Energy changes

Question 1

What is electrolysis?

Answer 1

The splitting up of a substance using electricity

Question 2

Why does the ionic substance have to be molten or in solution?

Answer 2

So that the ions are free to move to the electrodes

Question 3

What is the electrolyte?

Answer 3

The molten ionic substance or solution of the ions

Question 4

What are the positive and negative electrodes called?

Answer 4

Cathode - negative electrode
Anode - positive electrode

Question 5

What are electrodes made from?

Answer 5

Graphite, aluminium or carbon

Question 6

What happens during electrolysis?

Answer 6

Positively charged ions are attracted to the negative electrode and the negatively charged ions are attracted to the positive electrode. When the ions reach the electrodes they can lose their charge and become elements

Question 7

What’s different about the electrolysis of aqueous solutions?

Answer 7

They contain hydrogen ions (H+) and hydroxide ions (OH-) as well as the ions of the dissolved substance. These ions are formed from the water that the compound is dissolved in

Question 8

What happens if the metal ion is more reactive than hydrogen in the electrolysis of an aqueous solution?

Answer 8

The H+ ions go to the negative electrode to form hydrogen gas

Question 9

What happens if the metal is less reactive than hydrogen in the electrolysis of an aqueous solution?

Answer 9

The metal ions go to the negative electrode and form the metal

Question 10

What happens at the positive electrode in the electrolysis of an aqueous solution?

Answer 10

Oxygen is produced unless there is a halide ion present, which means the halogen is produced instead

Question 11

What happens in the electrolysis of brine?

Answer 11

The ions in the solution are H+ ions, Na+ ions, Cl- ions and OH- ions
The hydrogen ions go to the negative electrode, the chloride ions go to the positive electrode and this leaves a solution of sodium hydroxide

Question 12

What happens in an exothermic reaction?

Answer 12

Heat is transferred to the surroundings; this could be air or water and then the surroundings have more heat energy so the temperature increases

Question 13

What are some examples of exothermic reactions?

Answer 13

Combustion
Respiration
Neutralisation
Many oxidation reactions
Hand warmers

Question 14

What happens in an endothermic reaction?

Answer 14

Heat energy is taken away from the surroundings and stored as chemical energy, this means that the surroundings have less heat energy and so the temperature decreases

Question 15

What are some examples of endothermic reactions?

Answer 15

Thermal decompositions
Some dissolving processes
Photosynthesis
Sports injury cooling packs

Question 16

What is the activation energy?

Answer 16

The minimum amount of energy required for particles to react

Question 17

How can you tell if a reaction is endothermic?

Answer 17

If more energy is required to break the bonds than is released from making the bonds, the reaction is endothermic

Question 18

How can you tell if a reaction is exothermic?

Answer 18

If more energy is released from making the bonds than required to break the bonds, the reaction is exothermic

Question 19

When is cryolite used?

Answer 19

In the electrolysis of aluminium oxide in order to lower its melting point and save energy

Question 20

How is a cell made?

Answer 20

By connecting two different metals which are in contact with an electrolyte

Question 21

What does the voltage produced by a cell depend on?

Answer 21

The type of electrode and electrolyte used; the greater in reactivity between the two metals, the greater the voltage

Question 22

What happens in a non-rechargeable battery?

Answer 22

One of the reactants is used up, causing the reaction to stop and the battery no longer works

Question 23

What are non-rechargeable batteries also called?

Answer 23

Alkaline batteries

Question 24

What happens in a rechargeable battery?

Answer 24

A reversible reaction is used which can go forwards and backwards when an external electricity current is applied

Question 25

Why does a fuel cell never stop working?

Answer 25

Because the reactants are continually replaced

Question 26

How is energy released in a fuel cell?

Answer 26

In the form of electrical current and heat

Question 27

What happens at the negative electrode in a fuel cell and what is the equation?

Answer 27

The hydrogen molecules lose electrons to form H+ ions and become oxidised
H2 → 2H+ + 2e-

Question 28

How is electrical current created around the fuel cell?

Answer 28

The electrons move around the circuit to the positive electrode, creating an electrical current

Question 29

What happens at the positive electrode in a fuel cell?

Answer 29

The hydrogen ions and electrons react with the oxygen to form water and the oxygen atoms are reduced
4H+ + O2 + 4e- → 2H2O

Question 30

What is the overall reaction in a fuel cell?

Answer 30

2H2 + O2 → 2H2O

Question 31

What are the advantages of fuel cells?

Answer 31

Don’t need to be charged overnight
Continuous supply of electricity
No loss of efficiency over time
Only waste product is water so it’s good for the environment
Longer life than rechargeable cells or batteries that add to landfill
No toxic chemicals to dispose of at the end of the cell’s life

Question 32

What are the disadvantages of fuel cells?

Answer 32

Expensive and unreliable technology in motor vehicles
Hydrogen is difficult to store as it’s highly flammable
Hydrogen is produced from methane (a non-renewable resource) and the process releases carbon dioxide
Rechargeable cells and batteries are much cheaper
Hydrogen is produced by electrolysis which uses fossil fuels and carbon dioxide