Chem 3 - Energy Calculations - Energy Changes in Reactions

When we carry out a chemical reaction, we can think of the chemical bonds between the atoms in the reactants being broken. Then new chemical bonds can be formed to make the products.

- Because Energy has to be supplied to break chemical bonds, breaking bonds is an Endothermic process, energy is taken in from the surroundings.

- But when new bonds are formed, Energy is released, so making bonds is an Exothermic process.

In some reactions the Energy released when new bonds are formed is more than the Energy needed to break the bonds in the reactants. These reactions transfer Energy to the surroundings - they are exothermic.

In other reactions the Energy needed to break the bonds in the reactants is more than the Energy released when the new bonds are formed in the products. These reactions involve a transfer of Energy from the surroundings to the reacting chemicals - they are endothermic.

The balance between the Energy needed to break bonds and the Energy released when new bonds are made is what decides whether a reaction is endothermic or exothermic. We can find out more about what is happening in a particular reaction by looking at Energy level diagrams.

Energy level diagrams show us the relative amounts of Energy contained in the reactants and the products of this reaction. This Energy is measured in kJ/mol.

The difference between the Energy levels of the reactants and the products is the change during the reaction, measured in kJ/mol. We represent this Energy change by the symbol ΔH ('Delta H'). This simply means 'the difference in Energy between the products and the reactants'.

- In an exothermic reaction, the 'ΔH' is always negative, because the products are at a lower Energy level than the reactants. The temperature of the surroundings increases.

- In an endothermic reaction, the 'rg' is always positive, because the products are at a higher Energy level than the reactants. The temperature of the surroundings decreases.