What happens when alkali metals react with water?

When alkali metals react with water, they produce a hydroxide of the metal and hydrogen gas.

Alkali metals, found in Group 1 of the periodic table, are highly reactive due to their single electron in the outermost shell. This makes them eager to participate in chemical reactions, such as with water. When an alkali metal comes into contact with water, a displacement reaction occurs. This is a type of redox reaction where the alkali metal displaces the hydrogen in the water molecule, forming a hydroxide of the metal and releasing hydrogen gas.

The general equation for this reaction is 2M(s) + 2H2O(l) → 2MOH(aq) + H2(g), where M represents the alkali metal. The hydroxide produced is a strong base, making the solution alkaline. This is why these elements are called 'alkali' metals. The reaction is exothermic, meaning it releases heat. In fact, it's so exothermic that the hydrogen gas produced often ignites, causing a fiery explosion.

The reactivity of alkali metals increases down the group. This means that lithium, at the top of the group, reacts relatively slowly with water, while potassium and especially rubidium and caesium, further down the group, react much more violently. This is due to the increasing size of the atoms and the decreasing ionisation energy down the group, which makes it easier for the outermost electron to be lost and the metal to be oxidised.

In a laboratory setting, these reactions must be handled with care. Alkali metals are usually stored under oil to prevent them from reacting with moisture in the air. When demonstrating the reaction with water, safety measures such as using small amounts of the metal and standing well back are essential. The reaction produces a caustic solution and flammable hydrogen gas, both of which can be hazardous.

In conclusion, the reaction of alkali metals with water is a fascinating demonstration of the reactivity of these elements, and a vivid illustration of the principles of redox reactions and trends in the periodic table.