ElectrochemistryYear 10 Chemistry

ElectrochemistryGetting Started

You can make a simple electrical cell from a lemon, a think piece of copper wire and a trip of magnesium.1. Clean the copper wire and the magnesium strip with steel

wool2. Push the wire and strip into the lemon about 2cm apart,

making sure they do not touch.3. Connect the copper wire to the positive terminal of a

sensitive voltmeter or multimeter and the magnesium to the negative terminal. What voltage is produced? Does the voltage stay the same? How many lemons would you need to make a 12 volt battery?

4. Does it make any difference how far apart the copper wire and the magnesium strip are, or how far you push them into the lemon?

5. What happens if you use different metals or different fruits

Cells and batteries

What is an electric cell? Do you have any grey fillings in your teeth? If so have you ever bitten on

a piece of metal foil and felt a tingle? This is caused by a small electric current in your mouth. All that is

needed to produce the current is two different metals and a conducting solution.

When two different metals are placed in a conducting solution and connected together an electrochemical cell is made. It is called an electric cell. The conducting solution is called an electrolyte. It conducts electricity because it contains ions. The metal strips are called electrodes.

In the lemon activity; what formed the electric cell Electric cell – magnesium strip and the copper wire. The magnesium electrode produces electrons which flow through the

wire and voltmeter to the copper electrode where another chemical reaction uses up those electrons

Lemon battery

In the lemon activity; what formed the electric cell?

• Electric cell – magnesium strip and the copper wire.

The magnesium electrode produces electrons which flow through the wire and voltmeter to the copper electrode where another chemical reaction uses up those electrons

Investigation – Voltages of electric cells

Investigation – Voltages of electric cells

Investigation – voltage of electric cells

How an electric cell works When the negative terminal of the voltmeter is

connected to the zinc and the positive terminal to the copper, the cell produces about 1 volt (to start with)

The copper is the positive electrode and bubbles of gas are produced there.

The zinc is the negative electrode and slowly dissolves in the acid.

Dry Cells

The cells used in torches and portable radios are called dry cells. This is because the liquid electrolyte has been replaced by a moist electrolyte paste in a sealed container.

The outside case is made of zinc. This is the negative electrode, and it loses electrons (oxidation) to form zinc ions.

Dry Cells

Suppose you have a CD player that needs six 1.5V dry cells. The cells have to be connected in series so that the top positive terminal of one touches the bottom negative terminal of the next.

In the appliance illustrated to the right there are two rows of 3 cells, one row on top of the other. The total voltage is 1.5V x 6 = 9V

Rechargeable batteries

The cells discussed so far have a major disadvantage. Once the chemicals in them have reacted the cells ‘go flat’ and have to be thrown away.

However with some cells it is possible to reverse these reactions by passing electricity through them in the opposite direction.

Two common rechargeable cells are nickel-cadmium (NiCad) and lithium ion.

The most common rechargeable battery is the car battery. The battery produces an electric current to start the engine, but once the engine is running, an alternator (generator) is used to pass current back through the battery.

A Model Car Battery

A Model Car Battery

Questions

Electrolysis

Electroplating In many jewellery stores you will

notice that much of the silver jewellery is usually only coated with a thin coating of silver. This silver is usually put on the top of an inexpensive metal such as copper, zinc or metal.

The coating is put on by electrolysis, a process in which electricity is used to cause chemical reactions. (It is the reverse of what happens in electric cells)

Putting a layer of metal on the surface of another metal is called electroplating.

Hair removal by electrolysis

Corrosion of metals