Question

Gold has always been a highly prized metal, and it has been widely used from the beginning of history as a store of value.It does not rust like iron and does not become tarnished like silver. It is so chemically inert that it will not react with even the strongest concentrated acids. But it can be dissolved in aqua regia, a fresh-prepared mixture of concentrated HNO, and HC in a 1:3 ratio
When Germany invaded Denmark in World War II, the Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of Max von Laue and James Franck in aqua regia to prevent the Nazis from stealing them. He placed the jar with the solution on a shelf in his laboratory and, after the war, precipitated the gold from the acid solution and returned it to the Royal Swedish Academy of Sciences and the Nobel Foundation, who recast the medals and again presented them to Laue and Franck The unbalanced equation for the reaction of gold with aqua regia is given. Add the stoichiometric coefficients to the equation to balance it.
equation:Au(s)+ HNO, (ag)+HClagHAuCI(a)+No,(g)+H,00)
What is the function of HCI?
A) It acts as a reducing agent.
B) It supplies chloride ions to form a complex ion with the oxidized gold.
C) It acts as an oxidizing agent.

Answer 1

The equation given is incorrect, the correct equation is:

Au(s) + HNO₃(aq) + HCl(aq) → HAuCl₄(aq) + NO₂(g) + H₂O(l)

Answer:

B) It supplies chloride ions to form a complex ion with the oxidized gold.

Explanation:

The equation given is:

Au(s) + HNO₃(aq) + HCl(aq) → HAuCl₄(aq) + NO₂(g) + H₂O(l)

To balance the equation, we need to find the oxidation number (Nox) of the elements, and identify which substance is being oxided and which on is being reduced. Simple elements has Nox = 0, H has Nox = +1, O has Nox = -2. So:

Au(s): Nox Au = 0

HNO₃(aq): Nox H = +1; Nox O = -2, Nox N: +1 +x +3*(-2) = 0 -> x = +5

HCl(aq): Nox H = +1, Nox Cl = -1

HAuCl₄(aq): Nox H = +1, Nox Cl = -1, Nox Au: +1 +x +4*(-1) = 0 -> x = +3

NO₂(g): Nox O = -2; Nox N: x + 2*(-2) = 0 -> x = +4

H₂O(l): Nox H = +1; Nox O = -2

So, Au is being oxidezed from 0 to +3, and N is being reduced from +5 to +4:

ΔNox(Au) = 3

ΔNox(N) = 1

Both of them are single in the compound they are, so it's not necessary to multiply by the coefficient (because is 1). So, the ΔNox must be changed between the compounds:

Au(s) + 3HNO₃(aq) + HCl(aq) → HAuCl₄(aq) + NO₂(g) + H₂O(l)

Now, the balancing must be done by trial, knowing that the elements must be at the same number on both sides of the equation. So, we multiply HCl by 4, and NO₂ and H₂O by 3:

Au(s) + 3HNO₃(aq) + 4HCl(aq) → HAuCl₄(aq) + 3NO₂(g) + 3H₂O(l).

So, we can see that Au is being oxidized, so it's the reducing agent, and HNO₃ ins being reduced, so it's the oxidizing agent. The HCl supplies Cl ions to the complex formed with gold.