C6 - Fuel Cells, Redox and Rusting

Question 1

How does an hydrogen - oxygen fuel cell work?

Answer 1

The hydrogen-oxygen fuel cell uses hydrogen as its fuel. When oxygen or air is supplied, the hydrogen and oxygen react together. The energy released in the reaction is used to produce a potential difference or voltage. This causes a current to flow when the fuel cell is connected in a circuit.

Question 2

Give the word and symbol equation for the overall reaction in a fuel cell.

Answer 2

hydrogen + oxygen → water

2H2(g) + O2(g) → 2H2O(g)

Question 3

What happens to the electrons in hydrogen at the anode and in oxygen at the cathode? Give half equations. Sum up the overall reaction.

Answer 3

Hydrogen molecules lose electrons at the anode and become hydrogen ions:
H2(g) – 2e– → 2H+(aq)
This is an oxidation reaction because the hydrogen molecules lose electrons.

• The electrons pass through the electrical circuit, and the hydrogen ions pass through a special artificial membrane. This lets hydrogen ions through, but not hydrogen or oxygen gas.

When the hydrogen ions reach the cathode, they combine with oxygen and electrons from the electrical circuit:
4H+ + O2 + 4e– → 2H2O
This is a reduction reaction because electrons are gained.

• The overall reaction is an example of a redox reaction: reduction happens at the cathode while oxidation happens at the anode.

Question 4

What are some uses and advantages of fuel cells aboard a spacecraft?

Answer 4

Uses:

• provide electricity
• provide drinking water

Advantages:
• are lightweight
• are compact
• have no moving parts

Question 5

Why are cars not eco - friendly?

Answer 5

• Most vehicles use petrol or diesel which are hydrocarbon fuels made from crude oil, which is a non-renewable resource: it is being used up faster than it can form.
• In addition, the combustion of hydrocarbon fuels produces carbon dioxide which is a greenhouse gas that has been linked with global warming and climate change.

Question 6

What are the advantages of vehicles using hydrogen-oxygen fuel cells, rather than petrol or Diesel engines?

Answer 6

• Water vapour is the only waste product.
• There are no emissions of carbon dioxide from the car.
• There is a large potential source of hydrogen – water can
be decomposed by electrolysis.

Question 7

Why do fuel cells produce less pollution than things like coal-fired power station but still produce pollution?

Answer 7

Less pollution because : it transfers electrical energy directly as needed, whenever fuel and oxygen are supplied to it.

Still pollution because : - most hydrogen production involves fossil fuels. Hydrogen can be produced by reacting steam with coal or natural gas.
- Fuel cells often contain poisonous catalysts. These have to be disposed of safely at the end of the useful life of the fuel cell.

Question 8

What is a redox reaction? Define the words reduction and oxidation in terms of both oxygen and electrons. Define the terms reducing agent and oxidising agent.

Answer 8

Redox : reduction and oxidation happen at the same time.

Reduction : the removal of oxygen from a substance.
Oxidation : the gain of oxygen by a substance, or the reaction of a substance with oxygen.

• Oxidation is loss of electrons.
• Reduction is gain of electrons

Oxidising agent : a substance that can remove electrons from other substances, oxidising them.
Reducing agent : a substance that can give electrons to other substances, reducing them.

Question 9

What is the reactivity series for the 5 common metals? From most to least reactive?

Answer 9

Magnesium 
Zinc 
Iron 
Tin 
Copper

Question 10

What is a displacement reaction?

Answer 10

• A more reactive metal can displace a less reactive metal from its compounds.

Question 11

Displacement reactions can be explain in terms of redox. Write two half equations for the reaction between zinc and copper sulfate solution.

Answer 11

The sulfate ions act as spectator ions.
- Zn(s) – 2e– → Zn2+(aq) zinc atoms are oxidised
- Cu2+(aq) + 2e– → Cu(s) copper(II) ions are reduced

Question 12

What are the requirements for iron and steel to rust? What is the word equation?

Answer 12

Need to react with both water and oxygen.

iron + oxygen + water → hydrated iron(III) oxide

Question 13

Why is rusting a redox reaction?

Answer 13

The iron atoms lose electrons to become iron(III) ions, Fe3+. This means that they are oxidised.
The oxygen atoms gain electrons to become oxide ions, O2–.
This means that they are reduced.

Question 14

What are some methods to prevent rust?

Answer 14

Aim : stop air and water reaching metal surface
Methods :
• coating the metal part with oil or grease
• painting the surface of the metal part
• plating the surface with zinc (galvanising)
• plating the surface with tin.

Question 15

What is galvanisation?

Answer 15

Galvanising involves coating the surface of the iron or steel object with a layer of zinc. The zinc coating stops air and water reaching the metal below, but it also does something else. Zinc is more reactive than iron, so it is more likely to be oxidised. It sacrifices itself to protect the iron below.

Question 16

How does sacrificial protection work with other metals other than zinc?

Answer 16

• It works with other metals that are more reactive than iron, such as magnesium. The more reactive metal loses electrons more readily than iron does, so it is more readily oxidised.

Question 17

How are the insides of steel food cans protected? What are the disadvantages of using this metal?

Answer 17

Tin plating is used to protect the inside of steel food cans from rusting. It stops air and water reaching the surface of the iron. However, unlike zinc, tin is less reactive than iron. This means that tin loses electrons less readily than iron does, so it is less readily oxidised. Unfortunately, the result is that the iron rusts even faster if the tin layer is broken or scratched.