΔS
d) Plots of ΔS vs ΔfG°
Explanation: Ellingham diagram normally consists of plots of ΔfG° vs T for formation of oxides of elements. Similar diagrams can also be constructed for sulphides and halides of elements. These diagrams help us in predicting the feasibility of thermal reduction of an ore.
6. Under what condition can Al reduce MgO?
a) Above 1000°C
b) Below 1000°C
c) Above 1350°C
d) Below 1350°C
Explanation: For a reaction to be feasible, the change in the Gibbs free energy of the reaction (Δ G) must be negative. Lesser the ΔG, greater is the feasibility of the reaction. Above 1350°C, the standard Gibbs free energy of formation of Al2O3 from Al is less than that of MgO from Mg. Therefore, Al can reduce MgO above 1350°C.
7. Which of the following is false regarding the Ellingham diagram?
a) It consists of plots of ΔfG° vs T
b) Ellingham diagrams are based on thermodynamic concepts
c) They do not tell us anything about the kinetics of the reduction process
d) They do not assume reactant-product equilibrium
Explanation: The interpretation of ΔG° is based upon K (i.e., ΔG° = -RT ln K). Thus, it is presumed that the reactants and the products are in equilibrium. But, this is not always true because the reactant/product may be solid.
8. Any metal will reduce the oxide of other metals which lie below it in the Ellingham diagram.
Explanation: No, any element (metal) will reduce the oxide of other metals which lie above it in the Ellingham diagram because the free energy change (ΔrG°) for the combined redox reaction will be negative by an amount equal to the difference between the free energy of formation (ΔfG°) of the two oxides at that temperature.
9. Which of the following statements is true?
a) Al can reduce ZnO more readily than Mg
b) Zn can reduce Cu2O more readily than Al
c) Mg can reduce ZnO more readily than Al
d) Zn can reduce Cu2O more readily than Mg
Explanation: The relative tendency of these metals to act as reducing agents is Mg > Al > Zn > Fe > Cu. Mg can reduce Al2O3, ZnO, FeO and Cu2O more readily than Al can reduce ZnO, FeO and Cu2O. Similarly, Al can reduce ZnO, FeO and Cu2O more readily than Zn reduces FeO and Cu2O.
10. How are the oxides of less reactive metals like silver and mercury reduced?
a) By thermal decomposition
b) By electrolysis
c) By using reducing agents
d) Cannot be reduced
Explanation: In case of less reactive metals like silver and mercury, ΔfG° becomes positive at higher temperatures. This suggests that both silver oxide (Ag2O) and mercury oxide (HgO) are unstable and hence decompose at high temperatures to liberate the corresponding metal.
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