Reaction feasibilityAH Chemistry, Unit 2(d)

Thermodynamics• Helps understand and predict the

behaviour of substances and their reactions based on energy changes.

• Does not rely on knowing about atoms, molecules, ions etc.

Questions chemists are interested in…

1. Will a reaction go “spontaneously” in the direction written?

2. Will the composition of the reaction mixture contain enough product at equilibrium?

3. Will the reaction occur at an reasonable speed?

• Thermodynamics predicts if a reaction can happen, given enough time (sometimes a very long time e.g. billions of years).

• Kinetics predicts if a reaction happens at a reasonable rate.

• Thermodynamically favoured• But not kinetically favoured

Why do chemical reactions happen?

To make the Universe more disordered.

Important terms: system and surroundings

Important terms: spontaneous process

Hydrogen and oxygen• Is the reaction of hydrogen and oxygen

spontaneous?

• YES. It is a processes that will happen without any outside intervention.

True or false?

• A spontaneous process is one which moves to minimise it’s energy?

• All exothermic reactions are spontaneous?

• No endothermic reaction is spontaneous?

Ice• Ice forms liquid water

spontaneously at temperatures above 0C (an endothermic process) but the reverse is spontaneous – water to ice (an exothermic process) – at temperatures below 0C.

• There must be another factor (other than energy) which plays a part in determining whether changes or reactions happen.

Barium hydroxide and ammonium chloride

Entropy

• This factor is ENTROPY, S.

• A measure of disorder.

• The larger the entropy, the greater the disorder.

• ∆S = S(products) – S(reactants)

• To define the entropy of a substance in absolute terms, a reference point is needed.

• A perfectly ordered system would have zero entropy.

• The Third Law of thermodynamics states that:

“ the entropy of a perfect crystal at zero kelvin is zero”.

Standard Entropy• The standard entropy of a substance,

Sº, is the molar entropy at 298K and 1 atmosphere pressure.

• The units are J K-1 mol-1

Standard entropy change of a reaction

• ∆rSº = ∑Sº(products) – ∑Sº(reactants)

PractiseCalculate the standard entropy change of reaction at 298K for:

1. The reaction of hydrogen and oxygen

Why do chemical reactions happen?

• To make the Universe more disordered.

Important terms: system and surroundings

Second Law of Thermodynamics• Spontaneous processes are those

that increase the total entropy of the Universe.

• ∆S(total) = ∆ S(system) + ∆ S(surroundings)

Gibbs energy change• Gibbs energy change, ∆G, (free

energy change) combines changes in entropy and enthalpy into a single equation to describe the spontaneity of a process at constant temperature and pressure, using only information from the reaction system.

• ∆G = ∆H - T∆S

If ∆G < 0

If ∆G > 0

the reaction is spontaneous

The reaction is non-spontaneous (to any significant extent)

A negative ∆G means that a reaction CAN happen but does NOT mean that it WILL happen – the reaction is thermodynamically feasible but could be kinetically unfavourable i.e. it is very slow due to a high activation energy.

Practise

Gibbs energy and equilibrium• No chemical reaction proceeds in only

one direction.

• The reaction of hydrogen and oxygen at 298K nearly does (∆Gº = -237.1 kJ mol-1).

Gibbs energy and equilibrium• The reaction of dinitrogen tetroxide to

form nitrogen dioxide (∆Gº = -5.8 kJ mol-1) is different.

N2O4(g) 2NO2(g)

The reaction is spontaneous but does not go to completion.

Gibbs energy and equilibrium• When the Gibbs energy of the

reactants has become the same as the Gibbs energy of the products, the mixture is at equilibrium.

• Therefore, at a point in a reaction when ∆G = 0, the reaction is at equilibrium.

Coupling reactions• A positive ∆G does not mean that a

reaction can never happen, just that it will not occur spontaneously to any significant extent.

• If it is coupled to a reaction with a larger, negative ∆G, the reaction can be made to occur.

• This is common in biochemical processes.

The blast furnace

Ellingham diagrams

• The opposite of iron rusting.

• A +ve G doesn’t mean that it can’t happen, just that work must be done to make it happen.

• It can be coupled to a reaction which has a more negative G in order to make it happen.

2Fe2O3(s) → 4Fe(s) + 3O2(g) G = +1487 kJ mol-1

2CO(g) + O2(g) 2CO2(g) G = -514.4 kJ mol-1

2Fe2O3(s) 4Fe(s) + 3O2(g) G = +1487 kJ mol-1

x3

2Fe2O3(s) + 6CO(g) 4Fe(s) + 6CO2(g) G = -56 kJ mol-1

Overall, the reduction of iron(III) oxide has become thermodynamically feasible.