Reactions of Group 2 (AQA A-Level Chemistry): Revision Notes

2.2.3 Reactions of Group 2

Group 2 elements ($Mg$, $Ca$, $Sr$, $Ba$), also known as the alkaline earth metals, undergo reactions with water that follow a clear trend in reactivity as you move down the group. These reactions are important in understanding the redox behaviour of these metals and have practical uses, such as in industrial processes like the extraction of titanium. Let's explore the reactions and underlying principles.

General Reaction with Water

The general reaction of Group 2 metals with water produces a metal hydroxide and hydrogen gas. The metal is oxidised from an oxidation state of 0 to +2 during the reaction.

$$\text{M(s)} + 2H_2O(l) \rightarrow M(OH)_2(aq) + H_2(g)$$

Here, M represents any Group 2 metal. The hydroxides produced are alkaline, which is why these metals are termed "alkaline earth metals."

Reactivity Trend Down the Group

The reactivity of Group 2 metals with water increases as you move down the group.

This increase is explained by the decreasing ionisation energy due to additional electron shells, greater shielding, and weaker nuclear attraction, making it easier for the metal to lose its outer electrons.

Magnesium ($Mg$)

Reacts very slowly with cold water, but more vigorously with steam, forming magnesium oxide instead of hydroxide due to thermal decomposition at higher temperatures.

$$\text{Mg(s)} + H_2O(g) \rightarrow MgO(s) + H_2(g)$$

Calcium ($Ca$)

Reacts with cold water in an exothermic reaction. You will observe fizzing and the formation of calcium hydroxide, which is slightly soluble, producing a white precipitate.

$$\text{Ca(s)} + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + H_2(g)$$

Strontium ($Sr$)

Reacts more vigorously with cold water compared to calcium, producing strontium hydroxide and hydrogen gas in a highly exothermic reaction.

$$\text{Sr(s)} + 2H_2O(l) \rightarrow Sr(OH)_2(aq) + H_2(g)$$

Barium ($Ba$)

The most reactive of the Group 2 metals with water. It reacts violently with cold water, forming barium hydroxide and hydrogen gas.

$$\text{Ba(s)} + 2H_2O(l) \rightarrow Ba(OH)_2(aq) + H_2(g)$$

Explaining the Trend in Reactivity

As we move down Group 2, several factors explain the increase in reactivity:

• Increased shielding: Each successive element has more electron shells, which increases the distance between the nucleus and the outer electrons, reducing the effective nuclear charge felt by the outermost electrons.
• Weaker nuclear attraction: The greater shielding effect weakens the attraction between the nucleus and the outermost electrons.
• Easier electron loss: With less attraction between the nucleus and the outer electrons, these metals lose their outer electrons more easily, making them more reactive. Thus, as you move down the group, the metal becomes more reactive because it is easier to lose the two outermost electrons required to form a divalent cation ($M^{2+}$).