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Chapter 19: Problem 93

Gold will not dissolve in either concentrated nitric acid or concentrated hydrochloric acid. However, the metal does dissolve in a mixture of the acids (one part \(\mathrm{HNO}_{3}\) and three parts \(\mathrm{HCl}\) by volume ), called aqua regia. (a) Write a balanced equation for this reaction. (Hint: Among the products are \(\mathrm{HAuCl}_{4}\) and \(\mathrm{NO}_{2} .\) ) (b) What is the function of \(\mathrm{HCl} ?\)

Short Answer

Expert verified
The balanced reaction equation is \(\text{Au + 4 HNO}_3 + \mathrm{4HCl} \rightarrow \mathrm{HAuCl}_4 + \text{4NO}_2 + 2H{_2}O\). The function of \(\mathrm{HCl}\) in this reaction is to help form \(\mathrm{HAuCl}_{4}\), stabilizing the gold in solution.

Step by step solution

01

Identify the Products

Initially, it's important to list out the reactants and products. By the problem's hint, we know that \(\mathrm{HAuCl}_{4}\) and \(\mathrm{NO}_{2}\) are products of the reaction. The presence of nitric and hydrochloric acid, along with gold, signifies them as the reactants.
02

Write the Unbalanced Reaction

It's the combination of gold (Au), nitric acid (\(\mathrm{HNO}_{3}\)) and hydrochloric acid (\(\mathrm{HCl}\)) that forms \(\mathrm{HAuCl}_{4}\) and \(\mathrm{NO}_{2}\): \[ \text{Au + HNO}_3 + \mathrm{HCl} \rightarrow \mathrm{HAuCl}_4 + \text{NO}_2 \] Important to remember that this isn't balanced yet.
03

Balance the equation

For the balance equation, count the atoms of each element on both the reaction and product sides, and add coefficients where necessary to make them equal. The balanced equation is: \[\text{Au + 4 HNO}_3 + \mathrm{4HCl} \rightarrow \mathrm{HAuCl}_4 + \text{4NO}_2 + 2H{_2}O\] This equation tells us that it takes one atom of gold reacted with four molecules each of \(\mathrm{HNO}_3\) and \(\mathrm{HCl}\) to produce one molecule of \(\mathrm{HAuCl}_4\), four molecules of \(\mathrm{NO}_2\), and two molecules of \(\mathrm{H}_2\mathrm{O}\).
04

Function of \(\mathrm{HCl}\)

The function of \(\mathrm{HCl}\) in this reaction is to form the complex \(\mathrm{HAuCl}_{4}\), which stabilizes the gold in solution. Without the chlorides provided by the \(\mathrm{HCl}\), the gold would be reduced back to its metallic state by the \(\mathrm{HNO}_{3}\) and would precipitate out of solution.

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