Gold metal will not dissolve in either concentrated nitric acid (HNO 3 ) or conc

Question

Gold metal will not dissolve in either concentrated nitric acid (HNO3) or concentrated hydrochloric acid (HCl). It will dissolve, however, in aqua regia, a mixture of the two concentrated acids. The products of the reaction are the AuCl4 ion and gaseous NO. The two half-reactions are as follows:

As you can see, this is a non-spontaneous process with standard conditions (T = 25 ºC and all concentrations at 1.0 M).

Aqua regia is known to work as long as the concentration of hydrochloric acid is greater than that for nitric acid (in fact it is generally considered best when [HCl] is 3[HNO3]). However, at what ratio of HCl:HNO3 will this solution begin spontaneously dissolving gold?

Hint: Assume the concentration of nitric acid is kept at 1 M, which will simplify how much HCl to use. Also, assume NO(g) will not be a part of the equilibrium since it is a gas that will be allowed to escape (only to simplify things).

First, considering nitric acid and hydrochloric acids are strong acids: HNO' H+ (aq) + NOS (aq) ... NO3 + H-NO(g) ... 3e_ + NO3 (aq) +4H+(aq_ NO(g) +2 H2ow le" =0.96 v) HCl H+(aq) + Craq) ..Cl_ + Au(S-Aucia (aq) ... 4CT+Au(S-Auci-(aq) +3e- le" =-0.99 v) So combining the two half-reactions yields: 3( + NOs(aq) + 4H+(ag) + cr + Au(S-NO(g) + 2 H2OU) + Aucia(aq)x3é NO-(aq) + 4H+(aq) + 4C+Au(S-NO(g) + 2 H2OU) + Aucl-(aq) le"-0.96 v-0.99 v=-0.03 v) 4

Explanation / Answer

Applying Nernst equation,

Ecell = E0cell - (0.05916 / n) * Log[AuCl4-]/[NO3-][HCl]4

= - 0.03 - (0.05916 / 3) Log1/[HCl]4

Assume, [HCl] = 2.5 M

Ecell = - 0.03 - (0.0591/3)* Log(1/2.54)

Ecell = 0.0014 V

So, If conc. of HCl is about 2.5 times to conc. of HNO3, then the reaction would be spontaneous.

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