3.7 Entropy and feasibility of reactions

Question 1

What is entropy?

Answer 1

Entropy, S, of a system is a measure of the freedom possessed by particles within it. It is often referred to as a measure of disorder. The higher the entropy, the more freedom the particles possess so the more disordered the system.

Question 2

What is the general relation between natural processes and entropy?

Answer 2

All natural processes will always tend towards increasing entropy because they are more energetically stable when there is more disorder. Therefore there is an increase in entropy, towards a maximum, for all natural changes.

Question 3

This does physical state affect entropy?

Answer 3

The particles in a solid have much less freedom and disorder than those in a gas. Entropy increases across the physical states as follows:

S(solid) < S(liquid) < S(gas)

Question 4

Define standard entropy of a substance

Answer 4

The entropy of one mole of the substance under standard conditions.

Question 5

Give the equation for calculating the overall entropy change from absolute entropy values

Answer 5

Overall entropy change = S final - S initial

where S final can be found by adding up the standard entropy values of all the products and S initial can be found by adding up all the standard entropy values of the reactants.

Question 6

What are the units of entropy?

Answer 6

J K^-1 mol^-1

Question 7

What does it mean if a reaction is ‘spontaneous’?

Answer 7

A spontaneous process is capable of taking place without needing to be driven by an outside source of energy.

Question 8

Is a reaction with positive or negative entropy change more likely to be spontaneous?

Answer 8

A reaction with positive entropy change is more likely to occur spontaneously because reactions will naturally try to increase the amount of disorder.

Question 9

Is a reaction with positive or negative enthalpy change more likely to be spontaneous?

Answer 9

Negative enthalpy change.
Exothermic reactions are more likely to be spontaneous than endothermic reactions.

Question 10

What does it mean for a reaction to be ‘feasible’?

Answer 10

For a reaction to be feasible at a given temperature, the reaction must occur spontaneously. This means no external energy input is required for the reaction to take place.

Question 11

Explain why the following reaction is feasible despite being endothermic:
NaHCO3(s) + H+(g) —> Na+(aq) + CO2(g) + H2O(l)

Answer 11

There is an increase in entropy because the reaction produces carbon dioxide gas and water. Gases and liquids have a higher entropy than solids since they are more disordered. The reaction is also flavoured because there is an increase in the number of moles, which increases entropy. Therefore, the increase in entropy overcomes the fact that it is an endothermic reaction, making the reaction feasible.

Question 12

What is Gibbs free energy change?

Answer 12

Gibbs free energy change is a measure used to predict whether a reaction is feasible.

Question 13

Give the equation for Gibbs free energy change

Answer 13

ΔG = ΔH - TΔS

ΔG = free energy change
ΔH = enthalpy change
T = temperature
ΔS = entropy change

Question 14

How does the feasibility of a reaction relate to Gibbs free energy change?

Answer 14

For a reaction to be feasible, Gibbs free energy change must be negative or zero.

Question 15

Consider an endothermic reaction and explain the effect of T and ΔS on the feasibility of the reaction

Answer 15

ΔG = ΔH - TΔS
For an endothermic reaction, ΔH is positive.

• If ΔS is negative then ΔG is always positive so in this case the reaction is not feasible at any temperature.
• If ΔS is positive then the reaction will only be feasible at a high enough temperature, when TΔS>ΔH.

Question 16

Consider an exothermic reaction and explain the effect of T and ΔS on the feasibility of the reaction

Answer 16

ΔG = ΔH - TΔS
For an exothermic reaction, ΔH is negative.

-If ΔS is positive then ΔG is always negative so in this case the reaction is feasible at any temperature.
- If ΔS is negative then the reaction will only be feasible at lower temperatures, when TΔS < ΔH

Question 17

What are the key factors which tell you the sign of the entropy change of a reaction?

Answer 17

Number of moles in the reaction: More moles made in the product means there is an increase in entropy.

Changes of state: Changing to a state where the particles have a higher kinetic energy leads to an increase in entropy.

Question 18

What are the limitations of using Gibbs free energy change as an indicator of whether a reaction will occur?

Answer 18

Gibbs free energy only indicates if a reaction is feasible. It does not take into account the rate of reaction or the activation energy. Even if a reaction is theoretically feasible, it might still require a very high activation energy or be so slow that effectively no reaction is occurring.

Question 19

How can you calculate the temperature at which a reaction becomes feasible?

Answer 19

A reaction becomes feasible when ΔG is zero. The temperature at which this occurs can be found by rearranging the free energy equation:

ΔH - TΔS = 0 —> T = ΔH/ΔS