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Chapter 4: Problem 96

Gold metal will not dissolve in either concentrated nitric acid or concentrated hydrochloric acid. It will dissolve, however, in aqua regia, a mixture of the two concentrated acids. The products of the reaction are the AuCl, \(^{-}\) ion and gaseous NO. Write a balanced equation for the dissolution of gold in aqua regia.

Short Answer

Expert verified
The balanced chemical equation for the dissolution of gold in aqua regia is: \( Au (s) + 3 HNO_3 (aq) + 4 HCl (aq) \rightarrow AuCl_4^{-} (aq) + 3 NO (g) + 3 H_2O (l) \)

Step by step solution

01

Write the unbalanced equation

First, we will write the unbalanced equation representing the reaction of gold with aqua regia, which contains nitric acid and hydrochloric acid: Au (s) + HNO鈧� (aq) + HCl (aq) 鈫� AuCl鈧勨伝 (aq) + NO (g) Next, we need to balance this equation.
02

Balance the gold atoms

There is already one gold atom on both sides of the equation, so it is already balanced with respect to gold: Au (s) + HNO鈧� (aq) + HCl (aq) 鈫� AuCl鈧勨伝 (aq) + NO (g)
03

Balance the chlorine atoms

There are 4 chlorine atoms in AuCl鈧勨伝, so we need to put a coefficient of 4 in front of HCl to balance the chlorine atoms: Au (s) + HNO鈧� (aq) + 4 HCl (aq) 鈫� AuCl鈧勨伝 (aq) + NO (g)
04

Balance the nitrogen atoms

There is one nitrogen atom in the reactant side (HNO鈧�) and one in the product side (NO), so the nitrogen atoms are already balanced: Au (s) + HNO鈧� (aq) + 4 HCl (aq) 鈫� AuCl鈧勨伝 (aq) + NO (g)
05

Balance the hydrogen and oxygen atoms

From the reactants' side, we have one hydrogen atom from HNO鈧� and four from the 4 HCl molecules, giving 5 hydrogen atoms in total. There are none in the products side, so we need to add water (H鈧侽) as a product to balance the H atoms. We add 3 H鈧侽 molecules which contain a total of 6 H atoms and 3 O atoms. Also, to balance the O atoms, HNO鈧� should be changed to 3 HNO鈧� since it has 3 O atoms with 3 NO: Au (s) + 3 HNO鈧� (aq) + 4 HCl (aq) 鈫� AuCl鈧勨伝 (aq) + 3 NO (g) + 3 H鈧侽 (l) Now, we have a balanced chemical equation for the dissolution of gold in aqua regia: Au (s) + 3 HNO鈧� (aq) + 4 HCl (aq) 鈫� AuCl鈧勨伝 (aq) + 3 NO (g) + 3 H鈧侽 (l)

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Aqua Regia
Aqua regia is a fascinating chemical mixture that is renowned for its ability to dissolve gold, a metal known for its resistance to corrosion and reactions. This potent mixture consists of two strong acids: concentrated nitric acid (HNO鈧�) and concentrated hydrochloric acid (HCl). What makes aqua regia particularly unique is the combination ratio, where typically three parts of HCl are mixed with one part of HNO鈧�.
This combination results in a powerful oxidizing solution that produces an array of reactive chemical species that contribute to its reactivity. One of the critical aspects of aqua regia is its ability to generate nitrosyl chloride and chlorine gas, which are crucial in the dissolution process of metals, especially gold.
  • Aqua regia is named for its ability to "dissolve" noble metals like gold, with the name translating from Latin to "royal water."
  • This mixture is often used in laboratories and industries involving the purification and extraction of precious metals.
  • It should always be handled with extreme care due to its corrosive nature and release of toxic gases.
Gold Dissolution
Gold dissolution in aqua regia is a noteworthy chemical reaction because of gold's intrinsic resistance to both acids in their concentrated forms. This resistance is overcome when these acids are mixed, forming aqua regia, which transforms gold into soluble ions.
This chemical process involves multiple reactions where nitric acid acts as a powerful oxidizing agent. It oxidizes gold by converting it into gold ions, typically forming AuCl鈧勨伝 ions in solution.
  • Hydrochloric acid contributes chloride ions (Cl鈦�), which stabilize these gold ions, allowing gold to be solubilized.
  • The dissolving of gold is usually accompanied by the evolution of nitrogen dioxide (NO2) and nitrosyl chloride (NOCl), which are products of nitrosyl group reactions.
This ability to dissolve gold makes aqua regia essential in processes like gold refining and recycling, where it plays a critical role in extracting pure metal from its alloys and impurities.
Balancing Equations
Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is observed. In the context of gold dissolution in aqua regia, balancing is essential to accurately depict the chemical changes occurring.
When balancing an equation for the dissolution of gold, such as \[ Au (s) + HNO鈧� (aq) + 4 HCl (aq) \rightarrow AuCl_4^- (aq) + NO (g) + H_2O (l) \], it is critical to account for all atoms on both sides of the equation.
  • Start by counting atoms for each element present in reactants and products.
  • Match the number of gold, chlorine, nitrogen, hydrogen, and oxygen atoms on both sides of the equation.
  • Adjust coefficients as needed to equilibrate the numbers, maintaining the proportions of reactants and products.
At its heart, balancing ensures that all atoms are accounted for and that the equation reflects the true nature of the chemical reaction. Practice makes perfect, and mastering this skill is key for any aspiring chemist.

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Most popular questions from this chapter

What volume of 0.100\(M \mathrm{NaOH}\) is required to precipitate all of the nickel(Il) ions from 150.0 \(\mathrm{mL}\) of a \(0.249-M\) solution of \(\mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2} ?\)

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Assign the oxidation state for the element listed in each of the following compounds: \(\mathrm{S}\) in \(\mathrm{MgSO}_{4}\)_______ \(\mathrm{Pb}\) in \(\mathrm{PbSO}_{4}\)______ \(\mathrm{O}\) in \(\mathrm{O}_{2}\)___________ \(\mathrm{Ag}\) in Ag _________________________ \(\mathrm{Cu}\) in \(\mathrm{CuCl}_{2}\)_______

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You are given a 1.50-g mixture of sodium nitrate and sodium chloride. You dissolve this mixture into 100 mL of water and then add an excess of 0.500 M silver nitrate solution. You produce a white solid, which you then collect, dry, and measure. The white solid has a mass of 0.641 g. a. If you had an extremely magnified view of the solution (to the atomic- molecular level), list the species you would see (include charges, if any). b. Write the balanced net ionic equation for the reaction that produces the solid. Include phases and charges. c. Calculate the mass percent of sodium chloride in the original unknown mixture.
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