Coupled Reactions (College Board AP® Chemistry)

Making Thermodynamically Unfavorable Processes Occur

• A thermodynamically unfavorable reaction is one that does not occur spontaneously

  • It needs the additional input of energy after the reactants are mixed together 

  • It will have a positive ΔG° value.

•  We can make a thermodynamically unfavorable reaction favorable by:

  • Applying external energy sources

  • Coupling it with a thermodynamically favorable reaction that shares an intermediate 

• Examples of applying external energy include:

  • Electrical energy being used to drive an electrolytic cell 

  • Light driving the overall conversion of carbon dioxide to glucose in photosynthesis 

Coupling Reactions

• We can make a thermodynamically unfavorable reaction favorable by coupling it to a favorable reaction via an intermediate

• The sum of the two reactions will result in a reaction that overall has a negative ΔG° value.

• For example:

  • The decomposition of copper(I) sulfide to produce copper as the desired product and sulfur is thermodynamically unfavorable 

    Cu₂S (s) 2Cu (s) + S (s)   ΔG° = +85.1 kJ mol^-1    K < 1

  • The equilibrium constant is less than 1 which means there are more reactants than products at equilibrium so not much copper would be produced

• This reaction can be coupled to a thermodynamically favorable reaction such as the reaction between sulfur and oxygen to give sulfur dioxide 

  S (s) + O₂ (g) SO₂ (g)   ΔG° = –301.4 kJ mol^-1    K > 1

  • At equilibrium, the equilibrium constant is greater than 1 so there will be more products than reactants 

• Coupling these two reactions will allow us to make a significant amount of copper because they share the common intermediate, sulfur 

• Even if only a small amount, sulfur will still be produced in reaction 1 when copper(I) sulfide decomposes

  • This will then be used up in reaction 2 by reacting with oxygen to produce sulfur dioxide 

• Removing the sulfur from reaction 1 causes the equilibrium to shift to the right therefore producing more copper 

• The thermodynamically favorable second reaction is driving the thermodynamically unfavorable reaction 

• Adding the two reactions together, the sulfur will cancel out to give:

  Cu₂S (s) + O₂ (g) 2Cu (s) + SO₂ (g)

• Adding the two ΔG°values together:

  • +85.1 +(-301.4) = –216.3  kJmol^-1

  • A negative value shows now this overall reaction is thermodynamically favorable

  • It will have an equilibrium constant of above 1

  • Copper will be produced

• Coupling reactions are also used in the conversion of ATP to ADP in the body which helps drive protein synthesis 
  

  • Amino acids alanine and glycine will produce alanylglycine (a dipeptide) and water: 

alanine + glycine alanylglycine + H₂O   ΔG° = +31 kJ mol^-1

• This is thermodynamically unfavorable due to the positive ΔG° 

• The hydrolysis of ATP is thermodynamically favorable:

ATP + H₂O ADP + Pi   ΔG° = –33 kJmol^-1

• The reactions are coupled

• The water cancels out to give:

alanine + glycine + ATP alanylglycine + ADP + Pi

• Adding the two ΔG° values together gives an overall  ΔG° = –2  kJ mol^-1

• The reaction is now thermodynamically favored