Electrochemical Cells

From chemical reactions to electricity

A chemical cell converts energy stored in chemicals into electrical energy. Its chemical reactions create a potential difference, or voltage, between two electrodes. If the electrodes are joined through a complete external circuit, this potential difference drives electrons through the wire and can operate a device.

A simple cell needs two different metals acting as electrodes and an electrolyte in contact with both. The electrolyte contains mobile ions. These ions move within the liquid and allow charge to be transferred internally, while electrons travel through the metal electrodes and the external wires. The electrolyte does not supply electrons to the wire.

Why the metals must be different

Different metals do not release electrons equally readily. In a zinc–copper cell, for example, zinc is more reactive and releases electrons more readily than copper. This makes zinc the negative electrode in this simple model. Electrons then flow through the external circuit from zinc towards copper.

The difference between the chemical behaviour of the electrodes produces the voltage. If identical pieces of the same metal are used under identical conditions, there is no difference between the electrodes to produce a cell voltage.

The voltage depends on several factors, including the types of electrode and electrolyte. When the electrolyte and other conditions are kept the same, metals that are further apart in reactivity generally produce a greater voltage than metals that are closer together. For example, magnesium and copper would generally produce a greater voltage than zinc and copper because magnesium and copper differ more in reactivity. This is a useful pattern, not a reason to ignore the electrolyte: changing the electrolyte can also change the voltage.

Using voltage data to compare metals

Cells can provide evidence about relative reactivity. To compare unknown metals fairly, use the same reference metal, the same electrolyte and the same experimental conditions. Compare both the size and the polarity of each voltage reading: polarity identifies which electrode is negative, while the magnitude helps compare the separation between the metals' tendencies to release electrons.

Suppose metal X and copper give a larger voltage than metal Y and copper under the same conditions, and both X and Y form the negative electrode. X has the greater tendency to release electrons and can be placed as more reactive than Y. If the leads are reversed, the sign on a digital voltmeter reverses even though the chemistry of the cell has not changed, so polarity must be interpreted alongside how the leads were connected.

From a cell to a battery

One cell provides one cell voltage. A battery consists of two or more cells connected in series, so their voltages combine to provide a greater total voltage. In everyday speech, people often call a single packaged cell a “battery”, but the examination distinction is that a cell is one chemical unit whereas a battery contains multiple cells.

Non-rechargeable cells

A cell works only while its chemical reactions can continue. In a non-rechargeable cell, the reaction eventually stops when one of the reactants has been used up. The cell can no longer maintain its voltage and is described as “flat”. Its reaction cannot be usefully reversed by passing a charging current through it, so the cell must be replaced. Alkaline batteries are non-rechargeable.

Rechargeable cells

In a rechargeable cell, an external electrical supply drives the chemical reactions in the reverse direction. This converts products back towards reactants and restores the chemicals needed for the cell to supply electricity again. Charging therefore requires an input of electrical energy; it does not create energy.

The key distinction is not simply whether a cell has gone flat. Both types eventually need attention, but a rechargeable cell is designed so that an external current can reverse its reactions, whereas a non-rechargeable cell is not.

Evaluating cells

There is no single best cell for every purpose. Use the information in the question and consider factors such as the required voltage, whether repeated recharging is useful, how often a non-rechargeable cell would need replacing and whether access to a suitable external electricity supply is available. Rechargeable cells can be used through repeated charge–discharge cycles, but they must be recharged; a non-rechargeable cell is replaced once a reactant has been used up. A sound conclusion identifies the most important criterion for the stated application and supports the choice with the supplied data.

Simple cells

• Chemicals react to produce a potential difference and transfer electrical energy.
• A simple cell has two different metal electrodes in contact with an electrolyte and connected by an external circuit.
• Electrons flow through the wires; ions move through the electrolyte.
• The more reactive metal releases electrons more readily and is the negative electrode in the simple-cell model.

Voltage and reactivity

• Voltage depends on the electrode materials and the electrolyte.
• With the same electrolyte and conditions, a greater difference in metal reactivity generally gives a greater voltage.
• For reactivity comparisons, keep a common reference electrode and interpret voltage magnitude together with polarity.
• Identical electrodes under identical conditions give no potential difference.

Cells and batteries

• One chemical unit is a cell.
• Two or more cells connected in series form a battery and provide a greater total voltage.

Rechargeability

• Non-rechargeable: the reaction stops when a reactant is used up and cannot be usefully reversed by charging; alkaline batteries are an example.
• Rechargeable: an external electrical current reverses the chemical reactions, restoring reactants so the cell can be used again.

Evaluation

Use the supplied evidence to compare required voltage, need for replacement, opportunity to recharge and suitability for repeated use. Make a justified choice for the stated application.