Enthalpy & Entropy

What is entropy?

Measure of disorder in a system

Symbol for entropy?

S

Units for entropy?

J K⁻¹ mol⁻¹

What does higher entropy mean?

More disorder / energy dispersal

What does lower entropy mean?

More order

Do reactions always favour lower enthalpy?

No entropy also drives reactions

Why can some endothermic reactions occur spontaneously?

Entropy increase large enough

How does temperature affect entropy?

Increases entropy

Why does higher temperature increase entropy?

Particles gain energy and move more

Order of entropy: solid, liquid, gas?

Solid < Liquid < Gas

Why do gases have highest entropy?

Particles far apart and random

Entropy change when melting?

Sudden increase

Name for melting entropy change?

Entropy of fusion

Entropy change when evaporating?

Large increase

Name for evaporation entropy change?

Entropy of vaporisation

Which is larger: fusion or vaporisation entropy?

Vaporisation

Why is vaporisation entropy larger?

Gas far more disordered than liquid

Entropy change when more gas moles formed?

Increases

Why does more gaseous moles increase entropy?

More particle arrangements

Entropy change when gas moles decrease?

Decreases

Entropy change when lattice dissolves?

Increases

Why does dissolving increase entropy?

Ions free to move

Define entropy change of reaction?

Difference products − reactants

Equation for entropy change?

ΔS = ΣS(products) − ΣS(reactants)

If ΔS positive what does it mean?

Products more disordered

If ΔS negative what does it mean?

Products less disordered

What entropy direction is natural?

Towards greater disorder

Example of natural entropy increase?

Gas spreading in room

What is Gibbs free energy?

Balance of enthalpy and entropy

Symbol for Gibbs free energy?

G

Units for Gibbs free energy?

kJ mol⁻¹

Gibbs free energy equation?

ΔG = ΔH − TΔS

Unit requirement in Gibbs equation?

ΔH and TΔS same units

Temperature unit in equation?

Kelvin

Condition for spontaneous reaction?

ΔG ≤ 0

If ΔG negative what does it mean?

Reaction feasible

If ΔG positive?

Not feasible

If ΔG zero?

At equilibrium / feasible point

How find feasibility temperature?

Set ΔG = 0

Rearranged feasibility equation?

T = ΔH / ΔS

Graph form of Gibbs equation?

y = mx + c

Graph axes for Gibbs plot?

ΔG vs T

Gradient of Gibbs graph?

−ΔS

Intercept of Gibbs graph?

ΔH

Can endothermic reactions be feasible?

Yes if ΔS large positive

Why?

TΔS outweighs ΔH

What determines feasibility overall?

Both ΔH and ΔS

Define thermodynamics?

Energy feasibility of reaction

Define kinetics?

Rate of reaction

Can feasible reactions not occur?

Yes kinetically hindered

Why might feasible reaction be slow?

High activation energy

What is activation energy?

Energy barrier for reaction

If activation energy too high?

Reaction effectively doesn’t occur

Example concept?

Diamond → graphite feasible but slow