Enthalpy Change & Activation Energy (Cambridge O Level Chemistry)

• 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)
• There is a minimum amount of energy that the particles must collide with for the collision to be successful, that is for the particles to react together
  • This minimum amount of energy is called the activation energy (E_a)
• 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 a positive value for endothermic reactions and a negative value for exothermic reactions

• Reaction pathway diagrams (a.k.a energy level diagrams) are graphical representations of the relative energies of the reactants and products in chemical reactions
• The energy of the reactants and products are displayed on the y-axis and the reaction pathway (a bit like time) is shown on the x-axis
• The difference in height between the energy of reactants and products represents the overall energy change of a reaction.
  • This is usually a sketch but can be drawn to scale if data is provided
• Arrows on the diagrams indicate whether the reaction is exothermic (overall reaction arrow is downwards pointing, showing that the system has lost energy) or endothermic (overall reaction arrow is upwards pointing, showing that the system has gained energy)
• The initial increase in energy represents the activation energy (Ea), which is the minimum energy that colliding particles must have in order to react
• The greater the initial rise, the more energy that is required to get the reaction going e.g. more heat needed
• You can identify a reaction pathway diagram for an exothermic reaction as the energy of the product is lower than the reactants (as thermal energy has been transferred to the surroundings) 
• You can identify a reaction pathway diagram for an endothermic reaction as the energy of the product is higher than the reactants (as thermal energy has been taken in from the surroundings) 

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.