Potential energy diagrams
Chemical reactions involve a change in energy, usually a loss or gain of heat energy. The heat stored by a substance is called its enthalpy (H).
\(\Delta H\) is the overall enthalpy change for a reaction. Potential energy diagrams can be used to calculate both the enthalpy change and the activation energy for a reaction.
Exothermic reactions
An exothermic reaction is one in which heat energy is given out. The products must have less energy than the reactants because energy has been released.
This can be shown by a potential energy diagram:
EA is the activation energy (energy required to start the reaction)
\(\Delta H\) is the quantity of energy given out (ie the enthalpy change)
For exothermic reactions, \(\Delta H\) will always be negative.
Endothermic reactions
An endothermic reaction is one in which heat energy is absorbed. The products have more enthalpy than the reactants therefore \(\Delta H\) is positive.
Activated complex
The activated complex (high energy intermediate state where bonds are breaking and forming) can be shown on potential energy diagrams.
It is the 'energy barrier' that must be overcome when changing reactants into products.
Catalysts
A catalyst provides an alternative reaction pathway which involves less energy and so the catalyst lowers the activation energy.
The use of a catalyst does not affect the reactants or products, so \(\Delta H\) stays the same.
