Enthalpy Change & Activation Energy (CIE IGCSE Chemistry)
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Chemistry
Enthalpy Change & Activation Energy
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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
Reaction Pathway Diagrams
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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.
Exam 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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