Cells and batteries — AQA GCSE Chemistry

A cell is a device that converts chemical energy into electrical energy. Inside a simple cell, two different metals act as electrodes in contact with an electrolyte. The more reactive metal tends to lose electrons and is oxidised, while the less reactive metal provides a surface where electrons are gained and reduction occurs. These electron transfers happen at separate electrodes, so electrons flow through the external circuit as an electric current. The chemical reaction continues until one reactant is used up, which is why cells eventually run down. A battery consists of two or more cells connected together in series, giving a larger voltage. For example, a zinc–copper cell with a salt solution electrolyte produces a voltage because zinc is more reactive than copper.

The voltage of a simple cell is not fixed; it depends on the chemicals chosen. Using two metals with a larger difference in reactivity produces a larger voltage, because the more reactive metal has a stronger tendency to lose electrons. The electrolyte also matters: different ions and concentrations change the cell voltage. Connecting two or more cells in series increases the total voltage, which is why batteries can supply more than a single cell. Temperature can also affect the voltage in practical cells. For example, a zinc–copper cell produces a different voltage from a magnesium–copper cell because magnesium is more reactive than zinc, so the reactivity difference is greater.

A simple cell needs two different metals, each acting as an electrode, both dipped into an electrolyte such as a salt solution or dilute acid. The metals must be different so that one is more reactive than the other; this difference drives the electron transfer. The more reactive metal loses electrons and becomes oxidised, releasing electrons into the external circuit. These electrons travel through the wires to the less reactive metal, where they are gained by ions in the electrolyte in reduction. The electrolyte completes the circuit by allowing ions to move between the electrodes. If the two metals were the same, no overall voltage would be produced. For example, connecting zinc and copper strips in copper sulfate solution makes a working simple cell.

A single cell produces a voltage from the potential difference between its two different electrodes in an electrolyte. One cell gives only a small voltage, so a battery joins two or more cells in series: the positive terminal of one cell connects to the negative terminal of the next. In series, the voltages add, so two 1.5 V cells give 3 V and three give 4.5 V. The cells must be connected the same way round; reversing one cell subtracts its voltage instead of adding it. A battery therefore provides a greater voltage than one cell, which drives a larger current through a circuit and can power devices needing more energy per unit charge. The cells inside a battery are not separate objects in the circuit; together they form one source with a combined voltage.

A non-rechargeable cell, often called a primary cell, produces electricity from chemical reactions at its electrodes. The reactants are sealed inside and are used up as the cell discharges. When one reactant is exhausted, the reaction can no longer continue, so the voltage falls to zero and the cell is flat. The reaction is not easily reversed in the cell, so the cell cannot be recharged by passing current through it. Alkaline batteries are a common example of non-rechargeable batteries; they are discarded once flat. This differs from rechargeable cells, where the reactions can be reversed by an external current. The key idea is that a non-rechargeable cell has a limited amount of reactant, so its lifetime is finite.

A rechargeable cell, also called a secondary cell, produces electricity from chemical reactions like any cell. When it discharges, the reactions at the electrodes change the chemicals and the cell eventually goes flat. The important difference is that these reactions can be reversed. If an external electrical current is supplied in the opposite direction, for example from a charger, the products are converted back into the original reactants. The cell can then discharge again. This cycle can be repeated many times, although the cell eventually wears out. Recharging does not create energy; it stores electrical energy as chemical energy, which is later converted back to electrical energy.

The voltage of a simple cell depends on the difference in reactivity between its two electrodes. A more reactive metal and a less reactive metal in an electrolyte give a larger voltage than two metals close in the reactivity series. Students should read data tables or graphs showing voltage for pairs of metals and use the reactivity series to explain the pattern. For example, a cell with magnesium and copper gives a higher voltage than one with zinc and copper because magnesium is further from copper in the series. Evaluating the use of cells means weighing practical factors such as voltage, cost, lifetime, rechargeability and environmental impact to judge which cell suits a device.