GCSE / Chemistry
Energy Changes
Learn how energy is transferred during chemical reactions, including exothermic and endothermic reactions, reaction profiles, activation energy, bond energy calculations, chemical cells, batteries and hydrogen fuel cells.
Exothermic reactions transfer energy from the reacting chemicals to the surroundings. This causes the temperature of the surroundings to increase. Common examples include combustion, oxidation reactions, neutralisation reactions, self-heating cans and some hand warmers.
Endothermic reactions take in energy from the surroundings, causing the temperature of the surroundings to decrease. Examples include thermal decomposition reactions, the reaction of citric acid with sodium hydrogencarbonate and some instant cold packs.
Energy is conserved during chemical reactions. The total amount of energy in the universe remains constant, but energy can be transferred between the chemical reaction and its surroundings. Whether a reaction is exothermic or endothermic depends on the direction of this energy transfer.
Temperature changes can be used to distinguish between exothermic and endothermic reactions. A temperature rise indicates that energy has been transferred to the surroundings, while a temperature fall indicates that energy has been taken in from the surroundings. GCSE practical investigations measure these temperature changes for reactions such as neutralisation and displacement.
A reaction profile is an energy level diagram showing the relative energies of the reactants and products during a chemical reaction. It also shows the activation energy and allows a reaction to be identified as exothermic or endothermic.
Activation energy is the minimum amount of energy that reacting particles must have for a successful collision and therefore for a chemical reaction to occur. On a reaction profile, it is represented by the energy difference between the reactants and the highest point of the energy pathway.
In an exothermic reaction profile, the products are at a lower energy level than the reactants because energy has been released to the surroundings. The overall energy change is therefore associated with a transfer of energy from the reaction to the surroundings.
In an endothermic reaction profile, the products are at a higher energy level than the reactants because energy has been absorbed from the surroundings. Energy must therefore be supplied overall for the reaction to take place.
During a chemical reaction, energy is required to break existing chemical bonds in the reactants, while energy is released when new chemical bonds form in the products. Bond breaking is an endothermic process and bond formation is an exothermic process.
For Higher Tier, the overall energy change of a reaction can be calculated using bond energies. The energy change is found from: energy required to break bonds − energy released when new bonds form. A negative overall value indicates an exothermic reaction, while a positive value indicates an endothermic reaction.
Chemical cells contain chemicals that react to produce electricity. A simple chemical cell can be made using two different metals in contact with an electrolyte. The potential difference produced depends on factors including the materials used for the electrodes and the electrolyte.
The greater the difference in reactivity between two metals used as electrodes in a chemical cell, the greater the potential difference that can generally be produced. Experimental cell data can therefore be used to compare the relative reactivity of different metals.
A battery consists of two or more cells connected together in series to provide a greater potential difference. In non-rechargeable cells and batteries, the chemical reactions eventually stop when one of the reactants has been used up, so they can no longer produce electricity.
Rechargeable cells and batteries use chemical reactions that can be reversed when an external electrical current is supplied. This allows the reactants to be regenerated so that the cell can be used repeatedly after recharging.
Hydrogen fuel cells are supplied continuously with hydrogen fuel and oxygen or air. Hydrogen is oxidised and the overall reaction produces water while generating a potential difference. Fuel cells can operate for as long as fuel is supplied and may be compared with rechargeable batteries in terms of advantages, disadvantages, energy supply and environmental effects.