Thermodynamics Part 2

Define entropy

A measure of the disorder in a system

How is entropy determined?

By the number of ways the particles and the energy within the particles can be arranged

List the states of matter in decreasing order of entropy

1. Gas

2. Liquid

3. Solid

When are reactions feasible (spontaneously and thermodynamically possible)?

When the total entropy change (S) is positive

Why do larger molecules have higher entropy values than smaller ones?

The larger molecules provide more ways for the quanta of energy to be arranged

How do you work out entropy change of a system (S_system) ?

S_products - S_reactants

What are the units of S_system ?

J K^-1 mol^-1

How does change in the number of moles affect S_system ?

Increase in moles leads to an increase in S_system

What is the Gibbs Free Energy equation?

^G = ^H - T^S

What is the unit for ^G?

J mol^-1

What is the unit for ^H?

J mol^-1

What is the unit for T?

Kelvin

How do you know if a reaction is thermodynamically feasible?

If ^G is negative

Feasibility if a reaction is exothermic (- ^H) and has a postive entropy change ( + ^S)

Feasible at any temperature (^G is negative

Feasibility if a reaction is exothermic (- ^H) and has a negative entropy change (-^S)

Feasible below a certain temperature

Feasibility if a reaction is endothermic (+ ^H) and has a positive entropy change (+ ^S)

Feasible above

Feasibility if a reaction is endothermic (+ ^H) and has a negative entropy change (-^S)

Not feasible at any temperature (^G is positive)

Feasibility of a reaction if ^G is zero

Just feasible (chemical equilibrium)

What are the other factors (apart from Gibbs free energy) that determine thermodynamic feasibility of a reaction?

• Kinetic stability (high activation energy)

• Non-standard conditions (Gibbs is calculated using standard conditions)

On a free energy graph (^G against T), what does the gradient represent?

-^S