The shape of the lesson
By the end of the lesson, learners can
- How a fuel cell works: Hydrogen is fed to one electrode and oxygen, usually from the air, to the other.
- Fuel cells versus petrol: Extended.
- Reactants: hydrogen and oxygen.
- Product: water only; energy out as electricity.
- For: no CO2 at the point of use, efficient, quiet.
- Against: storage, flammability, hydrogen production, few stations, cost.
One picture every idea returns to
A hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity, with water as the only chemical product. The overall reaction is 2H2 + O2 → 2H2O. Extended: compared with a petrol engine, a fuel-cell vehicle produces no carbon dioxide or other pollutants at the point of use and is more efficient, but hydrogen is difficult to store, is often made using fossil fuels or electricity, and there are few refuelling stations.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Electricity with only water left over.. Electrolysis uses electricity to split water. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: How a fuel cell works | Hydrogen is fed to one electrode and oxygen, usually from the air, to the other. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Fuel cells versus petrol | Extended. | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Fuel cells versus petrol, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| "Fuel cells produce no pollution" is not quite right. | The vehicle emits only water, but making the hydrogen can release carbon dioxide, for example when it is produced from methane or when the electricity for electrolysis comes from burning fossil fuels. Say "no carbon dioxide at the point of use". |
Hydrogen-oxygen fuel cells: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
State the two reactants and the only chemical product of a hydrogen-oxygen fuel cell.
Write the balanced equation, with state symbols, for the overall reaction in a hydrogen-oxygen fuel cell.
Explain why a hydrogen fuel-cell bus does not release carbon dioxide from its exhaust.
(Extended) Give two advantages and two disadvantages of using hydrogen fuel cells, rather than petrol engines, to power cars.
(Extended) A company says its fuel-cell cars are "zero emission". Explain why this claim may be misleading.
Suggest why a fuel cell, unlike a battery, does not need to be recharged.
Answer key and marking notes
- Reactants: hydrogen and oxygen.
- Product: water.
- 2H2(g) + O2(g) → 2H2O(l)
- One mark for correct formulae and balancing, one for state symbols.
- The fuel, hydrogen, contains no carbon.
- The only product of its reaction with oxygen is water.
- Advantage: only water is produced, so no carbon dioxide, carbon monoxide or oxides of nitrogen at the point of use.
- Advantage: more efficient; no fossil fuel is burned in the car.
- Disadvantage: hydrogen is a flammable gas that is difficult to store; it needs large pressurised tanks.
- Disadvantage: hydrogen is mostly made from methane or by electrolysis using electricity, and there are few refuelling stations.
- The car itself emits only water.
- But the hydrogen is often made from methane, or by electrolysis using electricity from fossil fuels, which releases carbon dioxide.
- The reactants, hydrogen and oxygen, are supplied continuously from outside the cell.
- So it keeps producing electricity as long as the fuel supply lasts; it is refuelled rather than recharged.
9 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
The diagram shows a hydrogen-oxygen fuel cell powering a motor, M. What is gas X?
- Aoxygen
- Bnitrogen
- Ccarbon dioxide
- Dhydrogen
Which energy transfer takes place in a hydrogen-oxygen fuel cell?
- Achemical energy to electrical energy
- Belectrical energy to chemical energy
- Cthermal energy to electrical energy
- Dkinetic energy to chemical energy
Which statement correctly compares the electrolysis of water with a hydrogen-oxygen fuel cell?
- AElectrolysis uses electricity to split water; a fuel cell combines hydrogen and oxygen to produce electricity
- BElectrolysis combines hydrogen and oxygen; a fuel cell uses electricity to split water
- CBoth processes produce electricity from water
- DBoth processes release carbon dioxide
In a fuel cell, 2H2 + O2 → 2H2O. How many moles of oxygen are needed to react completely with 6 moles of hydrogen?
- A3 mol
- B6 mol
- C12 mol
- D2 mol
Where does the oxygen used by the fuel cell in a hydrogen-powered car usually come from?
- Athe air
- Ba tank of compressed oxygen carried in the car
- Cthe water produced by the cell
- Dthe petrol in a second tank
Which substance is released from the exhaust of a petrol car but not from a hydrogen fuel-cell car?
- Acarbon monoxide
- Bwater vapour
- Cnitrogen
- Dhydrogen
Which method of making hydrogen would give a fuel-cell car the lowest overall carbon dioxide emissions?
- Aelectrolysis of water using electricity from wind turbines
- Breacting methane with steam
- Celectrolysis of water using electricity from a coal-fired power station
- Dstoring the hydrogen at a higher pressure in the car
A fuel-cell car needs a much larger fuel tank than a petrol car to travel the same distance. Why?
- AHydrogen is a gas, so even when compressed it takes up a large volume
- BHydrogen releases less energy per gram than petrol
- CFuel cells waste most of the energy as heat
- DHydrogen reacts with the steel of the tank
Which statement about the efficiency of a hydrogen fuel cell compared with a petrol engine is correct?
- AThe fuel cell turns a larger fraction of the fuel's energy into useful energy, but some is still lost as heat
- BThe fuel cell is 100% efficient because its only product is water
- CThe petrol engine is more efficient because burning releases energy faster
- DBoth are equally efficient because both use oxygen from the air
Quiz key
A hydrogen-oxygen fuel cell reacts hydrogen with oxygen: 2H2 + O2 → 2H2O, so gas X is oxygen (often supplied from air). Nitrogen does not take part in the reaction. Carbon dioxide is not a reactant and contains carbon, which could not form water. Hydrogen is already supplied at the other inlet.
Energy stored in the chemicals (hydrogen and oxygen) is released by the reaction and transferred as electrical energy. Electrical to chemical is what happens in electrolysis, the reverse process. The cell does not need to be heated to produce electricity, and it does not rely on movement.
Electrolysis of water: 2H2O → 2H2 + O2, which needs an electrical energy input. Fuel cell: 2H2 + O2 → 2H2O, which releases electrical energy. They are the reverse of each other. The second option swaps them, water cannot be used as a source of electricity, and no carbon is involved in either process, so no carbon dioxide forms.
The equation shows 2 mol of H2 react with 1 mol of O2, a 2 : 1 ratio. So 6 mol of H2 needs 6 ÷ 2 = 3 mol of O2. 6 mol assumes a 1 : 1 ratio, 12 mol inverts the ratio, and 2 mol copies the coefficient from the equation without scaling it.
Oxygen is taken in from the air, which is about 21% oxygen, so only the hydrogen has to be stored in a tank. Carrying pure oxygen as well would add weight and cost and is unnecessary. Water is the product, not a source of oxygen for the cell, and a fuel-cell car does not use petrol.
Petrol is a hydrocarbon, and incomplete combustion in the engine produces carbon monoxide, along with carbon dioxide. Hydrogen contains no carbon, so a fuel cell cannot produce any carbon compounds. Water vapour comes out of both: the fuel cell makes water and so does burning a hydrocarbon. Nitrogen from the air passes through both. Neither car is designed to release hydrogen.
Wind turbines generate electricity without burning fuel, so hydrogen made by electrolysis with wind power releases almost no carbon dioxide. Making hydrogen from methane releases carbon dioxide, and so does generating the electricity for electrolysis by burning coal. Storage pressure affects how much hydrogen fits in the tank, not how much carbon dioxide was released making it.
Hydrogen is a gas with a very low density, so even at high pressure a given amount of it occupies a large volume; this is why storage is a key disadvantage. In fact hydrogen releases more energy per gram than petrol, and fuel cells are more efficient than petrol engines, not less. The tanks are designed to hold hydrogen safely; the problem is volume and pressure, not a reaction with the tank.
A fuel cell converts chemical energy directly into electrical energy and wastes less than a petrol engine, which loses most of its energy as heat in the exhaust and engine. But no device is 100% efficient: a fuel cell still warms up. The product being water says nothing about efficiency. Releasing energy faster does not make an engine more efficient, and using the same oxidant does not make two devices equally efficient.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.