nitric acid幾時用reducing or acid properties??

Problems usually occurs in the reacts between metals and nitric acid. The reasons are :
(1) Metals can act as a reducing agent (to form metal cations), and can also react with acids to give hydrogen gas.
(2) Nitric acid (not very dilute) can act as an oxidizing agent, and also an acid.

Case 1: Concentrated nitric acid is used.
Redox reactions occur. The metal is oxidized to metal ions and the conc. HNO3 is reduced to NO2 gas.
e.g. Mg(s) + 4H+(aq) + 2NO3(aq) → Mg2+(aq) + 2H2O(l) + 2NO2(g)
e.g. Cu(s) + 4H+(aq) + 2NO3(aq) → Cu2+(aq) + 2H2O(l) + 2NO2(g)

Case 2: Moderately concentrated to dilute nitric acid is used.
Redox reacts occur. The metal is oxidized to metal ions and the HNO3 is reduced to NO gas.
e.g. 3Mg(s) + 8H+(aq) + 2NO3(aq) → 3Mg2+(aq) + 4H2O(l) + 2NO(g)
e.g. 3Cu(s) + 8H+(aq) + 2NO3(aq) → 3Cu2+(aq) + 4H2O(l) + 2NO(g)

Case 3: Very dilute nitric acid is used.
Reactive metal reacts with the very dilute HNO3 to give H2 gas.
e.g. Mg(s) + 2HNO3(aq) → Mg(NO3)2(aq) + H2(g)
e.g. Cu(s) + HNO3(aq) → no reaction

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Consider a substance which can react with acid, but it is not a reducing agent. This substance will react with nitric acid (no matter what its concentration is) in a way similar to other common acids.

e.g. (1) Reaction with hydrogencarbonates to give CO2 gas:
e.g. (1) Mg(HCO3)­2(aq) + 2HNO3(aq) → Mg(NO3)2(aq) + 2H2O(l) + 2CO2(g)

e.g. (2) Neutralization with metal oxides:
e.g. (2) CuO(s) + 2HNO3(aq) → Cu(NO3)2(aq) + H2O(l)

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Consider a substance which can act as a reducing agent, but it does not react with acids. This substance with undergo redox reaction with nitric acid (provided it is NOT very dilute).

e.g. (1) Reaction between conc. HNO3 and FeSO4 solid:
e.g. (1) Fe2+(aq) + 2H+(aq) + NO3-(aq) → Fe3+(aq) + H2O(l) + NO2(g)

e.g. (2) Reaction between moderately conc. HNO3 and KI solution:
e.g. (1) 6I-(aq) + 8H+(aq) + 2NO3-(aq) → 3I2(aq) + 4H2O(l) + 2NO(g)

when nitric acid is in dilute form, it only has acidic properties.
but when nitric acid is in concentrated form, it has both
oxidizing properties as well as acidic properties.
when using concontrated nitric acid, you should consider
also what is the other reactant. when it is a bicarbonate
or a copper ii oxide which cannot be oxidized, nitric acid
only shows acidic properties. but the rate of reaction is
faster in concentrated acid than in dilute aicd.
ok?