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The trends in reactivity are not as clear in Group 2 as they are in Group 1. However, the reactivity does increase down the group. This can be obscured by a number of factors, including the formation of oxide layers on the surface of the reacting metal and the different melting points of the metals. Clearly the reaction of magnesium with cold water was much less vigorous than that of calcium, strontium or barium.
All the group 2 metals react on contact with the air and become tarnished. However, the oxide coating which forms limits further reaction. The magnesium oxide coating is quite tough and protects the underlying metal, and gives the impression of a not particularly reactive metal. When it is put in the Bunsen flame the metal melts and a vigorous reaction takes place. Bottles of calcium metal have a fairly short shelf life as most of the metal is converted to oxide when in contact with the air. Strontium and barium metals are kept under liquid paraffin (oil). The change in reactivity down the group is not particularly clear on observation.
The reaction with oxygen produces metal oxides such as magnesium oxide:
|
2Mg(s) + O2(g) ⇒ 2MgO(s) |
With water, metal hydroxides are produced and these make the water alkaline, as was shown by the blue colour of the full range indicator. We shall see in the next experiment that magnesium hydroxide does make the water alkaline. However, the reaction of magnesium with cold water is so slow that insufficient hydroxide ions were present in solution to have an effect on the indicator. Hydrogen gas was released in each case. The reaction is:
|
Mg(s) + 2H2O(l) ⇒ Mg(OH)2(aq) + H2(g) |
We also saw the much more vigorous reaction of magnesium with steam. At the temperature of the reaction any hydroxide decomposes to give the oxide, so the equation is:
|
Mg(s) + H2O(l) ⇒ MgO(s) + H2(g) |
The reason for the increase in reactivity as we descend the Group is the size of the atom involved. The Group 2 metals react by losing electrons to form positive ions. It takes energy to remove these electrons from the pull of the oppositely charged nucleus. As we go down the Group the atoms become larger, and the outer electrons are further away from the pull of the nucleus. This means they form ions more readily, and so react faster.