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Difficulty: MediumPhysical and Chemical Properties of Acids and Bases

When dilute tetraoxosulfate(VI) acid (H2SO4H_2SO_4) reacts with zinc metal, hydrogen gas (H2H_2) is liberated. However, when concentrated trioxonitrate(V) acid (HNO3HNO_3) reacts with zinc metal, hydrogen gas is not evolved. Which property of concentrated trioxonitrate(V) acid accounts for this difference in behavior?

  1. Its strong oxidizing property, which oxidizes the evolved hydrogen to waterAnswer
  2. B
    Its dehydrating property, which removes water molecules from the metal surface
  3. C
    Its weak acidic strength when present in concentrated solution
  4. D
    Its high volatility, which causes the acid to evaporate before reacting with the metal

Answer

Concentrated trioxonitrate(V) acid does not liberate hydrogen gas with zinc because of its strong oxidizing property, which oxidizes hydrogen to water.
Unlike typical mineral acids that liberate hydrogen gas when reacting with electropositive metals, concentrated trioxonitrate(V) acid (HNO3HNO_3) is a powerful oxidizing agent. It oxidizes hydrogen to water (H2OH_2O) as soon as it is produced, while the acid itself undergoes reduction to nitrogen dioxide (NO2NO_2).

Step-by-Step Solution

1
Analyze the typical reaction of metals with dilute acids
Dilute acids such as H2SO4H_2SO_4 or HClHCl act as typical acids where reactive metals displace hydrogen: Zn(s)+H2SO4(aq)→ZnSO4(aq)+H2(g)Zn_{(s)} + H_2SO_{4(aq)} \rightarrow ZnSO_{4(aq)} + H_{2(g)}.
This is a standard displacement reaction driven by hydrogen ion reduction.
2
Examine the behavior of concentrated trioxonitrate(V) acid (HNO3HNO_3)
Concentrated HNO3HNO_3 is a powerful oxidizing acid. Instead of releasing H2H_2 gas, any formed hydrogen is immediately oxidized to water (H2OH_2O), while HNO3HNO_3 is reduced to brown nitrogen dioxide gas (NO2NO_2).
The equation is: Zn(s)+4HNO3(aq)→Zn(NO3)2(aq)+2NO2(g)+2H2O(l)Zn_{(s)} + 4HNO_{3(aq)} \rightarrow Zn(NO_3)_{2(aq)} + 2NO_{2(g)} + 2H_2O_{(l)}.
3
Identify the chemical property responsible
The absence of hydrogen gas is due to the strong oxidizing nature of concentrated HNO3HNO_3.
The acid acts primarily as an oxidizing agent rather than a simple proton donor.

Key Concept

Oxidizing property of trioxonitrate(V) acid
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Topics

Physical and Chemical Properties of Acids and Bases

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