Energy changes

Endothermic reactions

Energy from the surroundings is transferred to the reacting chemicals, causing the temperature of the surroundings to decrease. Examples include:

• Thermal decomposition.
• The reaction between citric acid and sodium hydrogen carbonate (sodium bicarbonate).

Exothermic reactions 

Energy from the reacting chemicals is transferred to the surroundings, which often increase in temperature as a result. Examples include:

• Combustion.
• Neutralisation.
• Oxidation.

In exothermic reactions, energy from the reacting chemicals is transferred to the surroundings. This often leads to an increase in temperature. Examples include:

• Oxidation
• Hand warmers

• Disposable hand warmers use the energy released by iron oxidation.
• Reusable hand warmers source their energy from the crystallisation of salt solutions. Boiling the pack re-dissolves the crystals, so that it's ready to be activated once more.

• Combustion
• Neutralisation

In endothermic reactions, energy from the surroundings is transferred to the reacting chemicals. This decreases the temperature of the surroundings. Examples include:

• Sports injury packs

• When squeezed forcefully, ammonium nitrate and water mix in the pack, resulting in instant cooling.
• The speed of this reaction makes these packs ideal for scenarios when ice is not immediately available.

• Thermal decomposition
• Citric acid and baking soda

There are 2 main requirements for a successful reaction to take place:

• Collisions

• Reacting particles must collide.

• Activation energy

• Collisions must take place with sufficient energy (the activation energy).

Reaction profiles show the progress of reaction on the x axis and energy level on the y axis. They contain the following information:

• The relative energies of reactants and products.
• The activation energy of a reaction.
• The overall energy change of a reaction.

Catalysts can increase reaction rates by lowering the activation energy, which increases the likelihood of successful collisions.