Environmental chemistry
Acid deposition, driven by oxides of nitrogen and sulfur, results in environmental degradation. This process involves complex mechanisms, particularly the formation of nitric and sulfuric acids through reactions with hydroxyl radicals. Unfortunately, ammonia (NH3), although alkaline, exacerbates acid deposition by neutralizing these strong acids, resulting in ammonium salts. Furthermore, the hydrolysis of these salts in acid rain contributes to soil acidity, while excess NH4+ in the soil leads to nitrification, further increasing hydrogen ion concentration. Understanding these interactions is crucial for environmental protection.
Environmental chemistry
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Environmental chemistry E 11. acid deposition
Acid deposition • Describe the mechanism of acid deposition caused by the oxides of nitrogen and oxides of sulfur. • Explain the role of ammonia in acid deposition.
Mechanisms of acid deposition Hydroxyl free radicals are formed by the following two processes involving water, ozone and oxygen free radicals: H2O + O3 → 2HO• + O2 or H2O + O• → 2HO•
Mechanisms acid deposition Formation of nitric acid HO• + NO2 → HNO3 also (HO• + NO → HNO2)
Mechanism acid deposition Formation of sulphuric acid – 3 steps HO• + SO2 → HOSO2• then ( hydroxysulfonyl radical - intermediate) HOSO2• + O2 → HO2• + SO3 then: (hydroxylperoxyl radical) SO3 + H2O → H2SO4
Role of NH3 in acid deposition (1) • Atmospheric NH3, although an alkali, contributes to acid deposition. • NH3 neutralizes the strong acids in acid rain forming ammonium salts as shown in the equations below. 2NH3 (aq) + H2SO4 (aq) → (NH4)2SO4 (aq) 2NH3 (aq) + H2SO3 (aq) → (NH4)2SO3 (aq) NH3 (aq) + HNO3 (aq) → NH4NO3 (aq)
Role NH3 in acid deposition (2) • (NH4)2SO4,(NH4)2SO3 and NH4NO3 formed from strong acids and a weak alkali. • salt hydrolysis occurs in acid rain drops ensuring rain droplets are still acidic, making the soil onto which the rain falls also acidic. • salt hydrolysis equation: NH4+ + H2O → NH3 + H3O+
Role of NH3 in acid deposition (last one) • additional acidic effect • soil is further acidified as any NH4+ not hydrolysed reacts with oxygen in the soil to form the nitrate ion (=nitrification) and hydrogen ions, H+, as shown by the equation below: NH4+ +2O2 → 2H+ + NO3− +H2O
