Beyond Science returns with our final AQA Chemistry Paper 1 blog, exploring energy changes revision. Here, we’ll demystify each aspect of energy changes, so you can confidently tackle this aspect of your exams. Deep breaths…off we go.
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Key terms:
- Activation energy: the minimum amount of energy required for a chemical reaction to take place.
- Catalysts: increases the rate of a reaction. Catalysts provide an alternative pathway for chemical reactions to take place by lowering the activation energy. They are not used up during a reaction meaning they can be used over and over again.
Exothermic and Endothermic Reactions
When a chemical reaction takes place, energy is involved. Energy is transferred when chemical bonds are broken and when new bonds are made.
Exothermic reactions involve the transfer of energy from the reacting chemicals to the surroundings. During a practical investigation, an exothermic reaction would show an increase in temperature as the reaction takes place.
Examples of exothermic reactions include:
- Combustion
- Respiration
- Neutralisation
We can use exothermic reactions in a number of ways, examples of ways we use exothermic reactions include:
- Hand warmers
- Self heating cans used in camping
Endothermic reactions involve the transfer of energy from the surroundings to the reacting chemicals. During a practical investigation, an endothermic reaction would show a decrease in temperature as the reaction takes place.
Examples of endothermic reactions are rarer than exothermic but include:
Thermal decomposition
One use of endothermic reactions are in sports injury packs. The image below shows the change in temperature of an endo and exothermic reaction.
Reaction Profiles
Energy level diagrams show us what is happening in a particular chemical reaction. The diagram shows us the difference in energy between the reactants and products.
Exothermic Reactions
In an exothermic reaction, the reactants are at a higher energy level than the products.
In an exothermic reaction, the difference in energy is released to the surroundings so the temperature of the surroundings increases. This shows in the energy profile as an overall negative energy change as energy is lost from the system.
Endothermic Reactions
In an endothermic reaction, the reactants are at a lower energy level than the products.
In an endothermic reaction, the difference in energy is absorbed from the surroundings and so the temperature of the surroundings decreases. This is shown as an increase in energy on the energy profile as the products have more energy than the reactants.
Bond Making and Breaking
In all chemical reactions, energy is taken in to break chemical bonds and energy is released when new chemical bonds are made.
The overall energy change (ΔH) of a reaction depends on whether more energy is taken in or is released. You may also find, in some textbooks, ΔH referred to as the enthalpy change.
In an endothermic reaction, more energy is taken in than is released. The energy change (ΔH) in an endothermic reaction is positive.
In an exothermic reaction, more energy is released than is taken in. The energy change (ΔH) in an endothermic reaction is negative.
Bond energies are measured in kJ/mol (kilojoules per mole).
Calculations using Bond Energies
Bond energies are used to calculate the change in energy of a chemical reaction.
If we try to calculate the change in energy for the reaction:
2H202 → 2H2O + O2
Reactions will often be shown as displayed formulae so it is easier to work out which bonds are being broken and which bonds are being made.
H-O-O-H H-O-H
→ + O=O
H-O-O-H H-O-H
| Bond | Bond Energy (kJ/mol) |
| H – O | 464 |
| O – O | 146 |
| O = O | 498 |
On the left-hand side of the equation, the bonds are breaking.
There are four O-H bonds and two O-O bonds.
So, (4×464) + (2×146) = 2148
On the right-hand side there are four 0-H bonds and one O=O bond being made.
So, (4×464) + 498 = 2354
ΔH = sum (bonds broken) – sum (bonds made)
ΔH = 2148 – 2354 = -206 kJ/mol
The reaction is exothermic as ΔH is negative.
Chemistry Paper 1 Revision Resources from Beyond
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