entropy and gibbs free energy

Question 1

what is entropy

Answer 1

Entropy is a measure of the dispersal of energy at a specific temperature.

Entropy can also be thought of as a measure of the randomness or disorder of a system.

Entropy can also be thought of as a dispersal of energy, either from the system to the surroundings or from
the surroundings to the system.

Question 2

what happens to the energy of a system when the disorder is increased

Answer 2

the system becomes energetically more stable

Question 3

Explain the entropy change taking place when a reaction is exothermic

Answer 3

For an exothermic reaction, the energy released to the surroundings increases the number of ways of
arranging the energy. This is because the energy goes into rotation and translation (movement from
place to place) of molecules in the surroundings. So there is likely to be an increase in entropy and an
increased probability of the chemical change occurring spontaneously. In other words, the reaction
becomes more feasible.

Question 4

explain the entropy change taking place when a reaction is endothermic

Answer 4

For an endothermic reaction, the energy absorbed from the surroundings decreases the number of
ways of arranging the energy. So there is likely to be a decrease in entropy and a decreased probability
of the chemical change occurring spontaneously.

Question 5

calculate the entropy change of the system for the reaction
2Ca(s) + O2(g) —- 2CaO(s)

Answer 5

The standard entropy values are:
S⦵ [Ca(s)] = 41.40 J K
−1 mol−1
S⦵ [O2
(g)] = 205.0 J K
−1 mol−1
S⦵ [CaO(s)] = 39.70 J K
−1 mol−1
ΔS
⦵
system = ΣS
⦵
products − ΣS
⦵
reactants
= 2 × S⦵ [CaO(s)] − {2 × S⦵ [Ca(s)] + S⦵ [O2
(g)]}
= 2 × 39.70 − {(2 × 41.40) + 205.0}
= 79.40 − 287.8
ΔS
⦵
system = −208.4 J K
−1 mol−1
The negative value for the entropy change shows that the entropy of the system has decreased. We
know, however, that calcium reacts spontaneously with oxygen. So the entropy of the surroundings
must also play a part because the total entropy change must be positive for the reaction to be
feasible.