Bond Breaking & Forming

Specification Point 7.12:
  • Recall that the breaking of bonds is endothermic and the making of bonds is exothermic.
  • Energy is needed to break bonds which is absorbed from the reaction surroundings and making bonds releases energy back to the surroundings.
  • Both processes occur in the same chemical reaction, for example, in the production of ammonia:

N2  3H2 → 2NH3

  • The bonds in the N-N and H-H molecules must be broken which requires energy while the bonds in the NH3 molecule are formed which releases energy.
  • Most reactions occur in a number of steps and each reaction includes steps that are exothermic and steps that are endothermic.
  • Whether a reaction is overall endothermic or exothermic depends on the difference between the sum of the exothermic steps and the sum of the endothermic steps.

Heat Energy of a Reaction

Specification Point 7.13:
  • Recall that the overall heat energy change for a reaction is:
    a) exothermic if more heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants
    b) endothermic if less heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants

Endothermic 

  • If more energy is absorbed than is released, this reaction is endothermic.
  • More energy is required to break the bonds than that gained from making the new bonds.
  • The change in energy is positive since the reactants have more energy than the products.
  • The symbol ΔH (delta H) is used to show the change in heat energy. H is the symbol for enthalpy, which is a measure of the total heat of reaction of a chemical reaction.
  • Therefore an endothermic reaction has a positive ΔH value.

Bond breaking in endothermic reaction (1), Edexcel GCSE Chemistry

Breaking chemical bonds requires heat energy which is taken in from the surroundings 

Exothermic

  • If more energy is released than is absorbed, then the reaction is exothermic.
  • More energy is released when new bonds are formed than energy required to break the bonds in the reactants.
  • The change in energy is negative since the reactants have less energy than the products.
  • Therefore an exothermic reaction has a negative ΔH value.

Bond making in exothermic reaction (1), Edexcel GCSE Chemistry

Making new chemical bonds releases heat energy which is released to the surroundings

Activation Energy

Specification Point 7.15:
  • Explain the term activation energy.
  • Chemical reactions occur when particles collide with each other with sufficient energy.
  • The energy is needed to break the bonds in the reactant particles.
  • The amount of energy changes for every reaction, depending on the nature of the reactants and conditions.
  • The minimum amount of energy that particles must have to react is called the activation energy.
  • If the energy available for reaction is below the activation energy then there won’t be enough energy to start the reaction.
  • If the activation energy of a reaction is low it means that a lot of the particles will collide with enough energy to react, hence the rate of reaction will be fast.
  • If the activation energy of a reaction is high it means that fewer particles will collide with enough energy, hence the rate of reaction will be slow.
  • On a reaction profile diagram, the activation energy starts at the energy of the reactants and is the difference in energy between here and the top of the curve.

Reaction profile diagram for an exothermic reaction showing the activation energy

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.