Photochemical smog: forms when sunlight reacts with nitrogen oxides (NO, NO₂) and unburned hydrocarbons. PAN (peroxyacetyl nitrate) is a stable component that travels long distances and releases NOx and radicals again at higher temperature.
Acid rain: rain with pH less than 5 (book definition), caused by SO₂ and NO₂.
Oxides: binary compounds of an element with oxygen. Acidic oxide: oxide of a non-metal that forms an acid with water and reacts with bases (SO₂, CO₂). Basic oxide: oxide of a metal that reacts with acids to give salt and water (CuO, CaO, Na₂O). Amphoteric oxide: reacts with both acids and bases (Al₂O₃, ZnO).
PAN and acid rain
PAN formation: (1) 2NO + O₂ → 2NO₂. (2) NO₂ → NO + O (UV light, photolysis). (3) O + O₂ → O₃. (4) Unburned hydrocarbons (VOCs) react with O to give organic radicals RCO•. (5) RCO• + O₂ → RCO₃• (peroxyacyl radical). (6) RCO₃• + NO₂ → PAN. Three reactions are enough for a 1x3 answer.
Direct role of NOx in acid rain: 2NO + O₂ → 2NO₂ and 4NO₂ + 2H₂O + O₂ → 4HNO₃ (nitric acid).
Catalytic role of NO₂ with SO₂: SO₂ + NO₂ → SO₃ + NO; SO₃ + H₂O → H₂SO₄; the NO is re-oxidised, 2NO + O₂ → 2NO₂, so a small amount of NO₂ converts a lot of SO₂ into sulphuric acid.
Haber process (ammonia)
N₂ + 3H₂ ⇌ 2NH₃ (reversible, exothermic, volume decreases). Conditions: 450 °C, 200 atm (20 000 kPa), iron catalyst (Fe, with Mo as a promoter in the book). Hydrogen from methane (natural gas), nitrogen from air, ratio 1 : 3 by volume.
Le Chatelier: high pressure favours ammonia because the volume falls; a low temperature favours yield but is too slow, so 450 °C with a catalyst is the compromise. Gases leaving contain about 10 to 12% ammonia; it is liquefied by cooling (refrigerated brine) and the unreacted N₂ and H₂ are recycled.
Contact process (sulphuric acid)
SO₂ comes from burning sulphur (S + O₂ → SO₂) or roasting sulphide ores; oxygen from air. Key step: 2SO₂ + O₂ ⇌ 2SO₃ (reversible, exothermic). Conditions: 450 °C, 200 kPa (about 2 atm), vanadium(V) oxide catalyst (V₂O₅).
SO₃ is absorbed in 98% H₂SO₄ to make oleum: H₂SO₄ + SO₃ → H₂S₂O₇. Oleum diluted with water gives concentrated acid: H₂S₂O₇ + H₂O → 2H₂SO₄. (SO₃ is not added to water directly because that gives a fog of acid.) Sulphuric acid is called the King of Chemicals (fertilisers, plastics, paints, explosives, textiles).
Metallic oxides are basic, non-metallic oxides are acidic, and oxides of metals near the dividing line are amphoteric.
Reactions of metals
With dilute acid: metal + acid → salt + hydrogen (Mg + 2HCl → MgCl₂ + H₂). K and Na are explosive; Mg, Zn and Fe react steadily; Cu, Ag and Au do not react.
With cold water: metal + water → metal hydroxide + hydrogen (2Na + 2H₂O → 2NaOH + H₂; Ca + 2H₂O → Ca(OH)₂ + H₂). K, Na and Ca react vigorously; Mg very slowly; Zn and Fe not significantly.
With steam: metal + steam → metal oxide + hydrogen (Mg + H₂O → MgO + H₂; 3Fe + 4H₂O → Fe₃O₄ + 4H₂; Zn + H₂O → ZnO + H₂). Copper does not react.
With oxygen: metal + oxygen → metal oxide (2Mg + O₂ → 2MgO; 4Na + O₂ → 2Na₂O; 4Al + 3O₂ → 2Al₂O₃). Mg and Al form a protective oxide layer. Copper oxidises slowly; gold and platinum do not react.
Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au. Four groups: highly reactive (K, Na, Ca), moderately reactive (Mg, Al, Zn, Fe), less reactive (Pb, Cu, Ag), unreactive (Au, Pt). A metal that reacts with steam but not cold water sits in the moderate group (like Mg, Zn or Fe).
Past-paper questions with model answers
Catalyst in the Contact process and its role (B, 1+2): V₂O₅ (vanadium(V) oxide); it speeds up the oxidation of SO₂ to SO₃ without being used up, by lowering the activation energy.
Equation for conversion of SO₂ to SO₃ (B): 2SO₂ + O₂ ⇌ 2SO₃. Optimum conditions (D, M): 450 °C, 200 kPa (2 atm), V₂O₅ catalyst.
What is the Haber process, with its equation and conditions (D, M): industrial production of ammonia, N₂ + 3H₂ ⇌ 2NH₃, 450 °C, 200 atm, iron catalyst.
How NOx forms PAN, with three reactions (B, 1x3): use steps 1 to 3 or 4 to 6 above.
Type of compound formed when a metal reacts with oxygen, with an example (B, D): a metal oxide (basic oxide), e.g. 2Mg + O₂ → 2MgO.
What happens when a reactive metal reacts with dilute acid (B), or with steam (M): salt + hydrogen; metal oxide + hydrogen, with an example equation each.
Is CaO a basic oxide? Justify with an equation (B): yes, it reacts with acid to give salt and water, CaO + 2HCl → CaCl₂ + H₂O (or with water to form Ca(OH)₂). Is CO₂ an acidic oxide (D): yes, CO₂ + H₂O → H₂CO₃ or CO₂ + 2NaOH → Na₂CO₃ + H₂O. What are amphoteric oxides, two examples (M): oxides that react with both acids and bases, Al₂O₃ and ZnO.
Sec C: classify SO₂, CuO and Al₂O₃ with balanced equations (B, D, M, every set): SO₂ acidic, CuO basic, Al₂O₃ amphoteric (equations above).
Sec C: reactivity of metals with oxygen (Na, Al, Cu, Pt) (D): Na burns rapidly (4Na + O₂ → 2Na₂O); Al forms a protective oxide layer (4Al + 3O₂ → 2Al₂O₃); Cu oxidises slowly and only on heating (2Cu + O₂ → 2CuO); Pt does not react.
Sec C: reactions of metals with steam and oxygen (B), cold water and dilute acids (M): use the equations above.