Bonds & Energy Changes (Edexcel GCSE Chemistry)

Revision Note

Stewart Hird

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Bonds & Energy Changes

  • Energy is needed to break bonds which is absorbed from the reaction surroundings, so bond breaking is an endothermic process

  • The opposite occurs for forming bonds as it releases energy back to the surroundings in an exothermic process

  • 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 including 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

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 products have more energy than the reactants

  • Therefore an endothermic reaction has a positive change in energy

Bond breaking endothermic reaction, IGCSE & GCSE Chemistry revision notes

Energy must be absorbed from the surroundings for bonds to be broken 

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 more energy than the products

  • Therefore an exothermic reaction has a negative change in energy

Bond making exothermic reaction, IGCSE & GCSE Chemistry revision notes

Making new bonds gives off heat from the reaction to the surroundings

Examiner Tips and Tricks

Remember bond breaking is ENDothermic and results in the END of the bond.

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.