The alkali metals react with water to produce a metal hydroxide and hydrogen. For example, sodium reacts with water:
Sodium + water → sodium hydroxide + hydrogen
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Hydrogen gas is also produced. With very reactive metals such as potassium, the energy of the reaction causes the hydrogen to burn as the reaction takes place.
Write the word equation and balanced equation for the reaction of potassium with water.
Potassium + water → potassium hydroxide + hydrogen
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
The table shows observations when lithium, sodium and potassium are added to water. Notice that the reactivity of these metals increases going down the group. This pattern is seen with all reactions of group 1 elements. For example, the reaction of caesium with chlorine is more vigorous than the reaction of potassium with chlorine.
|Lithium, Li||Fizzes steadily, slowly becomes smaller until it disappears|
|Sodium, Na||Melts to form a ball, fizzes rapidly, quickly becomes smaller until it disappears|
|Potassium, K||Quickly melts to form a ball, burns violently with sparks and a lilac flame, disappears rapidly, often with a small explosion|
Rubidium is placed below potassium in group 1. Predict what is seen when rubidium is added to water.
Rubidium should melt very quickly. It should burn very violently, and disappear almost instantly with an explosion.
When an element in group 1 takes part in a reaction, its atoms lose their outer electron and form positively charged ions, called cations. The more easily these cations form, the more reactive the metal.
The reactivity of group 1 elements increases as you go down the group because: