Unit 9: Topic 5 - Free Energy and Equilibrium

Question 1

In a spontaneous reaction, we have ∆G < 0. Explain what relationship we can determine for K.

Answer 1

In a spontaneous reaction, the reaction should proceed towards the reactants. The products are more favored than the reactants, so this means K > 1.

Question 2

What is the formulaic relationship between G and K? Does this make sense intuitively?

Answer 2

The relationship between G and K is given by G = -RTlnK. Here, the constant R is 8.314 J/mol K, and the temperature T is in Kelvin (and always positive). Notice that lnK > 0 if K > 1 and lnK < 0 if 0 < K < 1. This makes sense intuitively, as if G < 0, the reaction is thermodynamically favorable, so K > 1 as products are favored.

Question 3

Explain why K is close to 1 if G is close to 0, and if G is far from 0 then K differs significantly from 1. Use the equation for support.

Answer 3

If K is close to 1, then one of the products or reactants is favored, but only very slightly. Since G is related to the favoring of reactants or product, if one of these is only very slightly favored, then G should be closer to 0. Similarly, if K differs significantly from 1, then either products or reactants are heavily favored, so G should differ significantly from 0. This is in agreement with the equation, where G is proportional to lnK.

Question 4

Intuitively explain why K < 1 if G > 0.

Answer 4

If K < 1, then the reaction favors the reactants. The reverse reaction would then favor the products, which means G < 0 for the reverse reaction. Therefore, G > 0 for the original reaction. This also makes sense because a reaction favoring reactants would not be thermodynamically favorable.