Na & Mg with water (AQA A-Level Chemistry): Revision Notes

6.1.1 Na & Mg with water

Sodium and Magnesium Reactivity

Sodium ($Na$) is an alkali metal from Group 1, while magnesium ($Mg$) is an alkaline earth metal from Group 2 of the periodic table.

The key difference in their reactions with water is due to the difference in their electron configurations and reactivity levels.

• Sodium needs to lose one electron to achieve a stable electron configuration, whereas magnesium needs to lose two. This difference affects their reactivity, with sodium being more reactive than magnesium because it requires less energy to lose a single electron compared to the two needed for magnesium.

Sodium's Reaction with Cold Water

Sodium reacts vigorously with cold water, forming a molten ball that skims along the surface, accompanied by vigorous fizzing due to hydrogen gas production.

Reaction:

$$2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2$$

• Product: Sodium hydroxide ($\text{NaOH}$) dissolves in water to create a strongly alkaline solution.
• Observation: The solution will have a high pH due to the formation of $\text{NaOH}$, a strong base, and the release of hydrogen gas, which may be observed as bubbles.

Magnesium's Reaction with Water

Magnesium reacts differently with water depending on its state (cold, warm, or steam).

Reaction with Cold Water

Magnesium reacts very slowly with cold water, forming a small amount of magnesium hydroxide and hydrogen gas.

Reaction:

$$\text{Mg} + 2\text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 + \text{H}_2$$

• Product: Magnesium hydroxide ($\text{Mg(OH)}_2$), which is only sparingly soluble in water, producing a weakly alkaline solution with a lower pH than sodium hydroxide.
• Observation: Reaction is slow, and the solution is only weakly alkaline due to the limited solubility of $\text{Mg(OH)}_2$

Reaction with Steam

Magnesium reacts vigorously with steam, as the added energy from steam provides enough activation energy to overcome the additional energy needed for magnesium to lose two electrons.

Reaction:

$$\text{Mg} + \text{H}_2\text{O (g)} \rightarrow \text{MgO} + \text{H}_2$$

• Product: Magnesium oxide ($\text{MgO}$), a white solid, and hydrogen gas.
• Observation: The reaction is vigorous, and magnesium burns with a bright white flame, indicative of high energy release.