Why do mg and mn react with hno3

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Last updated: April 8, 2026

Quick Answer: Magnesium (Mg) and manganese (Mn) react with nitric acid (HNO₃) due to their positions in the reactivity series and nitric acid's oxidizing properties. Magnesium typically produces magnesium nitrate and hydrogen gas in dilute nitric acid, but with concentrated nitric acid, it forms a protective oxide layer that can passivate the reaction. Manganese reacts vigorously with nitric acid to form manganese(II) nitrate, nitrogen oxides (like NO₂), and water, with the reaction being more pronounced in concentrated acid. These reactions are important in industrial processes like metal purification and fertilizer production.

Key Facts

Magnesium reacts with dilute nitric acid (1-2 M) to produce hydrogen gas and magnesium nitrate, but concentrated nitric acid (>15 M) causes passivation due to MgO layer formation.
Manganese reacts with concentrated nitric acid to produce brown nitrogen dioxide (NO₂) gas, visible as fumes, with the reaction accelerating above 60°C.
Nitric acid acts as an oxidizing agent in these reactions, with standard reduction potentials: NO₃⁻/NO at +0.96 V and NO₃⁻/NO₂ at +0.80 V vs SHE.
Industrial applications include using nitric acid to purify magnesium (removing impurities) and produce manganese nitrate for fertilizers since the early 20th century.
Safety protocols require handling these reactions in fume hoods due to toxic NOx emissions, with OSHA setting exposure limits at 5 ppm for NO₂.

Overview

The reaction of metals with nitric acid has been studied since the 18th century, with early work by chemists like Joseph Priestley (1772) identifying nitrogen oxides as products. Magnesium (atomic number 12) and manganese (atomic number 25) are both transition zone metals with distinct reactivities: magnesium is highly electropositive (E° = -2.37 V for Mg²⁺/Mg), while manganese has multiple oxidation states (+2 to +7). Nitric acid (HNO₃), first produced industrially via the Birkeland–Eyde process (1903), is a strong acid and oxidizing agent used in metallurgy. Historically, these reactions gained importance during World War I for munitions production, with manganese nitrate used in explosives. Today, they're relevant in sectors like agriculture (fertilizers) and electronics (metal etching), with global nitric acid production exceeding 60 million metric tons annually.

How It Works

The reaction mechanisms depend on acid concentration and metal properties. For magnesium with dilute HNO₃ (below 2 M), the typical acid-metal reaction occurs: Mg + 2HNO₃ → Mg(NO₃)₂ + H₂↑, where hydrogen gas evolves. With concentrated HNO₃ (above 15 M), passivation happens as a thin, adherent magnesium oxide (MgO) layer forms, slowing further reaction—this is due to HNO₃'s strong oxidation forming a protective film. Manganese reacts differently: with concentrated acid, it undergoes redox: Mn + 4HNO₃ → Mn(NO₃)₂ + 2NO₂↑ + 2H₂O, producing brown nitrogen dioxide. The process involves nitric acid reduction (NO₃⁻ to NO₂) and manganese oxidation (Mn to Mn²⁺), driven by HNO₃'s oxidizing power (standard potential ~ +0.80 V). Factors like temperature (reaction rate doubles per 10°C rise) and surface area influence kinetics, with manganese reacting faster due to its lower passivation tendency.

Why It Matters

These reactions have significant real-world impacts. In industry, nitric acid is used to purify magnesium by removing impurities like iron, crucial for aerospace alloys (e.g., in aircraft since the 1950s). Manganese nitrate production from these reactions supplies fertilizers, enhancing crop yields—manganese is essential for photosynthesis. Environmental concerns arise from nitrogen oxide (NOx) emissions, contributing to smog and acid rain; regulations like the US Clean Air Act limit emissions. Safety-wise, proper handling in fume hoods is vital due to toxic NO₂ exposure risks. Applications extend to battery manufacturing (manganese in alkaline batteries) and chemical synthesis, with global manganese consumption around 20 million metric tons yearly for steel production and beyond.