What you'll learn
Every time you use a phone, a torch or a remote control, a chemical reaction is producing electricity inside a cell. For AQA GCSE Chemistry you need to understand how simple cells and batteries work, why some cells are rechargeable and others are not, and how hydrogen fuel cells provide an alternative to conventional cells. This guide covers what determines the voltage of a cell, the difference between non-rechargeable and rechargeable cells, and the reactions and evaluation of hydrogen fuel cells. By the end you should be able to explain how a cell produces a potential difference, compare rechargeable and non-rechargeable cells, and evaluate fuel cells against ordinary batteries.
Key terms and definitions
Cell — A device that produces a potential difference (voltage) from a chemical reaction.
Battery — Two or more cells connected together in series.
Electrode — A conductor, usually a metal, that makes contact with the electrolyte in a cell.
Electrolyte — A solution or paste that conducts electricity and takes part in the reactions in a cell.
Potential difference (voltage) — The push that drives the current around a circuit, measured in volts.
Rechargeable cell — A cell whose reaction can be reversed by passing a current through it, so it can be used again.
Fuel cell — A cell supplied with a fuel and oxygen that reacts to produce a voltage, with water as the only product for a hydrogen fuel cell.
Oxidation — The loss of electrons by a substance.
Core concepts
How a simple cell works
A simple cell is made from two different metals (the electrodes) dipped into an electrolyte. Because the two metals are different, a chemical reaction takes place that produces a potential difference between them. If the electrodes are connected in a circuit, electrons flow from one electrode to the other, producing a current. The greater the difference in reactivity between the two metals, the larger the voltage produced. Using two metals with very similar reactivity gives only a small voltage.
What affects the voltage
The voltage of a cell depends on several things:
- The types of electrode used — a bigger difference in reactivity between the metals gives a larger voltage.
- The electrolyte used — different electrolytes produce different voltages.
Because the voltage depends on the reactivity difference, you can predict which combination of metals will give the largest voltage by looking at their positions in the reactivity series.
Batteries
A battery is simply two or more cells joined together in series. Connecting cells in series adds their voltages together, so a battery can supply a higher voltage than a single cell. This is why a torch that needs a higher voltage uses several cells stacked end to end.
Non-rechargeable cells
In a non-rechargeable cell, such as an ordinary alkaline battery, the chemical reactions happen in one direction only. Once one of the reactants is used up, the reaction stops and the cell can no longer produce a voltage. The reaction cannot be reversed, so the cell has to be thrown away or recycled. These cells are cheap and convenient for low-power devices.
Rechargeable cells
In a rechargeable cell, such as those in a phone or laptop, the chemical reactions can be reversed by connecting the cell to an external electrical supply and passing a current through it in the opposite direction. This restores the original reactants, so the cell can be used again many times. Rechargeable cells cost more to make but can be reused hundreds of times, which makes them cheaper over their lifetime and produces less waste.
Hydrogen fuel cells
A hydrogen fuel cell works differently from an ordinary cell. Instead of running down as reactants are used up, it is continually supplied with hydrogen fuel and oxygen (from the air). The hydrogen and oxygen react to produce a voltage, and the only product is water. The overall reaction is:
hydrogen + oxygen → water (2H₂ + O₂ → 2H₂O)
At the electrodes, hydrogen is oxidised (it loses electrons), and these electrons travel round the external circuit, producing the current, before oxygen is reduced. As long as fuel and oxygen are supplied, the fuel cell keeps producing electricity.
Evaluating hydrogen fuel cells
Fuel cells have advantages and disadvantages compared with rechargeable batteries:
Advantages:
- The only product is water, so there are no polluting gases such as carbon dioxide at the point of use.
- They do not run down or need recharging while fuel is supplied.
- They can be made lighter than batteries storing the same amount of energy, and there are fewer chemicals to dispose of.
Disadvantages:
- Hydrogen is a gas, which is difficult to store safely and takes up a large volume.
- Hydrogen is flammable and must be handled carefully.
- Producing hydrogen often uses energy from burning fossil fuels, which causes pollution elsewhere.
This is why fuel cells are a good example of an "evaluate" question — there are strong points on both sides.
Worked examples
Example 1: Predicting the largest voltage
A student makes simple cells using pairs of metals. Which pair gives the largest voltage: magnesium and zinc, or magnesium and copper? Magnesium and copper give the larger voltage, because copper is much less reactive than magnesium, so the difference in reactivity is greater than between magnesium and zinc.
Example 2: Explaining why a battery is used
A device needs 6 V but a single cell provides 1.5 V. How can 6 V be supplied? Connect four cells in series to make a battery. In series, the voltages add together: 4 × 1.5 = 6 V.
Example 3: Rechargeable versus non-rechargeable
Explain the difference between a rechargeable and a non-rechargeable cell. In a non-rechargeable cell the reaction cannot be reversed, so once a reactant is used up the cell stops working and is discarded. In a rechargeable cell the reaction can be reversed by passing a current through it, restoring the reactants so the cell can be used again.
Example 4: Evaluating a fuel cell
Give one advantage and one disadvantage of using a hydrogen fuel cell in a car instead of a petrol engine. An advantage is that the only product is water, so there are no polluting gases produced at the point of use. A disadvantage is that hydrogen is a flammable gas that is difficult to store safely.
Common mistakes and how to avoid them
A common error is thinking a bigger cell always gives a bigger voltage. Voltage depends on the types of electrode and electrolyte, not the size of the cell — a larger cell may last longer but does not necessarily give a higher voltage.
Students often say the fuel cell "burns" hydrogen. It does not burn it — the hydrogen and oxygen react electrochemically at the electrodes to produce a voltage, with water as the product. Burning would just release heat.
Another mistake is describing the fuel cell product as "water vapour and carbon dioxide". A hydrogen fuel cell produces only water — there is no carbon dioxide, because there is no carbon in the reactants.
When explaining rechargeable cells, be precise: the reaction is reversed by passing a current the opposite way, not simply "topped up" with energy.
Finally, remember that a battery is two or more cells in series. Calling a single cell a battery is technically incorrect, and voltage adds only when cells are in series.
Exam technique for "Cells, batteries and fuel cells"
For voltage questions, refer to the difference in reactivity between the two electrodes — the greater the difference, the larger the voltage. Using the reactivity series to justify your answer earns marks.
Fuel cell questions are very often "evaluate" or "compare" questions, so give a balanced answer with advantages and disadvantages, then a conclusion. Always mention that the only product is water and that no carbon dioxide is produced at the point of use, but also that storing hydrogen is difficult and that its production can cause pollution.
Be ready to write the overall equation for the hydrogen fuel cell reaction, 2H₂ + O₂ → 2H₂O, and to state that hydrogen is oxidised. When comparing with rechargeable batteries, focus on waste, weight, pollution and convenience.
Quick revision summary
- A cell produces a voltage from a chemical reaction between two different electrodes in an electrolyte; a battery is two or more cells in series.
- The voltage depends on the types of electrode (their reactivity difference) and the electrolyte.
- Non-rechargeable cells stop when a reactant is used up — the reaction cannot be reversed.
- Rechargeable cells can be reused because the reaction is reversed by passing a current through them.
- A hydrogen fuel cell reacts hydrogen and oxygen to make a voltage, with water as the only product: 2H₂ + O₂ → 2H₂O.
- Fuel cells produce no CO₂ at the point of use but hydrogen is hard to store and its production can cause pollution.