ENVIRONMENTAL CHEMISTRYE 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

HOSO2• + O2 → HO2• + SO3 then:

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