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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.
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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 + NO → HNO2 and 2HNO2 + O2→ 2HNO3
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