C3 - Energy Changes

Question 1

Describe what is meant by an endothermic and exothermic reaction

Answer 1

Exothermic reactions transfer energy from the reacting molecules to the surroundings. In exothermic reactions, the temperature of the surroundings increases e.g. combustion.

Endothermic reactions take in energy from their surroundings so during endothermic reactions, the surroundings get colder.

Question 2

Describe what is meant by the activation energy for a reaction

Answer 2

The activation energy is the minimum amount of energy that particles must have in order to react.

Question 3

Use bond energies to calculate the energy change for a chemical reaction

Answer 3

-In exothermic reactions, energy is transferred from the chemicals to the surroundings so exothermic energy changes are shown as negative.
-In endothermic reactions, energy is gained from the surroundings so endothermic energy changes are shown as positive.

When a chemical bond is broken, it requires energy so it is endothermic and making bonds releases energy so it is exothermic.
Every chemical bond has an energy value which tells us the energy required to break that bond.

To calculate final energy change for a reaction, you subtract the energy released from the energy taken in.
If the final energy change is positive, the reaction is endothermic and if it is negative, the reaction is exothermic.

Question 4

Endothermic and exothermic energy level diagrams

Answer 4

-In an endothermic energy level diagram, the reactants are lower than the products.

-In an exothermic energy level diagram, the reactants are higher than the product.