The concept of coupling reactions can be more understood knowing that Gibbs free energy is a state function. State function is a property that does not depend on the path taken to arrive to that value
What is Coupling reaction ?
A reaction where the the free energy of a thermodynamically favorable transformation, such as the hydrolysis of ATP, and a thermodynamically unfavorable one, are mechanistically joined into a new reaction (or may be envisaged to be so joined) is known as a coupled reaction.
How do you make an unfavorable reaction favorable?
Chemically unfavorable means non-spontaneous which means Gibbs' free energy change (ΔG) is positive, i.e. ΔG>0.
But we know that ΔG=ΔH−TΔS. Now ΔH and ΔS (enthalpy and entropy change, respectively) are state functions. So ΔG is also a state function.
Catalysts can only change the pathway of a reaction (in most of the cases, by lowering the activation energy of the reaction (Ea)) but after using the catalysts also, the energy states of the reactants and products don't alter. So ΔH and ΔS remain the same as uncatalysed reaction. So ΔG also doesn't change at all. So if the reaction is non-spontaneous, it remains like that.
That means, the thermodynamically unfavorable reaction can't be made favorable by introducing a catalyst.
But changing the temperature to a very high value can make ΔG<0, so the reaction can be made favorable by abruptly increasing temperature.
What are favorable and unfavorable reactions?
Reactions that release energy are energetically favorable. Conversely, reactions in which the products are higher in energy than the reactants require an input of energy to proceed; such reactions are energetically unfavorable
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