Potential Energy

• 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). ΔH is the overall enthalpy change 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.

Endothermic reactions

• An endothermic reaction is one in which heat energy is absorbed. The products have more enthalpy than the reactants therefore ΔH 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 that involves less energy and so the catalyst lowers the activation energy.

• Solid → Liquid means gaining energy.

• Gas → Liquid means losing energy so delta h will be negative.