Energy Changes

Question 1

How is energy conserved in chemical reactions?

Answer 1

The amount of energy in
the universe at the end of a chemical reaction is the same as before the reaction takes place. If a reaction transfers energy to the surroundings the product molecules must have less energy than the
reactants, by the amount transferred

Question 2

What is an exothermic reaction?

Answer 2

An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.

Exothermic reactions include combustion, many oxidation reactions and neutralisation.

Question 3

What is an endothermic reaction?

Answer 3

An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.

Endothermic reactions include thermal decompositions and the
reaction of citric acid and sodium hydrogen carbonate. Some sports injury packs are based on endothermic reactions.

Question 4

When can chemical reactions occur?

Answer 4

Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.

Question 5

What is activation energy?

Answer 5

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

Question 6

What are reaction profiles used for?

Answer 6

Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.

Question 7

What happens in a chemical reaction involving bonds?

Answer 7

• energy must be supplied to break bonds in the reactants
• energy is released when bonds in the products are formed.

Question 8

what are bond energies?

Answer 8

The energy needed to break bonds and the energy released when
bonds are formed can be calculated from bond energies.

Question 9

What is the overall energy change of a reaction?

Answer 9

The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when
bonds in the products are formed is the overall energy change of the reaction.

Question 10

what happens with bonds in an exothermic reaction?

Answer 10

In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.

Question 11

What happens with bonds in an endothermic reaction?

Answer 11

In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.