Entropy & Gibbs Free Energy

Question 1

Define: entropy

Answer 1

A measure of the ‘disorder’ of a system

A system becomes more stable when its energy is spread out in a more disordered state

Question 2

Equations for calculating entropy

Answer 2

S(total) = S(surroundings) + S(system)

S(system) = S(products) + S(reactants)

S(surroundings) = -H(reaction) / T

Question 3

Explain the entropy changes that occur during:

i) a change of state
ii) a change in temperature
iii) a change in the number of gas molecules

Answer 3

i) solid < liquid/aq < gas
The molecules become less ordered and move more randomly

ii) higher temperature = higher entropy
Greater energy to move rapidly and randomly

iii) more gas molecules = higher entropy
Gas molecules have high entropy, so having more gas molecules increases entropy

Question 4

Defining equation of Gibbs free energy

Answer 4

G = -TS(total)
= -T(-H(reaction)/T + S(system))
= H(reaction) - TS(system)

G can be thought of as the part of the enthalpy change needed to do work
H = G + TS

Question 5

Predicting spontaneity of a reaction

Answer 5

Negative G = spontaneous
Positive G = not spontaneous

Note: at equilibrium, G=0
i.e. H = TS