Comparing cells
exothermicReaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases. reactions transfer energyThe capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. to their surroundings. This is usually transferred by heating, but the energy can be transferred in other ways. Chemical cellA store of internal energy that can be transferred as an electric current in a circuit. use chemical reactions to transfer energy by electricityThe flow of charge through a conductor..
Chemical cells
Chemical cells include the familiar batteryA chemical supply of electrical energy. For example, common battery voltages include 1.5 V and 9 V. used in torches and mobile phones. There are different designs of chemical cells, with different reactions depending on the type of cell.
Chemical cells produce a voltageThe potential difference across a cell, electrical supply or electrical component. It is measured in volts (V). until one of the reactantA substance that reacts together with another substance to form products during a chemical reaction. is used up. When this happens, we say the battery ‘has gone flat’.
Fuel cells
fuel cellDevice that produces a voltage continuously when supplied with a fuel and oxygen. work in a different way than chemical cells. Fuel cells produce a voltage continuously, as long as they are supplied with:
- a fuelMaterial that is used to produce heat, like coal, oil or gas.
- oxygen (from the air)
Hydrogen-oxygen fuel cells
In a hydrogen-oxygen fuel cell, hydrogen and oxygen are used to produce a voltage. Water is the only product.
Two separate reactions happen, one on each side of the fuel cell.
1. On one side, hydrogen molecules lose electrons and form hydrogen ions:
2H2 → 4H+ + 4e-
The two products reach the other side of the cell, hydrogen ions through a membrane and electrons through the electrical circuit.
2. On the other side, hydrogen ions react with oxygen molecules and electrons to form water:
4H+ + O2 + 4e- → 2H2O
The overall reaction in a hydrogen-oxygen fuel cell is:
hydrogen + oxygen → water
2H2(g) + O2(g) → 2H2O(l)