Electrochemical cellsIB MYP Chemistry: Revision notes

An electrochemical cell turns chemical energy into electrical energy. A simple cell is made from:

• two different metals (the electrodes), for example zinc and copper
• an electrolyte, a solution containing ions, such as salt solution or dilute acid
• wires connecting the two metals to a voltmeter or a bulb

The more reactive metal (zinc) loses electrons more easily, so it forms ions: Zn → Zn²⁺ + 2e⁻. It becomes the negative terminal, and it slowly dissolves. The less reactive metal (copper) is the positive terminal.

Electrons flow through the external wire from the more reactive metal (negative) to the less reactive metal (positive). Inside the cell, ions move through the electrolyte and complete the circuit. Electrons do not travel through the solution.

At the less reactive metal, positive ions from the solution pick up the electrons (for example in acid, hydrogen ions form hydrogen gas).

The cell stops working when the more reactive metal has been used up, or the reactants run out.

The voltage depends on the difference in reactivity of the two metals. The further apart they are in the reactivity series, the bigger the voltage.

• magnesium and copper: a large difference, a high voltage
• iron and copper: a smaller difference, a lower voltage
• two identical metals: no difference, no voltage

You can use this to predict. A cell of zinc and iron would give a lower voltage than zinc and copper, because zinc and iron are closer in reactivity. The voltage also depends on the electrolyte used. You do not need to know standard electrode potentials.

The two types of cell are opposites.

• An electrochemical cell produces electricity from a chemical reaction. It has two different metals. There is no power supply.
• An electrolytic cell uses electricity from a power supply to cause a chemical change, such as decomposing a compound. It usually has inert electrodes.

In an electrochemical cell, the negative terminal is the more reactive metal, and electrons flow out of it. In an electrolytic cell, the cathode is the electrode connected to the negative terminal of the power supply, and it attracts positive ions.

A battery is two or more cells joined together. A non-rechargeable (disposable) battery stops working when its reactants are used up, and has to be thrown away or recycled. A rechargeable battery is recharged by passing a current through it the opposite way, which reverses the chemical reaction so that it can be used again.

Batteries contain metals that are mined, so they should be recycled and not put in landfill.

A fuel cell also produces electricity from a chemical reaction, but the reactants are supplied continuously from outside. In a hydrogen-oxygen fuel cell, hydrogen and oxygen (from the air) react to make water:

2H₂ + O₂ → 2H₂O

The cell keeps working as long as the fuel is supplied, unlike a battery that runs down. The only product is water, so there is no carbon dioxide at the point of use. Hydrogen fuel cells are used in some cars, buses and spacecraft.

Drawbacks: hydrogen is flammable and hard to store, and it is often made using fossil fuels or large amounts of electricity.