Reaction Pathway Diagrams & Activation Energy (Cambridge International IGCSE Science)

Activation energy

Spec point — Define activation energy, Ea, as the minimum energy that colliding particles must have to react

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

Reaction pathway diagrams

Spec point — Interpret reaction pathway diagrams showing exothermic and endothermic reactions Draw and label reaction pathway diagrams for exothermic and endothermic reactions using information provided, to include: reactants (b) products enthalpy change of the reaction, ∆H activation energy, Ea

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 energy 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 overall energy 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 overall energy 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.

Bond breaking and bond making

• Bond breaking is an endothermic process because energy is needed to break bonds.

• Bond making is an exothermic process because energy is released when new bonds form.

• Whether a reaction is exothermic or endothermic depends on the balance between:

  • the energy absorbed during bond breaking, and
  • the energy released during bond making

• A reaction is exothermic when more energy is released forming new bonds for the products than absorbed breaking the bonds in the reactants

• A reaction is endothermic when more energy is absorbed breaking the bonds in the reactants than released forming new bonds for the products

Exam Hint You must be able to draw these pathway diagrams and label the following parts:

• Reactants
• Products
• Overall energy change of the reaction, ΔH
• Activation energy, E_a