Magnesium carbonate, magnesium oxide, and magnesium hydroxide are all white solids that react with acidic solutions.
(a) Write a balanced molecular equation and a net ionic equation for the reaction that occurs when each substance reacts with a hydrochloric acid solution.
(b) By observing the reactions in part (a) could you distinguish any of the three magnesium substances from the other two? If so how?
(c) If excess HCl(aq) is added, would the clear solutions left behind after the reaction is complete contain the same or different ions in each case?
The correct answer and explanation is :
(a) Balanced Molecular and Net Ionic Equations
- Magnesium carbonate (MgCO₃) with HCl:
Molecular Equation:
[
MgCO₃ (s) + 2HCl (aq) \rightarrow MgCl₂ (aq) + CO₂ (g) + H₂O (l)
]
Net Ionic Equation:
[
CO₃^{2-} (s) + 2H⁺ (aq) \rightarrow CO₂ (g) + H₂O (l)
] - Magnesium oxide (MgO) with HCl:
Molecular Equation:
[
MgO (s) + 2HCl (aq) \rightarrow MgCl₂ (aq) + H₂O (l)
]
Net Ionic Equation:
[
O^{2-} (s) + 2H⁺ (aq) \rightarrow H₂O (l)
] - Magnesium hydroxide (Mg(OH)₂) with HCl:
Molecular Equation:
[
Mg(OH)₂ (s) + 2HCl (aq) \rightarrow MgCl₂ (aq) + 2H₂O (l)
]
Net Ionic Equation:
[
OH⁻ (s) + H⁺ (aq) \rightarrow H₂O (l)
]
(b) Distinguishing the Three Magnesium Compounds
Yes, these substances can be distinguished based on their reaction observations:
- Magnesium carbonate (MgCO₃): Produces bubbles of CO₂ gas (effervescence) when reacting with HCl, which is a visible indicator.
- Magnesium oxide (MgO): Reacts with HCl but does not produce gas, just dissolves while forming a clear solution.
- Magnesium hydroxide (Mg(OH)₂): Reacts similarly to MgO but dissolves more slowly since it is only sparingly soluble in water. No gas formation occurs.
Thus, MgCO₃ can be distinguished from the other two due to gas evolution. MgO and Mg(OH)₂ could be harder to differentiate without additional tests, though Mg(OH)₂ may be distinguished by its slower dissolution in acid.
(c) Ionic Composition of Final Solution
After the reaction, the solution will contain the same ions in all cases. The excess HCl will react with all solid reactants, forming magnesium chloride (MgCl₂) and water in each case. Since MgCl₂ is a soluble salt, the final solution will contain Mg²⁺ and Cl⁻ ions regardless of which solid was initially used. Therefore, the solution will be identical in ionic composition across all three cases.