Electrolysis - The aim of this experiment is to prove that by passing electric current through an aqueous copper sulphate solution changes the mass of each electrode. The anode's mass will increase and the cathode's mass will decrease.

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Solids, e.g. metals conduct electricity; the current is carried by electrons. The battery forces the electrons through the conductor. The metal may change for example it may become hot, but when the flow of current is stopped the metal is just as it was before. When a metal conducts electricity no chemical reactions occur. However when a molten salt conducts electricity, chemical changes occur, and new substances are formed.

This diagram shows an experiment, which aims to find out what happens when a direct electric current passes through a molten salt. A salt is a compound of a metallic element with a non-metallic or elements.

Lead (II) is a salt with a fairly low melting point. The container through which the current passes is called a cell. The rods, which conduct electricity into and out of the cell, are called electrodes. The electrode connected to the positive terminal of the battery is called the anode. The electrode connected to the negative terminal is called the cathode. The electrodes are usually made of elements such as platinum and graphite, which do not react with electrolytes.

When the salt melts, the bulb lights, showing that the molten salt conducts electricity. At the positive electrode (anode), bromine can be detected. It is a reddish-brown vapour and therefore a non-metallic element with a very pungent (safety- the vapour produce must not be inhaled). At the cathode (negative electrode), lead is formed. This can been seen after cooling.

Lead Bromide (II) has been split up by the electric current, it has been electrolysed. Electrolytes are the compounds, which conduct electricity. Bromine is negatively charged and only goes to the positive electrode and Lead is positively charged meaning is only goes to negative electrode. These charged particles are called ions. Positive ions are called cations because they travel to the cathode and negative ions are called anions because they travel to the anode.

A bromide ion, Br- differs from Br a bromide atom because it has one more electron. The extra electron gives it a negative charge.

A Lead ion Pb2+, differs from a Lead atom, Pb by having 2 more electrons.

When ions reach the electrodes they lose their charge, they are discharged. The positive electrode takes electrons form bromine ions so that they become bromine atoms.

Then the bromide atoms pair up to form bromine molecules:

The negative electrode gives electrons to the positively charged lead ions so that they become lead atoms.

The electrons, which are supplied to the anode by the discharge of bromine ions, travel round the external circuit to the cathode. At the cathode, they become lead (II) ions.

Lead Bromide is an uncharged substance because there are two Br- ions for each Pb2+ ions. The formula is PbBr2. When the lead bromide is solid it does not conduct electricity. This is because when the salt it a solid the ions cannot move. They are fixed in a three-dimensional ridged structure. When the solid is molten the ions can move and make their way to the electrodes.

A few common ions

When compounds are electrolysed, new substance are produced at the electrodes. For example when electricity is passed through molten sodium chloride, pale chlorine gas comes off at the anode and sodium forms at the cathode. When copper sulphate is electrolysed using the apparatus in the diagram below, a pink deposit of copper appears on the cathode. Bubbles of colourless gas stream off the anode and collect in the inverted test tube. This gas relights a glowing splint showing that oxygen is present.

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The table below list the products formed at the electrodes when various liquids and aqueous solutions are electrolysed.

 Information taken from Chemistry counts by Graham Hill, Second Edition, published by Hodder & Stoughton in 1997/1998.

When acids and metal/non metal conduct electricity-

  • Metals of hydrogen are formed at the cathode.
  • Non- metals (except hydrogen are formed at the anode).

The compounds are decomposed by electrical energy and an element I produced at each electrode. This is different from conduction of electricity by which ...

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