AQA A Level Chemistry: Thermodynamics - Gibbs free energy and entropy

Define entropy

The measure of disorder within a system

What is a feasible reaction ?

A reaction which can occur at a particular tempearture

Why is enthalpy change not enough to determine if a reaction is feasible ?

Some reactions are endothermic

Place the 3 states in order from most ordered to least ordered

solid

liquid

gas

What are the units of entropy ?

JK-1mol-1

What does absolute entropy assume ?

A crystalline form of the substance at 0K is a perfect crystal with entropy 0 (complete order)

What determines the feasibility of a reaction ?

The balance between enthalpy change and entropy change

Why is the entropy change from a liquid to a gas greater than from a solid to a liquid ?

Gases are very disordered compared to both liquids and solids

In general, why does entropy increase as temperature increases?

The particles gain energy and vibrate/move more and become more disordered

Why does the entropy increase at melting/boiling point but the temperature doesn't?

The energy is used to melt/boil the substance and break the bonds (endothermic)

What is the general rule linking Mr and entropy?

The higher the Mr of a substance, the higher the entropy value

SAME STATE

What does entropy change equal?

Entropy of products - Entropy of reactants

Remember absolute enthalpy and entropy values are always positive but enthalpy and entropy CHANGE can be positive or negative

Remember absolute enthalpy and entropy values are always positive but enthalpy and entropy CHANGE can be positive or negative

What is the equation for Gibbs free energy, ∆G ?

∆G = ∆H - T∆S

What does a negative ∆G value mean?

The reaction is feasible at that temperature

What does a positive ∆G value mean?

The reaction is NOT feasible at that temperature

How do you determine at what temperature a reaction becomes feasible ?

Put ∆G = 0

Rearrange to find T

What is the difference between a spontanous and feasible reaction ?

A feasible reaction is one which is possible at a certain temperature

A spontaneous reaction is one is feasible and will occur under standard conditions

Why do some feasible reactions not proceed?

The activation energy is too high

(i.e. melting of diamond)

What value does ∆G take at changes of state?

0 kJmol-1

What are the units of ∆G ?

kJmol-1

Why is ∆G positive for temperatures below the melting point ?

The reaction is not feasible

Why is ∆G negative for temperatures above the melting point ?

The reaction is feasible

When ∆G = 0, in what state is the system ?

Equilibrium

For a reaction to be feasible what must be true ?

∆G must be less than or equal to 0

Which parts of the gibbs free energy equation equate to which parts of the straight line equation ?

∆G = ∆H -T∆S

y = ∆G

x = T

m=-∆S

c = ∆H

What is the y- intercept of the ∆G = ∆H -T∆S graph equal to?

Enthalpy change, ∆H

What is the gradient of the ∆G = ∆H -T∆S graph equal to ?

Entropy change, -∆S

What does a positive gradient on the ∆G = ∆H -T∆S graph mean?

Negative entropy value

-T∆S is positive

so ∆S must be negative

What does a negative gradient on the ∆G = ∆H -T∆S mean?

Positive entropy value

-T∆S is negative

so ∆S must be positive