Exothermic reactions
- Energy is conserved in chemical reactions: the total energy of the universe is the same before and after.
- An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases.
- Examples: combustion, many oxidation reactions and neutralisation.
- Uses: self-heating cans and hand warmers.
Endothermic reactions
- An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases.
- Examples: thermal decomposition, and the reaction of citric acid with sodium hydrogencarbonate.
- Uses: some sports injury packs (instant cold packs).
Reaction profiles
- Chemical reactions happen only when particles collide with enough energy. The minimum energy needed is the activation energy.
- A reaction profile shows the energy of the reactants and products, the activation energy (the 'hump') and the overall energy change.
- Exothermic: the products are lower in energy than the reactants. Endothermic: the products are higher in energy than the reactants.
Required practical: temperature changes
- Put a measured volume of one solution (such as hydrochloric acid) in a polystyrene cup inside a beaker, and record its starting temperature.
- Add the other reactant (such as sodium hydroxide solution, a metal or a carbonate), stir, and record the highest (or lowest) temperature.
- The polystyrene cup and a lid reduce energy transfer to or from the surroundings.
- Repeat, changing one variable (such as the volume of alkali added), and keep the others the same.
Key terms
- Exothermic
- A reaction that transfers energy to the surroundings, so the temperature rises.
- Endothermic
- A reaction that takes in energy from the surroundings, so the temperature falls.
- Activation energy
- The minimum energy that particles must have to react when they collide.
- Reaction profile
- A diagram showing the energy of reactants and products during a reaction.
- Combustion
- Burning a substance in oxygen, an exothermic reaction.
- Thermal decomposition
- Breaking down a substance by heating it, an endothermic reaction.
- Surroundings
- Everything around the reacting chemicals, including the container and solvent.