5.25CCells and batteries
Chemistry only — not on the Combined Science specification
Cells contain chemicals which react to produce electricity.
A simple cell can be made by connecting two different metals in contact with an electrolyte. The voltage produced by a cell is dependent upon a number of factors, including the type of electrode and electrolyte.
Batteries consist of two or more cells connected together in series to provide a greater voltage.
You should be able to interpret data for relative reactivity of different metals, and evaluate the use of cells.
The pattern to look for in the data is the one that follows from the reactivity series: the further apart the two metals are in the reactivity series, the larger the voltage the cell produces. If two identical metals are used, the voltage is zero, because there is no difference between them for electrons to flow across. You do not need to know details of cells and batteries other than those specified.
Worked example
A cell made from magnesium and copper gives 2.7 V. One made from zinc and copper gives 1.1 V. Predict the voltage from an iron and copper cell, and explain your reasoning.
- The voltage depends on the difference in reactivity between the two metals.
- Copper is the same in all three cells, so the differences come from magnesium, zinc and iron.
- The reactivity order is magnesium, then zinc, then iron — iron is the closest of the three to copper.
- So the iron cell should give the smallest voltage of the three.
Answer: Less than 1.1 V — around 0.8 V — because iron is closer to copper in the reactivity series than zinc is, so the difference driving the cell is smaller.
In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up. Alkaline batteries are non-rechargeable.
Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.
That is the whole difference: in a rechargeable cell the reaction can be driven backwards to restore the original reactants, and in a non-rechargeable one it cannot.
Common mistake
A non-rechargeable battery does not "run out of electricity" — it runs out of one of its reactants. Say which idea you mean: the chemical reactions stop because a reactant has been used up.
- Cell
- a device containing chemicals which react to produce electricity, made by connecting two different metals in contact with an electrolyte
- Battery
- two or more cells connected together in series to provide a greater voltage
5.26C–5.27CFuel cells
Chemistry only — not on the Combined Science specification
Fuel cells are supplied by an external source of fuel, such as hydrogen, and oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.
The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water:
2H₂ + O₂ → 2H₂O
Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries. The important structural difference is that a fuel cell is not a store of energy at all — as long as fuel is supplied it keeps producing a potential difference, so it never needs recharging.
You should be able to evaluate the use of hydrogen fuel cells in comparison with rechargeable cells and batteries.
Worked example
A car manufacturer is choosing between hydrogen fuel cells and rechargeable batteries for a delivery van used all day in a city. Evaluate the two.
- Identify what matters for this use: long hours of running, little time to stop, and city air quality.
- Argue for the fuel cell: it refills in minutes rather than hours, so the van need not stop for long; it is lighter, so more load can be carried; and it produces only water at the point of use.
- Argue against it: hydrogen is hard to store safely, refuelling stations are scarce, and the hydrogen itself may have been made from fossil fuels.
- Reach a judgement that answers the question that was actually asked, and say what would change it.
Answer: For a van running all day, the fuel cell's minutes-long refuelling is the decisive advantage over hours of charging — provided a refuelling station is available on the route. Without that infrastructure, rechargeable batteries are the practical choice.
Common mistake
A hydrogen fuel cell is not automatically "clean". The cell itself produces only water, but if the hydrogen was made from fossil fuels or by electrolysis using fossil-fuel electricity, carbon dioxide was released elsewhere. A full evaluation says where the hydrogen came from.
- Fuel cell
- a cell supplied by an external source of fuel, such as hydrogen, and oxygen or air, in which the fuel is oxidised electrochemically to produce a potential difference; it is not a store of energy and keeps working while fuel is supplied
5.26CHalf equations in the hydrogen fuel cellHigher tier
Chemistry only — not on the Combined Science specification
Higher tier students should be able to write the half equations for the electrode reactions in the hydrogen fuel cell.
In an alkaline fuel cell the half equations are:
• negative electrode: 2H₂ + 4OH⁻ → 4H₂O + 4e⁻ — the hydrogen loses electrons, so it is oxidised • positive electrode: O₂ + 2H₂O + 4e⁻ → 4OH⁻ — the oxygen gains electrons, so it is reduced
Adding the two together and cancelling gives the overall reaction, 2H₂ + O₂ → 2H₂O, which is the oxidation of hydrogen to produce water.
Worked example
Show that the two half equations above combine to give the overall reaction.
- Write them one above the other so the electrons match: 4e⁻ are released at one electrode and 4e⁻ are used at the other.
- Add both sides: 2H₂ + 4OH⁻ + O₂ + 2H₂O → 4H₂O + 4OH⁻.
- Cancel the 4OH⁻ that appears on both sides.
- Cancel 2H₂O from each side, leaving 2H₂O on the right.
Answer: 2H₂ + O₂ → 2H₂O — the electrons and the hydroxide ions cancel, because they are made at one electrode and used at the other.
Before you practise
- A simple cell needs two different metals in an electrolyte; the bigger the difference in reactivity, the bigger the voltage.
- A non-rechargeable cell stops when one of the reactants is used up, and the reaction cannot be reversed.
- A rechargeable cell can be reversed by passing a current back through it.
- In a hydrogen-oxygen fuel cell, hydrogen and oxygen react to produce a voltage and the only product is water.
- The fuel cell keeps working as long as fuel is supplied, so it does not need recharging.
- Against that: hydrogen is hard to store and to distribute, and making it takes energy.
- Evaluate by comparing on named grounds — mass, lifetime, refuelling, pollution and cost — not by asserting one is better.