Energy changes

Exothermic Reaction

➤ A reaction that releases energy to the surroundings, usually as heat — the temperature of the surroundings increases.

Endothermic Reaction

➤ A reaction that absorbs energy from the surroundings — the temperature of the surroundings decreases.

Activation Energy (Ea)

➤ The minimum amount of energy needed to start a chemical reaction.

Reaction Profile (Energy Diagram)

➤ A diagram that shows the energy change during a chemical reaction.

Give two examples of exothermic reactions.

➤ Combustion, neutralisation, many oxidation reactions.

Give two examples of endothermic reactions.

➤ Thermal decomposition, photosynthesis, citric acid + sodium hydrogencarbonate.

What happens to temperature in an exothermic reaction?

➤ It increases (because energy is released).

What does a reaction profile for an exothermic reaction look like?

➤ The products are lower in energy than the reactants; energy is released.

What is activation energy?

➤ The energy barrier that must be overcome to start a reaction

Bond Energy (Bond Enthalpy)

➤ The amount of energy needed to break one mole of a specific bond in a gaseous molecule.

Overall Energy Change

➤ The difference between energy used to break bonds and energy released when new bonds form.

Is bond breaking endothermic or exothermic?

➤ Endothermic – energy is required to break bonds.

Is bond forming endothermic or exothermic?

➤ Exothermic – energy is released when bonds form.

How is the overall energy change in a reaction calculated?

Overall energy change = Energy in (bonds broken) − Energy out (bonds made)

If more energy is released making bonds than used breaking them, what type of reaction is it?

➤ Exothermic

If more energy is used to break bonds than is released making them?

➤ Endothermic