AS Chemistry 9701: Reactions of Sodium and Magnesium with Water and Steam

A student reacts small samples of sodium and magnesium separately with water under different conditions.

(a) Sodium reacts vigorously with cold water. Construct the balanced symbol equation, including state symbols, for this reaction. [2]

(b) When a piece of magnesium ribbon is placed in cold water, only a very slow reaction is observed, with a few small bubbles of gas forming after several minutes. State the name of this gas and construct the balanced symbol equation, including state symbols, for the slow reaction between magnesium and cold water. [2]

(c) When magnesium ribbon is instead heated so that it burns in a stream of steam, it reacts vigorously to form a white solid and the same gas as in (b). Construct the balanced symbol equation, including state symbols, for this reaction with steam. [2]

(d) Suggest why sodium reacts vigorously with cold water at room temperature, whereas magnesium reacts only very slowly with cold water under the same conditions. [2]

Part (a): Sodium with cold water

Sodium reacts with cold water to form sodium hydroxide solution and hydrogen gas: $2\text{Na(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)}$

Checking the balance: $2$ Na atoms, $4$ H atoms ( $2\times2$ on the left; $2$ in $2\text{NaOH}$ $+ 2$ in $\text{H}_2$ on the right) and $2$ O atoms on each side.

Part (b): Magnesium with cold water

The gas produced is hydrogen. Only a very slow reaction occurs at room temperature, forming a thin layer of magnesium hydroxide on the metal surface: $\text{Mg(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(s)} + \text{H}_2\text{(g)}$

Checking the balance: $1$ Mg atom, $4$ H atoms ( $2\times2$ on the left; $2$ in $\text{Mg(OH)}_2$ $+ 2$ in $\text{H}_2$ on the right) and $2$ O atoms on each side.

Part (c): Magnesium with steam

When magnesium is heated in steam, it burns vigorously. The product is magnesium oxide, not the hydroxide, because the water is present as a gas rather than as liquid water: $\text{Mg(s)} + \text{H}_2\text{O(g)} \rightarrow \text{MgO(s)} + \text{H}_2\text{(g)}$

Checking the balance: $1$ Mg atom, $1$ O atom and $2$ H atoms on each side.

Part (d): Why the reactivities differ

Sodium’s outer $3s^1$ electron is further from the nucleus and screened by the same inner shells that magnesium has, but magnesium’s outer electrons additionally experience a greater nuclear charge (12 protons rather than 11) pulling on a smaller atomic radius. This means magnesium’s outer electrons are held considerably more tightly than sodium’s single outer electron.

As a result, sodium loses its outer electron to water almost immediately, releasing enough energy to sustain a vigorous, self-propagating reaction at room temperature. Magnesium’s more tightly held electrons mean the reaction with cold water has a much higher activation energy, so at room temperature only a very slow reaction is observed; supplying additional energy (by heating magnesium in steam) allows the reaction to proceed rapidly.

Final answers

(a) $2\text{Na(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)}$
(b) Hydrogen gas; $\text{Mg(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(s)} + \text{H}_2\text{(g)}$
(c) $\text{Mg(s)} + \text{H}_2\text{O(g)} \rightarrow \text{MgO(s)} + \text{H}_2\text{(g)}$
(d) Sodium’s outer electron is more loosely held than magnesium’s (smaller nuclear charge, less tightly bound electron), so sodium reacts almost instantly with cold water while magnesium’s reaction has a much higher activation energy and proceeds only very slowly at room temperature.