Higher chemistry: Enthalpy

Chemical reactions involve a change in energy, usually a loss or gain of heat energy.

The heat stored by a substance is called its enthalpy (H).

ΔH is the overall enthalpy change for a reaction.

Potential energy diagrams can be used to calculate both the enthalpy change and the activation energy for a reaction.

Examples: Exothermic Reactions – release energy to the surroundings and there will be a temperature rise

Examples of exothermic reactions are neutralisation reactions and burning magnesium

Endothermic Reactions – absorb energy from the surroundings and there will be a decrease in temperature

Examples: reaction of barium hydroxide and ammonium thiocyanate

Enthalpy

Change

Heat

Temperature

of

Surroundings

Reason

Exothermic

Endothermic

Gets hot

Gets cold

increases

Decreases

Heat is given out

Heat is taken in

Exothermic Reactions

An exothermic reaction is one in which heat energy is given

out. The products must have less energy than the reactants

because energy has been released.

This can be shown by a potential energy diagrams

EA is the activation energy (energy required

to start the reaction)

ΔH is the quantity of energy given out (ie

the enthalpy change)

The following equation is used to calculate

enthalpy change:

Enthalpy change = enthalpy of products -

enthalpy of reactants

ΔH = Hp -Hr

For exothermic reactions, ΔH will always be

negative.

Example

Endothermic reactions

An endothermic reaction

is one in which heat

energy is absorbed.

The products have more

enthalpy than the

reactants

therefore ΔH is

positive.

A catalyst provides an

alternative reaction

pathway which involves

less energy and so the

catalyst lowers the

activation energy.

Activated Complex

The activated complex (high

energy intermediate state

where bonds are breaking

and forming) can be shown

on potential energy

diagrams.

It is the 'energy barrier'

that must be overcome when

changing reactants into

products.