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How a simple cell makes a voltage, how batteries differ, and how a hydrogen fuel cell compares with a rechargeable cell.
Your phone, remote control and torch all run on cells. A cell turns the energy stored in chemicals into electricity. Some cells can be recharged, and some run on a fuel.
It matters because we use more and more batteries, in phones, laptops and electric cars. Chemists are trying to make cells that last longer and cause less pollution.
By the end you will be able to explain how a simple cell works and what decides its voltage. You will compare batteries with hydrogen fuel cells and weigh up their good and bad points.
A chemical cell makes electricity from a chemical reaction. A simple cell has two different metals dipped into an electrolyte. The metals are the electrodes. An electrolyte is a liquid that conducts electricity.
A wire joins the two metals. The more reactive metal loses electrons more easily. So electrons flow through the wire from the more reactive metal to the less reactive one. This flow is the electric current.
The reaction carries on until one of the reactants is used up. Then the cell stops working, like a torch with flat batteries.
The voltage depends on the two metals and the electrolyte. The bigger the difference in reactivity between the two metals, the bigger the voltage.
A magnesium and copper cell gives a bigger voltage than an iron and copper cell, as the bars show. Magnesium is far from copper in the reactivity series. Iron is closer.
Think of a ball rolling down a slope. A bigger drop gives a bigger push. If both metals are the same there is no drop, so the voltage is zero.
A battery is two or more cells connected together. This gives a higher voltage than one cell on its own.
In a non-rechargeable battery, such as an alkaline battery, the reaction cannot be reversed. It stops when the reactants are used up. Then the battery is thrown away.
In a rechargeable battery, such as a lithium-ion battery, a current is passed through the opposite way. This reverses the reaction and makes the reactants again. Think of a phone you plug in overnight and use again the next day.
A fuel cell makes electricity from a fuel and oxygen. In a hydrogen fuel cell, hydrogen and oxygen are supplied from outside. They react to make water and produce a voltage.
The cell keeps working for as long as the fuel is supplied. It does not need recharging. Think of a car: it keeps going as long as you keep filling the tank.
The overall reaction is 2H₂ + O₂ → 2H₂O. The only product is water, so the cell itself makes no pollution. Reactions at the electrodes make the electrons flow.
This step is for Higher tier. A half equation shows just the electrons lost or gained at one electrode.
At the negative electrode, hydrogen is oxidised, which means it loses electrons: H₂ → 2H⁺ + 2e⁻. At the positive electrode, oxygen is reduced, which means it gains electrons: O₂ + 4H⁺ + 4e⁻ → 2H₂O.
Double the first equation so four electrons are lost and four are gained: 2H₂ → 4H⁺ + 4e⁻. Adding gives 2H₂ + O₂ → 2H₂O. The electrons flow round the outer circuit and the ions move through the electrolyte.
Fuel cells make only water and need no mains charging. They suit a vehicle that needs long, continuous use. They can be lighter than a big battery for the same range.
But hydrogen is a flammable gas that is hard to store. Most hydrogen is made from fossil fuels, which uses energy and causes pollution. Fuel cells can also be expensive.
Rechargeable cells are easy to carry and charge from the mains. They need no flammable gas. But they take time to charge, store less energy for their size and eventually wear out. Their metals need careful disposal.
A simple cell has two different metals in an electrolyte. Electrons flow from the more reactive metal.
The bigger the difference in reactivity, the bigger the voltage.
Non-rechargeable batteries run out. Rechargeable batteries are recharged by reversing the reaction.
A hydrogen fuel cell is supplied with hydrogen and oxygen. The overall reaction is 2H₂ + O₂ → 2H₂O.
Fuel cells and rechargeable cells each have good and bad points.
State the only product of a hydrogen fuel cell.
Answer: water
A student makes two cells. Cell A uses zinc and copper. Cell B uses magnesium and copper. Predict which cell has the larger voltage and explain why.
Answer: Cell B, because magnesium and copper are further apart in the reactivity series.
Evaluate the use of a hydrogen fuel cell and a rechargeable battery to power a car.
Answer: A fuel cell makes only water but hydrogen is hard to store. A battery is easy to charge but takes time and has a limited range.
Chemical cells and fuel cells practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
What is the name for the solution that conducts electricity in a cell?
Answer: electrolyte
Which two substances react in a hydrogen fuel cell?
Answer: hydrogen and oxygen
What is made when hydrogen reacts with oxygen in a fuel cell?
Answer: water
Can a non-rechargeable battery be recharged? Answer yes or no.
Answer: no
What do we call the two different metals in a simple cell?
Answer: electrodes
Which gas is the fuel in a hydrogen fuel cell?
Answer: hydrogen
One cell uses magnesium and iron. Another cell uses magnesium and copper. Which has the larger voltage?
Answer: magnesium and copper
Why does a fuel cell not need recharging?
Answer: the fuel and oxygen are supplied continuously
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
A student makes a simple cell using a piece of zinc and a piece of copper in salt solution. (a) Name the part of the cell that is the salt solution. [1 mark] (b) The student replaces the zinc with magnesium. State and explain what happens to the voltage. [2 marks] (c) State one way that a fuel cell differs from this cell. [1 mark]
Answer: (a) electrolyte (b) The voltage increases because magnesium is more reactive than zinc, so the difference in reactivity from copper is bigger. (c) A fuel cell is supplied with fuel from outside and does not run out.
Hydrogen fuel cells and rechargeable batteries can both be used to power vehicles. Evaluate the use of each. [6 marks]
Answer: In a hydrogen fuel cell, hydrogen and oxygen react to make only water, so there is no pollution at the point of use, and the vehicle can be refuelled quickly. However, hydrogen is flammable and difficult to store, there are few refuelling stations, and most hydrogen is made using fossil fuels which causes carbon dioxide emissions. A rechargeable battery is easier to charge, has no flammable gas and is efficient, but it takes a long time to recharge, has a limited range and wears out, and the metals in it must be disposed of safely. Neither is completely clean, because the electricity to charge the battery may come from fossil fuels. The best choice depends on how the hydrogen or electricity is produced.
Saying a fuel cell has to be recharged
It works as long as hydrogen and oxygen are supplied. It does not need electrical charging.
Saying a fuel cell makes no pollution at all
It only makes water, but making the hydrogen can cause pollution.
Saying the voltage depends on the size of the electrodes
It depends on the metals and the electrolyte, not on the size.
Saying a non-rechargeable battery runs out because the metal is destroyed
The reactants are used up and the reaction cannot be reversed.
Chemical cells and fuel cells is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 6.
Saying a fuel cell has to be recharged. It works as long as hydrogen and oxygen are supplied. It does not need electrical charging.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
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