Trends in Reactivity with Water

This section explores how the Group 2 elements – beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba) – react with water (or steam). We will use these reactions to understand the trend in reactivity within the group.

Beryllium

Beryllium stands out as the least reactive member. It shows no reaction with water even at high temperatures (red hot).

Magnesium

Magnesium burns in steam to produce white magnesium oxide and hydrogen gas:
Mg_(s) + H₂O_(g) → MgO_(s) + H_2(g)
Magnesium reacts slowly with cold water, but the reaction quickly slows down due to the formation of a protective layer of magnesium hydroxide (Mg(OH)₂). This layer is insoluble and hinders further contact between the metal and water.
Mg_(s) + 2H₂O_(l) → Mg(OH)_(s) + H_2(g)

Calcium, Strontium, and Barium

These elements react vigorously with cold water, with increasing intensity as you move down the group. The reaction produces the metal hydroxide and hydrogen gas. The metal hydroxides are slightly soluble, forming a visible white precipitate in the solution.
Strontium and barium have reactivities similar to lithium in Group 1 of the Periodic Table.
Calcium, for example, reacts fairly vigorously with cold water in an exothermic reaction. Bubbles of hydrogen gas are given off, and a white precipitate of calcium hydroxide forms, together with an alkaline solution (also of calcium hydroxide – calcium hydroxide is slightly soluble).
The general equation for the reactions of these metals with water is:
X_(s) + 2H₂O_(l) → X(OH)_(aq) + H_2(g)

Reactivity with Water

This trend is reflected in their reactivity with water. Beryllium, with the smallest atomic radius, is the least reactive. As we move down the group, the increasing atomic radius leads to a greater willingness to lose electrons and react with water, resulting in progressively more vigorous reactions.

Summarizing the Trend Down the Group

As the metal atoms get bigger, any bonding pair gets further and further away from the metal nucleus and is less strongly attracted towards it. In other words, as you go down the Group, the elements become less electronegative. As you go down the Group, the bonds formed between these elements and other things such as chlorine become more and more ionic. The bonding pair is increasingly attracted away from the Group 2 element towards the chlorine (or whatever).

The hydroxides are not very soluble, but they get more soluble as you go down the Group. The calcium hydroxide formed shows up mainly as a white precipitate (although some does dissolve). You get less precipitate as you go down the Group because more of the hydroxide dissolves in the water.

Explaining the Trend in Reactivity

Beryllium Hydroxide (Be(OH)₂

Beryllium hydroxide is amphoteric, meaning it reacts with acids to form salts and with alkalis to give beryllates:
Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O
Be(OH)₂ + 2NaOH → Na₂BeO₂ + 2H₂O

Magnesium Hydroxide (Mg(OH)₂

Magnesium hydroxide is weakly basic.

Other Group 2 Hydroxides

The basic character of the hydroxides increases down the group. The increase in basic character is due to the increase in the size of the M²⁺ cation, which both decreases the polarity of the M-OH bond and increases the distance between the oxygen of the OH⁻ ion and the metal atoms. As a result, the ions are more easily liberated, leading to greater ionization of M(OH)₂ and hence increased basic character.
The smaller and more highly polarizing Be²⁺ ion results in Be(OH)₂ being covalent, whereas other hydroxides are ionic.

Solubility of Hydroxides

The solubility of the formed hydroxides also increases down the group. The larger cations (positively charged metal ions) in the hydroxides of calcium, strontium, and barium have a weaker attraction to the hydroxide (OH⁻) ions compared to the smaller beryllium and magnesium cations. This weaker attraction allows the hydroxides of the larger elements to dissolve more readily in water.
Be(OH)₂ and Mg(OH)₂ are almost insoluble in H₂O, while the hydroxides of other metals are slightly soluble.

Table: Solubility increases down the group, as shown by the increasing values of the solubility products (Ksp) of these hydroxides:

Hydroxide	Ksp value
Be(OH)₂	1.6 x 10⁻²⁶ (very insoluble)
Mg(OH)₂	8.9 x 10⁻¹² (slightly soluble)
Ca(OH)₂	1.3 x 10⁻⁴ (slightly soluble)
Sr(OH)₂	3.2 x 10⁻⁴ (slightly soluble)
Ba(OH)₂	5.4 x 10⁻² (more soluble)

Summary of the Trend in Reactivity

The Group 2 elements exhibit a clear trend in reactivity with water. Beryllium is the least reactive, while the reactivity increases significantly down the group due to the increasing atomic radius and weaker attraction between the nucleus and valence electrons. The solubility of the metal hydroxide also increases down the group due to the weaker attraction between the larger cations and hydroxide ions.