Enthalpy and Entropy

Question 1

Describe entropy?

Answer 1

• The measure of disorder in a system.
• Symbol ‘S’
• the more disorder the higher the S
• If moles increase then S also increases.

Question 2

Explain how an endothermic reaction can spontaneously occur?

Answer 2

• The reaction is endothermic and enthalpically not favourable (+164kjmole-1)
• There are x(3) number of moles on the LHS and x(13) moles on the RHS
• it is entropically favourable
• start with solids but make a gas.
• Increases disorder so entropically favourable.
• reaction occurs spontaneously if changes in entropy overcomes enthalpy changes

Question 3

How to calculate entropy change?

Answer 3

∆S = S˚products - S˚reactants

Units: JK-1mol-1

Question 4

Describe Gibbs free energy?

Answer 4

- tells us if a reaction is feasible or not 
∆G = ∆H - T∆S
- H = kJmol-1
- T = K
- S = kJ K-1mol-1
- G = Jmol-1

Question 5

Using GFE, how would we know if a reaction is feasible?

Answer 5

• If ∆G is negative or 0, a reaction is feasible.

- Entropy needs to be larger than enthalpy so G can be negative.

Question 6

How to calculate ∆H?

Answer 6

q = m x c x ∆T
and then
q / n

q = J
m = g
c = 4.18jg-1k-1
T = K

Question 7

Explain why the second ionisation energy of calcium is more endothermic
than the first ionisation energy of calcium?

Answer 7

• Ca+ is smaller than Ca

- There is a greater attraction from the nucleus.

Question 8

Suggest why the second electron affinity of oxygen is positive?

Answer 8

• The O- ion and electron are both negative

- energy is needed to overcome the repulsion.

Question 9

Explain why the first ionisation energy of calcium is endothermic

Answer 9

• IE refers to removing electron that is attracted to the nucleus
• energy needed to overcome the force of attraction between outer electron and nucleus.

Question 10

Explain why the first electron affinity for oxygen is exothermic?

Answer 10

• electron affinity involves an electron being gained

- experiences attraction to the nucleus.

Question 11

Explain why the first ionisation energy of calcium is less positive than the second
ionisation energy?

Answer 11

• For second IE the electron lost is closer to the nucleus
• for second IE there is one more proton than electron
• so outer electron is more firmly attracted to the nucleus.

Question 12

When can a reaction be feasible at any temperature?

Answer 12

-ΔH and +ΔS means ΔG will always be negative

Question 13

When can a reaction never be feasible at any temp?

Answer 13

+ΔH and -ΔS means ΔG will always be positive

Question 14

When can a reaction be feasible at lower temperatures?

Answer 14

-ΔH and -ΔS means ΔG will be negative at lower temp

Question 15

When can a reaction be feasible at higher temperatures?

Answer 15

+ΔH and +ΔS means ΔG will be negative at higher temperatures.