Other examples of coupled reactions:

Copper smelting

Cu2S (s) ? 2 Cu (s) + S (s) DGorxn= +86.2 kJ

(FORBIDDEN)

Couple this with:

S (s) + O2 (g) ? SO2 (s) DGorxn= -300.1 kJ

Overall: Cu2S (s) + O2 (g) ? 2 Cu (s) + SO2 (s)

DGorxn= +86.2 kJ + -300.1 kJ = -213.9 kJ (ALLOWED)

Coupled reactions VERY COMMON in Biochemistry :

e.g. all bio-synthesis driven by

ATP ? ADP for which DHorxn = -20 kJ

DSorxn = +34 J/K

DGorxn = -30 kJ @ 37oC

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