5.2.2: Enthalpy and entropy

Question 1

Entropy units

Answer 1

J mol⁻¹ K⁻¹

Question 2

Entropy (S)

Answer 2

• Disorder
• More disordered = greater entropy
• Overall tendency (towards disorder)

Question 3

How does entropy of a substance vary with temperature?

Answer 3

• 3rd law thermodynamics = entropy of a substance is zero (or close to) at absolute zero and increases with temperature
• Higher temp = faster particles vibrate/move = greater entropy
• Large increases in entropy on state changes (vertical regions)

Question 4

How does the entropy of different substances compare?

Answer 4

• More ordered = lower entropy

* Diamond and graphite = v. highly structured and so have v. low entropy

Question 5

How to calculate entropy change?

Answer 5

ΔS = (ΣS products) - (ΣS reactants)

Question 6

Trends in/factors that affect entropy

Answer 6

• Entropy increases with temperature

* There is a large increase in entropy with any state change –> Change is greater from L–>G than S–>L

Question 7

Effect of ionic charge on lattice enthalpy

Answer 7

• Larger ionic charge = stronger electrostatic attractions between ions
• Lattice enthalpy = more exothermic

Question 8

Effect of ionic size on lattice enthalpy

Answer 8

• Smaller ionic radius = more exothermic lattice enthalpy

Question 9

Effect of ionic charge on hydration enthalpy

Answer 9

• Higher charge = electrostatic attractions between ions and water are stronger
• Enthalpy of hydration is more exothermic

Question 10

Effect of ionic radius on enthalpy of hydration

Answer 10

• Smaller = more exothermic

Question 11

ΔG =

Answer 11

ΔH - TΔS

Question 12

When is a reaction feasible

Answer 12

When ΔG = 0 or -ve

Question 13

How to find the ‘tipping point’ temperature at which a reaction becomes feasible

Answer 13

Make G= 0

0 = ΔH - TΔS

T = ΔH/ΔS

Question 14

Why might a reaction that is feasible not take place?

Answer 14

Because of a very high activation energy