C5 Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

A reaction where energy is transferred to the surroundings.

Question 2

Give examples of exothermic reactions.

Answer 2

Combustion, respiration and many neutralisation reactions

Question 3

What happens to the temperature of the surroundings during an exothermic reaction?

Answer 3

They increase

Question 4

What is an endothermic reaction?

Answer 4

A reaction where energy is transferred from the surroundings.

Question 5

Give examples of endothermic reactions.

Answer 5

Thermal decomposition reactions, the reaction between citric acid and sodium hydrogencarbonate and respiration

Question 6

What happens to the temperature of the surroundings during an endothermic reaction?

Answer 6

They decrease

Question 7

State two uses of exothermic reactions

Answer 7

Self-heating cans, hand warmers

Question 8

State two uses of endothermic reactions

Answer 8

Some cooling sports injury packs

Question 9

What are reactants?

Answer 9

The substances involved in a chemical reaction

Question 10

What are products?

Answer 10

The substances formed when reactants have a chemical reaction

Question 11

What is a reaction profile?

Answer 11

A diagram which shows whether the reactants have more or less energy than the products.

Question 12

Why does the reaction profile always go up at the start?

Answer 12

A reaction starts with bonds being broken

Question 13

What is the activation energy?

Answer 13

The energy needed to break bonds to start a reaction

Question 14

Why does the reaction profile always go down at the end?

Answer 14

A reaction always ends with bonds being made

Question 15

If the reactants have more energy than the products, what kind of a reaction must have taken place?

Answer 15

An exothermic one. Energy has been transferred to the surroundings.

Question 16

If the reactants have less energy than the products, what kind of a reaction must have taken place?

Answer 16

An endothermic one. Energy has been taken in from the surroundings.

Question 17

Is breaking bonds endothermic or exothermic?

Answer 17

Endothermic. Chemical bonds are strong so require energy to break (like when you have to put energy in to separate magnets from each other)

Question 18

Is making bonds endothermic or exothermic?

Answer 18

Exothermic. Energy is released when chemical bonds are formed (like how two magnets move together when close and generate kinetic energy)

Question 19

How do we work out the overall energy change of a reaction?

Answer 19

Work out the difference between the energy needed to break all the bonds in the reactants and the energy released to form all the bonds in the products.

Question 20

How do we work out the energy change from a reaction profile?

Answer 20

The difference between the energy of the reactants and the energy of the products

Question 21

How do we work out the activation energy from a reaction profile?

Answer 21

The difference between the energy of the reactants and the highest point on the profile

Question 22

What is a chemical cell?

Answer 22

A unit which contains chemicals which produce electricity through a reaction

Question 23

What are the main components of a chemical cell?

Answer 23

An anode, a cathode (made of 2 different metals) and an electrolyte

Question 24

What is an electrolyte?

Answer 24

A solution containing ions

Question 25

What does the voltage of a cell depend on?

Answer 25

The type of electrode and the electrolyte.

Question 26

What is a battery?

Answer 26

Two or more cells connected in series

Question 27

What happens to the electrodes in a chemical cell?

Answer 27

The more reactive metal is used up and the less reactive one increases in size.

Question 28

In non-rechargeable cells, why do the chemical reactions stop over time?

Answer 28

One of the reactants becomes used up.

Question 29

How can certain cells be recharged?

Answer 29

Applying an external electric current

Question 30

How does the reactivity of the metal electrodes affect the size of the potential difference?

Answer 30

The greater the difference in reactivity, the greater the voltage

Question 31

Which way do electrons move in a cell?

Answer 31

From the more reactive metal to the less reactive metal

Question 32

What is a fuel cell?

Answer 32

A cell which uses a fuel and oxygen (or air) to generate electricity.

Question 33

What are the products in a hydrogen fuel cell?

Answer 33

Water

Question 34

State advantages of hydrogen fuel cells

Answer 34

Do not need to be recharged, no pollutants are produced

Question 35

State three disadvantages of hydrogen fuel cells

Answer 35

Hydrogen is highly flammable, hydrogen is often made from fossil fuels, hydrogen is difficult to store

Question 36

What reaction occurs at the negative electrode of a hydrogen fuel cell

Answer 36

H2 → 2H + 2e⁻

Question 37

Which reaction occurs at the positive electrode of a hydrogen fuel cell

Answer 37

O2 + 4H+ + 4e- → 2H2O