Notice, from the table, that
- the speed of reaction increases down the group
- the solubility of the alkali produced increases down the group. (The actual variation is shown as a graph below).
Why does the solubility of the hydroxides produced increase down the group?
All of the group 2 elements will produce 2+ ions (as they all have two electrons in their outer shells that they can lose easily).
However the smaller metal ions (for example beryllium, Be2+ and magnesium Mg2+) have high charge densities since the 2+ charge is spread over a small area.
This high charge density means that the small ions are able to strongly attract the hydroxide ions, OH- , and hold them in a strong lattice structure which water molecules find very difficult to break down. Hence magnesium hydroxide tends to dissolve poorly in solution.
Strontium and Barium ions (Sr2+ and Ba2+) respectively are much larger, The 2+ charge is spread over a much greater area (these ions are said to have a much lower charge density). This will mean that the ions will have less attraction for the hydroxide ions and the much weaker structure is able to break down in water.
For example, barium hydroxide dissolves readily in water.
The solubility of the Group II Sulphates.
All of the group II elements will form sulphates.
The solubility of these sulphates decreases as the group is decended. Notice that this is the opposite of the trend that we saw for the solubilities of the group II hydroxides.
Magnesium sulphate, MgSO4 is very soluble in water. In fact ,magnesium sulphate is the main component of Epsom salts a water soluble laxative!.
On the other hand barium sulphate, BaSO4 is insoluble in water. This fact is used in an important qualitative test
What is a qualitative test?
A qualitative test (as opposed to a quantitative test ) in one that simply tell the observer whether or not a particular component is present. It does not tell us how much is present.
For example : you may have tested for the presence of acids in solution using either litmus paper or litmus solution. We know if there is acid present because the litmus would give a distinctive red colour. However this test does ot tll us how much acid is present.
Test for sulphate ions.
We can test for the presence of sulphate ions in solution using barium chloride solution. Barium chloride solution would contain both barium ions, Ba2+ and chloride ions, Cl-. The free barium ions would combine with any sulphate in the test solution and would immediately form barium sulphate which (due to its low solubility) would ‘crash out’ of solution as a white precipitate.
Ba2+(aq) + SO42-(aq) ➔ BaSO4(s)
This test is a useful analytical tool.
In the 1970’s a tanker accidentally unloaded its entire supply of aluminium sulphate into a reservoir (aluminium sulphate is a chemical used in low amounts in water treatment works) near Camelford, Southern England.
Unfortunately, this proved a particular problem since aluminium sulphate is poisonous!!
Various measures were used to attempt to remove the aluminium sulphate from the water supply and testing with barium chloride solution was be one effective method of determining whether the extraction methods had been successful.
In addition, barium sulphate is given to patients as a ‘barium meal’. The chemical is bulky and helps to show up the alimentary canal during X-ray examination. Barium sulphate is poisonous but the barium meal is relatively harmless as the insoluble barium sulphate is unable to cross the walls of the alimentary canal (as other water soluble materials would) and so never enters the system!
The atypical nature of beryllium.
Although beryllium is a group II metal and as such has two electrons in its outermost electron shell it still behaves rather differently from the other members of the group.
For example :
- Beryllium tends to form compounds that are largely covalent in character, while the chemistry of the other group two metal’s compounds are essentially ionic.
- This leads to the point that beryllium chloride dissolves in organic solvents (covalent solvents – like dissolves like)
- Beryllium chlorides is also a poor conductor of electricity when molten
- The other group II metal hydroxides are basic in character as they dissolve in acid :
Mg(OH)2(s) + 2H+(aq) + 4H2O(l) ➔ [Mg(H2O)6]2+(aq)
ACID
Beryllium hydroxide is amphoteric, which means that it is capable of dissolving in both acid and alkali.
Be(OH)2(s) + 2H+(aq) + 2H2O(l) ➔ [Be(H2O)4]2+(aq)
Be(OH)2(aq) + 2OH-9aq) ➔ [Be(OH)4]2-(aq)
OPEN QUESTION!!!
Can you explain why :
Be(OH)2 is not soluable in water while [Be(H2O)4]2+ is?
Why magnesium ions for [Mg(H2O)6]2+ (i.e six water molecules surrounding one magnesium ion) while beryllium forms only [Be(OH)4]2+ 9i.e. only four water molecules around the central beryllium ion)??
