C5

Question 1

What is conserved in chemical reactions?

Answer 1

Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.

Question 2

What does it mean if a reaction transfers energy to its surroundings?

Answer 2

If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.

Question 3

What is an exothermic reaction?

Answer 3

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

Question 4

State examples of exothermic reactions

Answer 4

Exothermic reactions include combustion, many oxidation reactions and neutralisation.

Question 5

State everyday uses of exothermic reactions

Answer 5

Everyday uses of exothermic reactions include self-heating cans and hand warmers.

Question 6

What is an endothermic reaction?

Answer 6

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

Question 7

State examples of endothermic reactions

Answer 7

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

Question 8

Note:

Answer 8

AQA says that students should be able to:

• distinguish between exothermic and endothermic reactions on the basis of the temperature change of the surroundings
• evaluate uses and applications of exothermic and endothermic reactions given appropriate information.

Question 9

Note:

Answer 9

Limited to measurement of temperature change. Calculation of energy changes or AH is not required.

Question 10

Practical 10

Answer 10

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Question 11

When do chemical reactions only occur?

Answer 11

Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy.

Question 12

What can reaction profiles be used to show?

Answer 12

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 13

Whar is the energy needed for a reaction to occur?

Answer 13

Activation energy is the energy needed for a
reaction to occur.

Question 14

Note:

Answer 14

AQA says that students should be able to:

• draw simple reaction profiles (energy level diagrams) for exothermic and endothermic reactions showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved line to show the energy as the reaction proceeds
• use reaction profiles to identify reactions as exothermic or endothermic
explain that the activation energy is the energy needed for a
reaction to occur.

Question 15

What is needed during a chemical reaction?

Answer 15

During a chemical reaction:

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

Question 16

What can you calculate with bond energies?

Answer 16

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

Question 17

What is the overall energy change of the reaction

Answer 17

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 18

What is needed in an exothermic reaction? (In terms of energy)

Answer 18

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

Question 19

What is needed in an endothermic reaction? (In terms of energy)

Answer 19

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

Question 20

Note:

Answer 20

AQA says that students should be able to calculate the energy transferred in chemical reactions using bond energies supplied.