Topic 5 (Energy Changes)

Question 1

What is an exothermic reaction?

Answer 1

A reaction that transfers energy to the surroundings, usually as heat, causing the temperature to increase.

Question 2

What is an endothermic reaction?

Answer 2

A reaction that takes in energy from the surroundings, usually as heat, causing the temperature to decrease.

Question 3

Give two examples of exothermic reactions.

Answer 3

Combustion

Neutralisation (acid + alkali)

Question 4

Give two examples of endothermic reactions.

Answer 4

Thermal decomposition

The reaction between citric acid and sodium hydrogencarbonate

Photosythsis

Question 5

What is energy transferred as in these reactions?

Answer 5

Mostly as heat energy.

Question 6

What are some everyday uses of exothermic reactions?

Answer 6

Hand warmers

Self-heating cans

Question 7

What are some everyday uses of endothermic reactions?

Answer 7

Instant cold packs (used in sports injuries)

Question 8

In terms of bonds, when is energy released?

Answer 8

Energy is released when bonds are formed.

Question 9

In terms of bonds, when is energy taken in?

Answer 9

Energy is taken in when bonds are broken.

Question 10

What happens to temperature in an exothermic reaction?

Answer 10

The temperature of the surroundings increases.

Question 11

What happens to temperature in an endothermic reaction?

Answer 11

The temperature of the surroundings decreases.

Question 12

What is activation energy?

Answer 12

The minimum amount of energy needed for a reaction to start.

Question 13

: What does a reaction profile show?

Answer 13

The energy levels of reactants and products during a chemical reaction.

Question 14

In a reaction profile, how can you tell if a reaction is exothermic?

Answer 14

The products are at a lower energy level than the reactants.

Question 15

In a reaction profile, how can you tell if a reaction is endothermic?

Answer 15

The products are at a higher energy level than the reactants.

Question 16

What is meant by bond energy (or bond enthalpy)?

Answer 16

The energy needed to break 1 mole of a specific type of bond in a gaseous molecule.

Question 17

Is bond breaking endothermic or exothermic?

Answer 17

Endothermic – energy is taken in to break bonds.

Question 18

Is bond forming endothermic or exothermic?

Answer 18

Exothermic – energy is released when new bonds form.

Question 19

What is the formula to calculate the overall energy change in a reaction?

Answer 19

Energy change = Total energy in (bonds broken) – Total energy out (bonds formed)

Question 20

What does it mean if the energy change is negative?

Answer 20

The reaction is exothermic – more energy is released forming bonds than is taken in to break them.

Question 21

What does it mean if the energy change is positive?

Answer 21

The reaction is endothermic – more energy is taken in to break bonds than is released when forming them.

Question 22

What units are bond energies measured in?

Answer 22

kJ/mol (kilojoules per mole)

Question 23

Why do we use average bond energies in calculations?

Answer 23

Because actual bond energies vary depending on the compound they’re in, so we use mean values.

Question 24

How do you calculate the energy needed to break bonds?

Answer 24

Multiply the bond energy value (from data) by the number of bonds broken.

Question 25

If a reaction has higher bond energies in the reactants than the products, what type of reaction is it?

Answer 25

Endothermic, because more energy is required to break bonds than is released forming them.

Question 26

What is a chemical cell?

Answer 26

A device that produces a voltage from a chemical reaction between two different metals in an electrolyte.

Question 27

What three components are needed to make a simple cell?

Answer 27

Two different metals (electrodes) An electrolyte Wires to complete the circuit

Question 28

What causes the voltage in a cell?

Answer 28

The difference in reactivity between the two metals. The bigger the difference, the higher the voltage.

Question 29

What is the function of the electrolyte?

Answer 29

It allows ions to move, enabling the chemical reactions that produce electricity.

Question 30

What affects the voltage of a cell?

Answer 30

The types of metals used The electrolyte used The concentration of the electrolyte

Question 31

What is a battery?

Answer 31

A group of two or more cells connected in series to produce a higher voltage.

Question 32

What is a non-rechargeable battery?

Answer 32

A battery where the chemical reactions are irreversible – once the reactants are used up, it can’t be recharged (e.g. alkaline batteries).

Question 33

What is a rechargeable battery?

Answer 33

A battery where the chemical reactions can be reversed by applying an external current, allowing it to be used multiple times (e.g. lithium-ion batteries).

Question 34

Why do batteries go flat?

Answer 34

The reactants are used up, so the cell can no longer produce a voltage.

Question 35

What are the advantages of rechargeable batteries?

Answer 35

Reusable – reduce waste Cheaper over time Less environmental impact than disposing of many single-use batteries

Question 36

What is a fuel cell?

Answer 36

A device that produces electricity through a chemical reaction between a fuel (often hydrogen) and oxygen, without combustion.

Question 37

What is the most common type of fuel cell in the GCSE spec?

Answer 37

The hydrogen-oxygen fuel cell.

Question 38

What is the only product of a hydrogen fuel cell?

Answer 38

Water (H₂O).

Question 39

What are the half-equations at the electrodes in a hydrogen-oxygen fuel cell?

Answer 39

Anode (−): H₂ → 2H⁺ + 2e⁻ Cathode (+): O₂ + 4H⁺ + 4e⁻ → 2H₂O

Question 40

What is the overall equation for the hydrogen-oxygen fuel cell?

Answer 40

2H₂ + O₂ → 2H₂O

Question 41

Is the reaction in a fuel cell exothermic or endothermic?

Answer 41

Exothermic – energy is released as electricity and heat.

Question 42

What are the advantages of hydrogen fuel cells?

Answer 42

Only product is water (no CO₂) Continuous supply of energy if fuel is provided Lightweight and compact No recharging required

Question 43

What are the disadvantages of hydrogen fuel cells?

Answer 43

Hydrogen is flammable and difficult to store Hydrogen production may involve fossil fuels Expensive to manufacture the fuel cell system

Question 44

What are fuel cells commonly used for?

Answer 44

Spacecraft Electric vehicles Backup power systems

Question 45

How is a hydrogen fuel cell different from a battery?

Answer 45

Fuel cells need a constant supply of fuel and oxygen Batteries store a limited amount of chemicals and need recharging Fuel cells don’t run down if fuel is supplied