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.
