Which of the following pairings is correct?

Question

Examoogle · Accepted Answer

The correct answer is Option A: Magnesium (Mg) and manganese (Mn). Very dilute nitric acid ($HNO_3$) is a powerful oxidizing agent for most metals, but it can act as an acid to release hydrogen gas only with metals like Magnesium ($Mg$) and Manganese ($Mn$). These two metals are highly reactive and, in a very dilute state, the oxidizing power of $HNO_3$ is sufficiently suppressed to allow for the standard metal-acid displacement reaction.

Step-by-Step Solution & Concepts:
Very dilute nitric acid ($HNO_3$) is a powerful oxidizing agent for most metals, but it can act as an acid to release hydrogen gas only with metals like Magnesium ($Mg$) and Manganese ($Mn$). These two metals are highly reactive and, in a very dilute state, the oxidizing power of $HNO_3$ is sufficiently suppressed to allow for the standard metal-acid displacement reaction.

Working Principle: In most cases, nitric acid ($HNO_3$) oxidizes the hydrogen gas produced to water, resulting in the evolution of nitrogen oxides ($NO_x$) instead of hydrogen. However, with Magnesium and Manganese, the reaction with very dilute nitric acid occurs such that the standard displacement $M + 2HNO_3 \rightarrow M(NO_3)_2 + H_2$ proceeds because the low concentration of $H^+$ ions and the specific reduction potentials of these metals favour hydrogen evolution over the reduction of the nitrate ion.

Key Formulas: $Mg(s) + 2HNO_3(aq) \rightarrow Mg(NO_3)_2(aq) + H_2(g)$ — Reaction with Magnesium; $Mn(s) + 2HNO_3(aq) \rightarrow Mn(NO_3)_2(aq) + H_2(g)$ — Reaction with Manganese

Key Points: Nitric acid is generally an oxidizing acid that produces water and nitrogen oxides instead of hydrogen.; Magnesium and Manganese are the only common metals that show this unique behaviour due to their position in the reactivity series.; The concentration must be 'very dilute' (approx 1%) to prevent the oxidizing action.

Advantages: Allows laboratory collection of Hydrogen gas.

Disadvantages: Requires precise concentration control; higher concentrations will fail to produce hydrogen.

Applications: Laboratory gas preparation; Study of metal reactivity

Additional Info: Copper and Cobalt react with nitric acid to produce Nitrogen oxides (e.g., $NO_2$, $NO$, $N_2O$), not hydrogen.. This phenomenon is a classic exception taught in Class 10 Chemistry (NCERT).

Answer: A is correct — Magnesium and Manganese are the only metals that produce hydrogen gas when reacted with very dilute nitric acid.

Exam Tip: Always remember that in competitive chemistry, 'Nitric Acid' is almost always an oxidizing agent—except for the specific case of Mg and Mn with very high dilution!

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Option B: Copper (Cu) and cobalt (Co)

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Option C: Manganese (Mn) and cobalt (Co)

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Option D: Magnesium (Mg) and copper (Cu)

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