What is coupling of reaction? How is it useful in metallurgy?

Explanation

If the value of `Delta`G is positive for any reaction, then to make such reaction spontaneous, it is coupled with another reaction of large negative `Delta`G value, so that the sum of two `Delta`G becomes negative. This is known as coupling of reaction.
In the metallurgy, thermodynamically infeasible reaction is coupled with a reaction which has more negative `Delta`G, so that net `Delta`G becomes negative.