Question

Explain the principle of coupling in metallurgical operations.

Answer 1

1. A chemical reaction is spontaneous only when the change in Gibbs free energy (ΔG) is negative.
2. ΔG is given by the equation: ΔG = ΔH – TΔS, where ΔH is change in enthalpy and ΔS is change in entropy.
3. If ΔG for a reaction is positive, the reaction is not feasible in the forward direction by itself.
4. Such a non-feasible reaction can be made feasible by coupling it with another reaction with a sufficiently large negative ΔG.
5. When coupled, the sum of the ΔG values of the two reactions becomes negative, thus making the overall process spontaneous.
6. This coupling principle is particularly useful in metallurgical reductions, where an otherwise non-spontaneous reaction is coupled with a spontaneous one to drive it forward.
7. The concept is well illustrated using the Ellingham diagram, which plots ΔG° vs temperature for oxide formation and helps in selecting suitable reducing agents.
8. Thus, coupling assists in carrying out metallurgical operations that are otherwise thermodynamically unfavorable.