The chlor-alkali industry

If an electric current is passed through concentrated sodium chloride solution, hydrogen gas forms at the negative electrode and chlorine gas forms at the positive electrode. A solution of sodium hydroxide forms.

You might have expected sodium metal to be deposited at the negative electrode. But sodium is too reactive for this to happen, so hydrogen is given off instead.

During electrolysis:

• Hydrogen ions H⁺ (from the water) are discharged at the negative electrode as hydrogen gas, H₂
• Chloride ions Cl^– are discharged at the positive electrode as chlorine gas, Cl₂
• Sodium ions Na⁺ and hydroxide ions OH^– (from the water) stay behind - they form sodium hydroxide solution, NaOH

Electrolysis of sodium chloride solution

The three products of electrolysis

The three products of the electrolysis of concentrated sodium chloride solution have important uses in the chemical industry:

• Hydrogen is used as a fuel and for making ammonia
• Chlorine is used to kill bacteria in water, and to make bleach and plastics
• Sodium hydroxide is used to make soap and bleach

Now try a Test Bite.

Read on if you're taking the higher paper.

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Predicting the products of electrolysis

Ionic substances in solution break down into elements during electrolysis. Different elements are released depending on the particular ionic substance.

At the negative electrode

At the negative electrode, positively charged ions gain electrons. This is reduction, and you say that the ions have been reduced.

Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series:

• The metal will be produced if it is less reactive than hydrogen
• Hydrogen will be produced if the metal is more reactive than hydrogen

The reactivity series of metal - carbon and hydrogen are not metals, but they are shown for comparison

So the electrolysis of copper chloride solution produces copper at the negative electrode. But the electrolysis of sodium chloride solution produces hydrogen.

At the positive electrode

At the positive electrode, negatively charged ions lose electrons. This is oxidation, and you say that the ions have been oxidised. The table summarises some of the elements you should expect to get during electrolysis.

 

Negative ion in solution	Element given off at positive electrode
Chloride, Cl–	Chlorine, Cl2
Bromide, Br–	Bromine, Br2
Iodide, I–	Iodine, I2
Sulfate, SO42–	Oxygen, O2

Putting it together

The table shows some common ionic compounds, and the elements released when their solutions are electrolysed.

 

Ionic substance in solution	Element at the negative electrode	Element at the positive electrode
Copper chloride, CuCl2	Copper	Chlorine
Copper sulfate, CuSO4	Copper	Oxygen
Sodium chloride, NaCl	Hydrogen	Chlorine
Hydrochloric acid, HCl	Hydrogen	Chlorine
Sulfuric acid, H2SO4	Hydrogen	Oxygen

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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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