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

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 en endothermic reaction
  • Negative for an exothermic reaction

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 exothermic reaction has a positive value for enthalpy, ΔH 
• The reaction pathway diagram for an endothermic reaction is:

The reaction pathway diagram for endothermic reactions.