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

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Alexandra Brennan

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Enthalpy change & activation energy

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  • 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, Ea 
  • 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

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:

Exothermic reaction profile diagram

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:

Endothermic reaction profile diagram

The reaction pathway diagram for endothermic reactions.

Examiner Tip

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

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

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.