Electroplating

Electrolysis is used to electroplate objects. This is useful for coating a cheaper metal with a more expensive one, such as copper or silver.

How it works

• The negative electrode should be the object that is to be electroplated
• The positive electrode should be the metal that you want to coat the object with
• The electrolyte should be a solution of the coating metal, such as its metal nitrate or sulfate

Here are two examples.

Electroplating with silver

The object to be plated, such as a metal spoon, is connected to the negative terminal of the power supply. A piece of silver is connected to the positive terminal. The electrolyte is silver nitrate solution.

Electroplating with copper

The object to be plated, such as a metal pan, is connected to the negative terminal of the power supply. A piece of copper is connected to the positive terminal. The electrolyte is coppersulfate solution.

This arrangement can also be used to purify copper during copper manufacture. In this case, both electrodes are made from copper. The negative electrode gradually gets coated with pure copper as the positive electrode gradually disappears. The animation shows how this works:

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Making insoluble salts

Insoluble salts do not dissolve in water. They can be made by mixing appropriate solutions of ions together.

Soluble and insoluble salts

 

Soluble	Insoluble
All nitrates	None
Most sulfates	Lead sulfate, barium sulfate
Most chlorides, bromides and iodides	Silver chloride, silver bromide, silver iodide, lead chloride, lead bromide, lead iodide
Sodium carbonate, potassium carbonate	Most other carbonates
Sodium hydroxide, potassium hydroxide	Most other hydroxides

Notice that all nitrates and most chlorides are soluble. This is why many of the chemicals you use in the laboratory are nitrates or chlorides. If you want to make an insoluble salt, you can react together two soluble salts in a precipitation reaction.

Making an insoluble salt

Silver chloride is insoluble - you can see this from the table. You need a soluble silver salt and a soluble chloride salt to make it. Silver nitrate and sodium chloride are both soluble. When you mix their solutions together, you make soluble sodium nitrate and insoluble silver chloride:

• silver nitrate + sodium chloride → sodium nitrate + silver chloride
• AgNO₃(aq) + NaCl(aq) → NaNO₃(aq) +AgCl(s)

The silver chloride appears as tiny particles suspended in the reaction mixture - it forms a precipitate. The precipitate can be filtered, washed with water on the filter paper, and then dried in an oven.

Remember: if you want to make an insoluble salt XY, mixing X nitrate with sodium Y will always work. In the example above, X is silver and Y is chloride.

Using precipitation reactions

Precipitation reactions can be used to remove unwanted ions in solution. This is useful for treating drinking water and waste water.

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Electrolysis

Electrolysis is the process by which ionic substances are broken down into simpler substances using electricity. During electrolysis, metals and gases may form at the electrodes.

What is electrolysis?

Ionic substances contain charged particles called ions. For example, lead bromide contains positively charged lead ions and negatively charged bromide ions.

Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.

For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or when melted. For example, if electricity is passed through molten lead bromide, the lead bromide is broken down to form lead and bromine.

Electrolysis

Here is what happens during electrolysis:

• Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced.
• Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised.

The substance that is broken down is called the electrolyte.

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