Chemguide: Support for CIE A level Chemistry

Learning outcome 10.1

Group 2

Similarities and trends in the properties of the Group 2 metals (magnesium to barium) and their compounds

Learning outcome 10.1.2

This statement wants you to be able to describe the behaviour of the Group 2 oxides, hydroxides and carbonates with water and with dilute hydrochloric and sulfuric acids.

Before you go on, you should find and read the statement in your copy of the syllabus.

The oxides

Reaction of the oxides with water

All of the oxides from magnesium oxide to barium oxide react with water, at least to some extent, to form hydroxides.

Magnesium oxide

If you shake some white magnesium oxide powder with water, nothing seems to happen - it doesn't look as if it reacts. However, if you test the pH of the liquid, you find that it is somewhere around pH 9 - showing that it is slightly alkaline.

There must have been some slight reaction with the water to produce hydroxide ions in solution. Some magnesium hydroxide is formed in the reaction, but this is almost insoluble - and so not many hydroxide ions actually get into solution.

Calcium oxide

If you add water to calcium oxide ("quicklime"), there is a very exothermic reaction to produce calcium hydroxide ("slaked lime"). The calcium oxide swells up and a lot of steam is produced as the heat boils the water.

If you add enough water (or use a very small amount of calcium oxide), a colourless solution of calcium hydroxide is formed with a pH around 12. The solution is often called "lime water".

The pH isn't any higher because calcium hydroxide is only slightly soluble. A saturated solution of calcium hydroxide is only about 0.015 mol dm-3 at room temperature.

Strontium and barium oxides

These will be similar to calcium oxide, except that the hydroxides are rather more soluble, and so the pH of the solutions formed increase slightly - to about 13.5 for barium hydroxide.

The equations are just like the magnesium oxide or calcium hydroxide ones.

Reaction of the oxides with dilute acids

These are all basic oxides, reacting with acids to give a salt and water.

They will react with dilute hydrochloric to give colourless solutions of the chloride, and with dilute sulfuric acid to give colourless solutions of the sulfate.

With dilute sulfuric acid, the situation is more confusing because calcium sulfate is only sparingly soluble, and strontium or barium sulfates are virtually insoluble. The precipitates formed will coat the oxides, and the reactions will stop. It would be impossible to make, say, barium sulfate like this - you couldn't separate the unreacted barium oxide from the barium sulfate coating.

Sample equations:

The hydroxides

Reaction of the hydroxides with water

The hydroxides don't react with water. None of them are very soluble, but the solubility increases as you go down the Group. See the beginning of the page about the solubility of the hydroxides (etc).

Reaction of the hydroxides with dilute acids

The hydroxides behave in exactly the same way as the oxides - with the same problem of the only slight solubility of calcium sulfate and the insolubility of strontium and barium sulfates.

Sample equations:

The carbonates

Reaction of the carbonates with water

The carbonates don't react with water. All of them are virtually insoluble in water. You will find a bit more detail on the page about the solubility of the hydroxides (etc).

Reaction of the carbonates with dilute acids

Carbonates react with acids to give a salt, carbon dioxide and water.

All of these carbonates will react with dilute hydrochloric acid to give a colourless solution of the chloride and carbon dioxide gas.

For example:

But the same problems exist as before with sulfuric acid. If you add dilute sulfuric acid to a marble chip (calcium carbonate), you will get a few bubbles of carbon dioxide for a very short time, and then the reaction stops. The marble chip becomes coated with insoluble calcium sulfate, so stopping the reaction.

The only Group 2 carbonate to react properly with dilute sulfuric acid is magnesium carbonate, because magnesium sulfate is soluble.

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© Jim Clark 2020