Mercury(II) hydroxide, Hg(OH)2, is so insoluble in water that only about 2 formula units per billion dissolve. It does, however, neutralize hydrochloric acid. Explain, with appropriate net-ionic equations, how the methods in this experiment can be used to obtain the enthalpy of solution for Hg(OH)2 even when direct measurements of that process is not possible.
The Correct Answer and Explanation is :
To determine the enthalpy of solution ((\Delta H_{\text{sol}})) of mercury(II) hydroxide ((\text{Hg(OH)}_2)) indirectly, we can use a thermochemical cycle based on Hess’s Law. Since direct measurement is impractical due to its extremely low solubility, we can utilize its neutralization with hydrochloric acid ((\text{HCl})) and other measurable reactions.
Relevant Net Ionic Equations:
- Dissolution of Mercury(II) Hydroxide (Hypothetical Process)
[
\text{Hg(OH)}2 (s) \rightleftharpoons \text{Hg}^{2+} (aq) + 2\text{OH}^- (aq) ] This process has an unknown enthalpy change ((\Delta H{\text{sol}})). - Neutralization of Hydroxide Ions by Hydrochloric Acid
[
\text{OH}^- (aq) + \text{H}^+ (aq) \rightarrow \text{H}2\text{O} (l) \quad (\Delta H{\text{neutralization}})
]
This reaction is well known to have an enthalpy change of approximately (-55.8 \text{ kJ/mol}). - Overall Reaction of Mercury(II) Hydroxide with Hydrochloric Acid
[
\text{Hg(OH)}2 (s) + 2\text{H}^+ (aq) \rightarrow \text{Hg}^{2+} (aq) + 2\text{H}_2\text{O} (l) ] The enthalpy change of this reaction ((\Delta H{\text{rxn}})) can be measured calorimetrically.
Methodology and Explanation:
- The experiment involves measuring the enthalpy change when a known amount of (\text{Hg(OH)}_2) is reacted with an excess of hydrochloric acid.
- The total heat released ((\Delta H_{\text{rxn}})) corresponds to the combination of the dissolution process and the neutralization of hydroxide ions.
- Since (\Delta H_{\text{neutralization}}) is known, we can use Hess’s Law: [
\Delta H_{\text{sol}} + 2\Delta H_{\text{neutralization}} = \Delta H_{\text{rxn}}
] Solving for (\Delta H_{\text{sol}}): [
\Delta H_{\text{sol}} = \Delta H_{\text{rxn}} – 2\Delta H_{\text{neutralization}}
] - This allows us to determine (\Delta H_{\text{sol}}) for mercury(II) hydroxide indirectly.
Conclusion:
By using Hess’s Law and calorimetric measurements of the reaction between (\text{Hg(OH)}_2) and hydrochloric acid, we can bypass direct dissolution measurement and determine the enthalpy of solution accurately.
Now, I’ll generate an image illustrating this thermochemical cycle.
I’ve generated an image illustrating the thermochemical cycle for determining the enthalpy of solution ((\Delta H_{\text{sol}})) of mercury(II) hydroxide. It includes dissolution, neutralization, and the overall reaction with Hess’s Law application. Let me know if you need any modifications!
