Enthalpy Change and Activation Energy

Section 1 of 7Activation energy (HT only)

Collisions must be successful

• Collisions must be successful For two particles to react they must collide with enough energy and in the correct orientation.
  • Collisions that do not meet both conditions simply bounce apart.

Definition

• Definition The activation energy, E_a, is the minimum energy that colliding particles must have in order to react.

Different reactions, different barriers

• Different reactions, different barriers A reaction with a high activation energy needs more energy to get started than one with a low value,
  • so it is slower at the same temperature.

Section 2 of 7Enthalpy change, ΔH

Meaning

• Meaning The enthalpy change is the overall transfer of thermal energy to or from the surroundings during a reaction.
  • It is given the symbol ΔH and is measured in kJ/mol.

Exothermic

• Exothermic Energy is released to the surroundings, so the temperature of the surroundings rises.
  • ΔH is negative.

Endothermic

• Endothermic Energy is absorbed from the surroundings, so the temperature falls. ΔH is positive.

Section 3 of 7Drawing a reaction pathway diagram (HT only)

1

Label the vertical axis 'energy' and the horizontal axis 'progress of reaction'.

2

Draw a horizontal line for the reactants at the starting energy and label it.

3

Draw a hump rising above the reactants and then falling to a second horizontal line for the products.

4

Mark activation energy, E_a, with an upward arrow from the reactants' level to the top of the hump.

5

Mark ΔH with an arrow from the reactants' level to the products' level: downward for exothermic, upward for endothermic.

Section 4 of 7Comparing the two shapes (HT only)

Feature	Exothermic	Endothermic
Energy of products	Lower than reactants	Higher than reactants
Sign of ΔH	Negative	Positive
Arrow for ΔH	Points down	Points up
Effect on surroundings	Get warmer	Get colder
Activation energy	Reactants up to the top of the curve	Reactants up to the top of the curve

Section 5 of 7Examples

Energy change in practice

Exothermic examples

• Exothermic examples Combustion, neutralisation and respiration all release heat energy.

Endothermic examples

• Endothermic examples Thermal decomposition and photosynthesis take in energy from the surroundings.

Reading a diagram

• Reading a diagram E_a is measured from the reactants to the peak, never from the reactants to the products.
  • The difference between reactants and products is ΔH.

Section 6 of 7Key terms

Term	Meaning
Activation energy (Ea)	The minimum energy colliding particles need for a reaction to occur.
Enthalpy change (ΔH)	The overall energy transferred to or from the surroundings in a reaction.
Exothermic	A reaction that releases energy to the surroundings; ΔH is negative.
Endothermic	A reaction that takes in energy from the surroundings; ΔH is positive.
Reaction pathway diagram	A graph of energy against progress of reaction showing reactants, products, Ea and ΔH.

Section 7 of 7Exam technique

1

Measuring the activation energy from the reactants to the products instead of from the reactants to the top of the curve.

2

Giving an exothermic reaction a positive ΔH; exothermic changes are negative.

3

Drawing the products at the same level as the reactants when the question describes an energy change.

4

Leaving out axis labels or the labels for reactants, products, ΔH and E_a, all of which carry marks.