Entropy, Enthalpy and

Spontaneous Reactions!

Entropy (S), Ethalpy (H) and Gibbs Free Energy

(G)

What do we know about chemical reactions?

We can write chemical reactions and show how atoms and molecules come together to form products.

We can determine the energy change in a chemical reaction ΔH.

We can determine if a reaction is fast or slow (rate).

We can determine how to increase the rate of the reaction.

How do we know if a reaction will occur or not?

Do the reaction in the lab!

Use thermodynamic qualities to determine spontaneity.

What is spontaneous?

Going downhill happens without having to put any energy into it.

Spontaneous

Going uphill only occurs if you continually add energy to the process.

Non-spontaneous

Enthalpy

Nature favours a decrease in potential energy. (NATURE IS LAZY!)

Exothermic reactions are favoured over endothemic reactions.

-ΔH means the reaction is favoured.

+ΔH means the reaction is not favoured.

Entropy

Nature favours an increase in Entropy. (NATURE IS MESSY)

This is the systems state of disorder.

It is dependent on the temperature.

Entropy = S, Change in Entropy = ΔS, kJ/K

Increase in entropy

+ ΔS, Favoured

Decrease in entropy

-ΔS, Unfavoured.

When does entropy increase or decrease?

Are these reactions favoured in

terms of entropy?2 K(s) + Cl2(l) 2KCl(s)

2LiBr(s) 2Li(s) + Br2(l)

N2(g) + 3H2(g) 2NH3(g)

Unfavoured

Favoured

Unfavoured

Spontaneous?

NaOH(s) Na+(aq) + OH-(aq)ΔH = -44.4 kJ

Enthalpy

Entropy

Spontaneity

-Δ H Favoured

+ΔS Favoured

This reaction will ALWAYS be spontaneous!

Spontaneous? 2Al(s) + 3O2(g) 2Al2O3(s) ΔH = -3351 J

Enthalpy

Entropy

Spontaneity?

-ΔH Strongly favoured.

-ΔS Not favoured.

Spontaneous at very low temperatures.

Spontaneous? KNO3(s) K+(aq) + NO3

-(aq) ΔH = +34 kJ

Enthalpy

Entropy

Spontaneous?

+ΔH unfavourable

+ΔS favourable

Spontaneous at very high temperatures!

Spontaneous? 6CO2(g) + 6H2O(g) C6H12O6(s) + 6O2(g) ΔH = +2868

kJ/mol

Enthalpy

Entropy

Spontaneous?

+ΔH unfavoured

-ΔS unfavoured

This reaction is NEVER spontaneous and requires a constant input of energy!

ΔG = ΔH-TΔS

ΔG < 0 the reaction is spontaneous.

ΔG> 0 the reaction is not spontaneous.

Gibbs Free energy is the amount of energy available to do work.

Classwork

Complete handout and reading for tomorrow.