Enthalpy Change & Activation Energy (Cambridge International IGCSE Chemistry)

Spec point — 5.1.2 · State that the transfer of thermal energy during a reaction is called the enthalpy change, ∆H, of the reaction. ∆H is negative for exothermic reactions and positive for endothermic reactions Define activation energy, Ea, as the minimum energy that colliding particles must have to react

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

Spec point — 5.1.2 · 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

Exothermic reactions

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

• So, the products have less energy than the reactants

  • This means that the change in energy is negative

• Therefore, an exothermic reaction has a negative value for enthalpy, ΔH

• The reaction pathway diagram for an exothermic reaction is:

The reaction pathway diagram for exothermic reactions

Endothermic reactions

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

• So, the products have more energy than the reactants

  • This means that the change in energy is positive

• Therefore, an endothermic reaction has a positive value for enthalpy, ΔH

• The reaction pathway diagram for an endothermic reaction is:

The reaction pathway diagram for endothermic reactions.

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

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