AS Chemistry 9701: Sulfate Solubility and Reactions with Dilute Sulfuric Acid

Excess powdered magnesium and excess powdered barium are added to separate test tubes, each containing dilute sulfuric acid.

(a) Write a balanced symbol equation, including state symbols, for the reaction of magnesium with dilute sulfuric acid. [2]

(b) The magnesium reacts steadily, with bubbles of gas produced continuously until all the magnesium has dissolved. The barium, however, reacts quickly for only a few seconds before the reaction becomes very slow and appears almost to stop, even though a large excess of barium metal is still present. Using the solubility trend of the Group 2 sulfates, suggest an explanation for this difference. [2]

Part (a): Magnesium with dilute sulfuric acid

Magnesium is a reactive Group 2 metal and reacts with dilute sulfuric acid in a simple metal–acid reaction, releasing hydrogen gas:

$\text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)}$

The equation is already balanced: one magnesium atom, one sulfate group and two hydrogens appear on each side.

Part (b): Why the barium reaction becomes self-limiting

Barium also reacts with dilute sulfuric acid in the same general way, forming barium sulfate and hydrogen gas. However, $\text{BaSO}_4$ is essentially insoluble in water. This is well below $\text{MgSO}_4$ in the Group 2 sulfate solubility trend. As soon as barium starts reacting, a thin layer of solid $\text{BaSO}_4\text{(s)}$ forms directly on the surface of the metal. Because this layer does not dissolve away, it physically coats the remaining unreacted barium, preventing fresh acid from reaching the metal underneath. The reaction therefore becomes very slow (essentially stops), even though plenty of barium metal and dilute sulfuric acid are still present.

$\text{MgSO}_4$ , in contrast, is soluble, so it dissolves away into solution as fast as it forms, constantly exposing fresh magnesium surface to the acid, which is why the magnesium reaction continues steadily until the metal is completely used up.

Part (c): The solubility trend of the Group 2 sulfates

Down Group 2, the sulfates become progressively less soluble: $\text{MgSO}_4$ is freely soluble in water, $\text{CaSO}_4$ is only sparingly soluble, $\text{SrSO}_4$ is even less soluble, and $\text{BaSO}_4$ is essentially insoluble. (This is the opposite direction to the Group 2 hydroxide solubility trend, which increases down the group. A common point of confusion.)

Final answers

(a) $\text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)}$
(b) The insoluble $\text{BaSO}_4$ formed coats the barium metal, blocking further contact with the acid, so the reaction becomes very slow/stops; the soluble $\text{MgSO}_4$ formed with magnesium does not coat the metal, so that reaction continues to completion.
(c) Sulfate solubility decreases down the group, from soluble $\text{MgSO}_4$ to essentially insoluble $\text{BaSO}_4$ .