10.02.4 Cells and Batteries

Syllabus

- Cells contain chemicals which react to produce electricity.
- The voltage produced by a cell is dependent upon a number of factors including the type of electrode and electrolyte.
- A simple cell can be made by connecting two different metals in contact with an electrolyte.
- Batteries consist of two or more cells connected together in series to provide a greater voltage.
- Alkaline batteries are non-rechargeable. In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up.
- Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.
- Students should be able to interpret data for relative reactivity of different metals & evaluate the use of cells.

Students do not need to know details of cells and batteries other than those specified.

What does this mean?

What is a cell?

When chemicals reacts there is always an energy change - usually energy is released (exothermic).

But the energy doesn't have to be released as heat, it could be electrical energy.

And energy can also be released as light or sound in some cases.

In the cell (right) there are two electrodes of different metals separated by an electrolyte - a conducting solution, often an acid.

All metals "want to" react by losing electrons but can only do this if there is something they can force their extra electrons on to.

Zinc is more reactive than Copper.

It can lose electrons and they flow towards the Copper, ions move through the electrolyte to complete the circuit.

This is an electrical cell.

What affects the voltage?

A cell like this will generate a very small voltage, which can be changed by:

Using a different electrolyte
Using different electrodes

We don't have to predict anything about the electrolyte.

But we should know that the greater the difference in Reactivity of the metal electrodes the higher the voltage.

So our cell of Zinc and Copper electrodes would generate a decent voltage but Silver and Potassium electrodes would generate a higher voltage - if expensively and dangerously.

However, a cell will stop generating a voltage at all when one of the electrodes or the electrolyte becomes used up.

What is a battery?

Often multiple cells are connected in a battery - such as the ones used in cars.

Car batteries are rechargeable - when the car is moving electrical energy from the alternator reverses the chemical reactions so that the battery should last for years.

The same thing happens with phone batteries when we plug them in.

But the familiar Alkaline batteries are not rechargeable - they will simply stop when the reaction is over.

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