Chemistry Unit 3 Topic 2

C3-2 : Energy Calculations

Energy & Reactions
An exothermic reaction releases energy. We use exothermic reactions in burning fuels as a source of energy. However, some reactions give off more energy than others, so we can calculate how much energy is released in a given reaction. There is apparatus available to do this called a calorimeter. In a school lab, a simple calorimeter might be used – but a more accurate instrument is available, called a bomb calorimeter.

A bomb calorimeter works by measuring the temperature of the water inside it – because the energy produced in an exothermic reaction increases the temperature of its surroundings, in this case the water. The change in energy is calculated using the temperature change and amount of water (see later on for calculations using energy).

A simple calorimeter however involves very basic apparatus. We don’t use this to measure energy change necessarily because it isn’t very accurate – but we can use it to compare energy changes from different fuels.

When a reaction takes place, bonds are broken and new chemical bonds are made:

• breaking bonds is an endothermic process, because energy has to be taken in from the surroundings to break the bonds (remember energy is needed to break bonds)
• making bonds is an exothermic process, because energy is released in the formation of new chemical bonds

Because a reaction makes and breaks bonds, reactions are sort of both exo- and endothermic. For this reason, it is the balance between exo- and endothermic reactions which decides the overall reaction type; for example if more energy is released in the making of new bonds than is taken in to break the bonds, it is overall exothermic – because the exothermic > endothermic.

Energy Level Diagrams
We can draw energy level diagrams to show energy changes in a reaction. These diagrams show the relative amounts of energy stored in the products and reactants of a reaction, measured in kJ/mol.

This is the energy level diagram for an exothermic reaction. The products are at a lower energy level than the reactants, so energy has been released as the reactants form the products. In this release of energy,temperature of the surroundings increases. In such an exothermic reaction, we say that the change in energy is negative – which we write as ΔH -ve (see below). This is so because energy is released – so there is less energy in the products than the reactants.

So this is the energy level diagram for an endothermicreaction. With an endothermic reaction, more energy is needed to break to bonds of the reactants than is released in forming products. Here,temperature pf the surroundings decreases. Because the change in energy this time is positive, we say ΔH +ve (see below).

The Greek letter “delta” (written as Δ) is often used in the sciences and maths to represent change. In chemical energies, we use ΔH to abbreviate energy change…