Group 2: Question 1

Worked solution

Why reactivity with water increases down the group

Going down Group 2 from magnesium to barium, the atomic radius and the ionic radius both increase, and the first and second ionisation energies decrease (the outer $ns^2$ electrons are further from the nucleus and more shielded, so they are easier to remove). This makes the atoms more willing to lose their two outer electrons to a water molecule, so both the rate of reaction and the vigour of the reaction (bubbling, heat given out) increase down the group.

What is actually observed for each metal

• Magnesium reacts only extremely slowly with cold water: a thin, barely-visible layer of $\text{Mg(OH)}_2$ forms on the surface, which is only sparingly soluble and further slows the reaction. (Magnesium does, however, react vigorously with steam, forming $\text{MgO}$ and $\text{H}_2$.)
• Calcium reacts steadily with cold water, giving a visible stream of bubbles and a cloudy suspension of the slightly soluble $\text{Ca(OH)}_2$.
• Strontium reacts noticeably faster and more vigorously than calcium.
• Barium reacts vigorously with cold water, giving rapid effervescence; $\text{Ba(OH)}_2$ is soluble enough to give a fairly concentrated, strongly alkaline solution.

$\text{M(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{M(OH)}_2 + \text{H}_2\text{(g)} \quad (\text{M} = \text{Ca, Sr, Ba})$

So the rate and vigour of reaction with cold water increase down the group, from magnesium (slowest) to barium (fastest).

Why the other options are wrong

• B reverses the actual trend.
• C is wrong on both counts: magnesium is the least reactive of the four with cold water, and barium does react with it, vigorously.
• D ignores the systematic increase in reactivity that comes from increasing atomic/ionic radius and decreasing ionisation energies down the group; members of the same group do not automatically react at identical rates.

Final answer

A. Reactivity (rate and vigour) with cold water increases from magnesium to barium.