Coupled reactions | Applications of thermodynamics | AP Chemistry | Khan Academy
Coupled Reactions: Driving Unfavorable Processes
Understanding Coupled Reactions
- Coupled reactions utilize a thermodynamically favorable reaction to drive a thermodynamically unfavorable one, exemplified by reactants A and B forming products C and i, where the standard change in free energy (ΔG°) is greater than zero.
- When ΔG° is positive, the reaction is unfavorable, leading to an equilibrium constant (K) of less than one, indicating more reactants than products at equilibrium.
- In contrast, another hypothetical reaction involving intermediate i reacting with D to form E and F has ΔG° much less than zero, making it thermodynamically favorable with K significantly greater than one.
- By coupling the unfavorable first reaction with the favorable second reaction that shares common intermediate i, we can produce significant amounts of product C despite the initial unfavorability.
- The removal of intermediate i from the first reaction shifts its equilibrium rightward, enhancing production of desired product C through this coupling mechanism.
Practical Example: Copper Extraction
- A practical example involves extracting solid copper from copper(I) sulfide. The ΔG° for this process is +86.1 kJ/mol, indicating it's thermodynamically unfavorable without coupling.
- To facilitate this extraction, it’s coupled with a favorable reaction converting sulfur into sulfur dioxide (ΔG° = -300.4 kJ/mol).
- Combining these reactions cancels out sulfur as an intermediate; thus copper(I) sulfide plus oxygen yields solid copper and sulfur dioxide as products.
- The overall ΔG° for this coupled equation becomes -214.3 kJ/mol, rendering it thermodynamically favorable and allowing substantial copper production.
Biochemical Importance of Coupled Reactions
- Coupled reactions are crucial in biochemistry; for instance, amino acids alanine and glycine combine to form dipeptides—a process that is typically thermodynamically unfavorable.
- To make dipeptide formation feasible for protein synthesis in the body, ATP hydrolysis provides necessary energy since its ΔG° is -31 kJ/mol—indicating favorability.
- The combination of alanine and glycine forms alanylglycine along with water; however, this formation has a positive ΔG°, marking it as unfavorable without enzyme assistance.
- Enzymes couple the unfavorable dipeptide formation with ATP hydrolysis to drive the overall process forward effectively.