Enthalpy Change & Activation Energy (CIE IGCSE Chemistry)

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• For atoms or particles to react with each other in a chemical system they must first of all come into contact with each other in a collision
• A number of factors come into play when analysing collisions such as energy, orientation, and number of collisions per second (the frequency of collisions)
• In terms of the energy of the collision, there is a minimum amount of energy required for the collision to be successful, that is for the particles to react together
• This minimum amount of energy is called the activation energy (Ea)
• Different reactions have different activation energies, depending on the chemical identities involved
• Reactions which have higher activation energies require more energy to start than those with lower activation energies
• The transfer of thermal energy during a reaction is called the enthalpy change, ΔH, of the reaction. 
• ΔH is either a positive or a negative value depending on whether the reaction is exothermic or endothermic 

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Exothermic reactions

• 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 products have less energy than the reactants
• Therefore an exothermic reaction has a negative ΔH value
• The reaction pathway diagram for an exothermic reaction is shown below

The reaction pathway diagram for exothermic reactions

Endothermic reactions

• If more energy is absorbed to break bonds than is released to form new bonds, this reaction is endothermic overall
• The change in energy is positive since the products have more energy than the reactants
• Therefore an endothermic reaction has a positive ΔH value, which is shown on the energy level diagrams and in calculations

The reaction pathway diagram for endothermic reactions.