I need to know this chemical reaction! please help me chemistry geniuses!!?

Question

when gold is given for polishing, 3 parts of nitric acid and a part of Hcl is used for polishing. after it's polished, a bit of the weight remains in the solution. i need to know the chemical reaction for this as it's a question in my book.

Answer 1

Here's the stuff I got from Wikipedia on aqua regia:

Aqua regia dissolves gold, even though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au3+). The hydrochloric acid provides a ready supply of chloride ions (Cl-), which react with the gold to produce chloraurate anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which favors formation of chloraurate anions (AuCl4-). This results in a removal of gold ions from solution and allows further oxidation of gold to take place, and so the gold is dissolved. In addition, gold may be oxidized by the free chlorine present in aqua regia. Appropriate equations are

    Au (s) + 3 NO3- (aq) + 6 H+ (aq) → Au3+ (aq) + 3 NO2 (g) + 3 H2O (l) and

    Au3+ (aq) + 4 Cl- (aq) → AuCl4- (aq).

The oxidation reaction can also be written with nitric oxide as the product rather than nitrogen dioxide:

    Au (s) + NO3- (aq) + 4 H+ (aq) → Au3+ (aq) + NO (g) + 2 H2O (l).

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Selena comments: Technically, though, the stuff you described (3 parts nitric acid and 1 part HCl) is not aqua regia (which is 1 part nitric and 3 parts HCl), but it comes close. The reason is: true aqua regia will, much to everyone's panic, dissolve most of the gold.

Since the liquid described in the problem is only for cleaning, it comes in a different proportion and is much weaker.

Answer 2

No idea. It certainly isn't gold, as that doesn't react.

Answer 3

Au + 4H+ NO3(-) → Au(3+) + NO + 2H2O

Au(3+) + 4Cl- → AuCl4(-)

The nitric acid dissolves a tiny bit of gold, and the chloride complexes the dissolved gold.