Enthalpy Change & Activation Energy (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Enthalpy change & activation energy

Extended Tier Only

• For atoms or particles to react with each other in a chemical system they must collide together

• A number of factors affect the success of a collision:

  • Energy

  • Orientation

  • Number of collisions per second - the frequency of collisions

What is activation energy?

• In terms of the energy of collisions, there is a minimum amount of energy required for a successful collision

  • A successful collision is where the particles in the reactant(s) are rearranged to form the products

• This minimum amount of energy is called the activation energy, E_a 

• Different reactions have different activation energies, depending on the chemical identities involved

• Reactions with higher activation energies require more energy to start than those with lower activation energies

What is enthalpy change?

• The transfer of thermal energy during a reaction is called the enthalpy change, ΔH, of the reaction. 

• ΔH is:

  • Positive for an endothermic reaction

  • Negative for an exothermic reaction

Reaction pathway diagrams

Extended Tier Only

• Reaction pathway diagrams are graphical representations of the relative energies of the reactants and products in chemical reactions

• On a reaction pathway diagram:

  • Progress of the reaction is shown on the x-axis

  • Energy is shown on the y-axis

  • The difference in energy between the energy of reactants and products is the overall enthalpy change of a reaction

Reaction pathway diagram of an exothermic reaction and an endothermic reaction

Exothermic reactions

• Energy is given out to the surroundings

• The energy of the products will therefore be lower than the energy of the reactants

• The enthalpy change is negative

• This is represented on the reaction profile with a downwards-arrow as the energy of the products is lower than the reactants

• The activation energy is the energy difference between the reactants and the highest point on the curve

• The reaction pathway diagram for an exothermic reaction is:

The reaction pathway diagram for exothermic reactions

Endothermic reactions

• Energy is taken in from the surroundings

• The energy of the products will be higher than the energy of the reactants

• The enthalpy change is positive

• This is represented on the reaction profile with an upwards-arrow as the energy of the products is higher than the reactants

• The activation energy is the energy difference between the reactants and the highest point on the curve

• The reaction pathway diagram for an endothermic reaction is:

The reaction pathway diagram for endothermic reactions.