Energy Changes

Question 1

How is energy transferred in a chemical reaction?

Answer 1

From the chemicals or from the surroundings, therefore many reactions are accompanied by an energy change.

Question 2

What are exothermic reactions?

Answer 2

Reactions accompanied by a temperature rise- they transfer (heat) energy from the chemicals to the surroundings.

Question 3

Exothermic reactions include:

Answer 3

• Self-heating cans
• hand warmers
• combustion
• neutralisation
• displacement

Question 4

What are endothermic reactions?

Answer 4

Reactions accompanied by a fall in temperature, heat energy is transferred from the surroundings to the chemicals.

Question 5

Endothermic reactions include:

Answer 5

• Thermal decompositions
• Reaction if Citric acid and sodium hydrogencarbonate
• Some sports injury packs

Question 6

If a reversible reaction is endothermic in one direction…

Answer 6

It is exothermic in the other direction, the same amount of energy is transferred in each case.

Question 7

When can chemical reactions occur?

Answer 7

Only when reacting particles collide with each other and with sufficient energy.

Question 8

What is activation energy?

Answer 8

The minimum amount of energy that particles must have to react.

Question 9

What so reaction profiles show?

Answer 9

The relative energies of all reactants and products- the activation and overall energies change the reaction.

Question 10

In an exothermic reaction:

Answer 10

• Energy is given out to surroundings

- Products have less energy than reactants

Question 11

In an endothermic reaction:

Answer 11

• Energy is being taken from surroundings

- Products have more energy than reactants

Question 12

What do cells contain?

Answer 12

Chemicals which react to produce energy.

Question 13

What is the voltage produced by a cell dependent on?

Answer 13

A number of factors including the type of electrode and electrolyte

Question 14

How is a simple cell made?

Answer 14

By connecting two different metals in contact with an electrolyte.

Question 15

What do batteries consist of?

Answer 15

Two or more cells connected together in series to provide greater voltage.

Question 16

In non-rechargeable cells and batteries when do the reactions stop?

Answer 16

When one of the reactants has been used up- alkaline batteries are non-rechargeable.

Question 17

Why can rechargeable cells and batteries be recharged?

Answer 17

Because the chemical reactions are reversed when an external electrical current is supplied.

Question 18

What are fuel cells?

Answer 18

A very efficient way of producing electrical energy.

Question 19

What do most fuel cells use?

Answer 19

Hydrogen, but other fuels can be used.

Question 20

In a hydrogen fuel cell:

Answer 20

• Hydrogen and oxygen are supplied to the fuel cell.
• The fuel is oxidised to produce a pd (voltage).
• Overall, the hydrogen is oxidised to form water.

Question 21

What effect do catalysts have on activation energy?

Answer 21

They reduce it.

Question 22

How can the energy needed to break bonds and the energy released when bonds are formed be calculated?

Answer 22

From bond energies.

Question 23

Energy in an exothermic reaction:

Answer 23

The energy released from forming bonds is greater than the energy needed to break existing bonds.

Question 24

Energy in an endothermic reaction:

Answer 24

The energy needed to break existing bonds is greater than the energy released from forming new bonds.

Question 25

What are the advantages of fuel cells?

Answer 25

• Hydrogen fuel cells produce water which is non-polluting
• Lightweight and small
• Have no moving parts so are very unlikely to break down