Nitrogen dioxide, \(\text{NO}_2\), is an atmospheric pollutant and is formed in car engines.
(a) Explain how nitrogen dioxide is formed in car engines.
(b) Nitrogen dioxide causes respiratory problems.
State one other adverse effect of nitrogen dioxide.
(c) Nitrogen dioxide emissions can be reduced by adding an aqueous solution of urea, \((\text{NH}_2)_2\text{CO}\), to car exhaust gases.
The heat of the exhaust gases breaks down the urea into simpler substances.
(i) Name the type of reaction which occurs when a substance is heated and breaks down into simpler substances.
(ii) One molecule of urea breaks down to form one molecule of ammonia and one other molecule.
Complete the chemical equation to show the formula of the other molecule formed in this reaction.
\((\text{NH}_2)_2\text{CO} \rightarrow \text{NH}_3 + \)
(iii) State the test for ammonia.
(d) The ammonia formed reacts with nitrogen dioxide to form nitrogen and water.
(i) Balance the equation for this reaction.
\(\text{NO}_2 + \text{NH}_3 \rightarrow \text{N}_2 + 12\text{H}_2\text{O}\)
(ii) State how the equation shows that the nitrogen in nitrogen dioxide is reduced.
(iii) This reaction is a redox reaction.
State the meaning of the term redox.
(e) 135 moles of urea, \((\text{NH}_2)_2\text{CO}\), is stored in the tank of a car.
Calculate the mass, in kg, of the stored \((\text{NH}_2)_2\text{CO}\).
(f) Another oxide of nitrogen formed in car engines is nitrogen monoxide, NO. A catalytic converter removes NO by reacting it with a gas formed by incomplete combustion of the fuel. Two non‑toxic gases are formed.
(i) Name the gas formed by incomplete combustion of the fuel.
(ii) Name the two non‑toxic gases formed.
▶️ Answer/Explanation
(a) Nitrogen dioxide (\(\text{NO}_2\)) forms in car engines when nitrogen (\(\text{N}_2\)) and oxygen (\(\text{O}_2\)) from the air react at high temperatures during combustion. The reaction is: \[ \text{N}_2 + \text{O}_2 \rightarrow 2\text{NO} \quad \text{(nitrogen monoxide)}, \quad \text{NO} + \text{O}_2 \rightarrow \text{NO}_2. \]
(b) Another adverse effect is acid rain, as \(\text{NO}_2\) dissolves in rainwater to form nitric acid.
(c) (i) The reaction is thermal decomposition (breaking down due to heat).
(ii) The balanced equation is: \[ (\text{NH}_2)_2\text{CO} \rightarrow \text{NH}_3 + \text{HNCO} \quad \text{(isocyanic acid)}. \]
(iii) Test for ammonia: Damp red litmus paper turns blue (ammonia is alkaline).
(d) (i) The balanced equation is: \[ 6\text{NO}_2 + 8\text{NH}_3 \rightarrow 7\text{N}_2 + 12\text{H}_2\text{O}. \]
(ii) Nitrogen in \(\text{NO}_2\) is reduced (its oxidation state decreases from +4 to 0).
(iii) Redox means reduction and oxidation occur simultaneously.
(e) Molar mass of urea = 60 g/mol. Mass of 135 moles: \[ 135 \times 60 = 8100\, \text{g} = 8.1\, \text{kg}. \] Final Answer: 8.1 kg
(f) (i) The gas formed by incomplete combustion is carbon monoxide (\(\text{CO}\)).
(ii) The non-toxic gases formed are nitrogen (\(\text{N}_2\)) and carbon dioxide (\(\text{CO}_2\)).
Zinc is extracted from an ore containing zinc sulfide.
(a) State the name of this zinc ore.
(b) This ore is converted to zinc oxide, ZnO.
(i) Write a chemical equation for the reaction of zinc oxide with carbon.
(ii) State what type of chemical change happens to the zinc in zinc oxide in this reaction.
Explain your answer.
(iii) Explain why aluminium is not extracted from aluminium oxide by heating with carbon.
(iv) Suggest an alternative method for the extraction of zinc from zinc oxide.
(c) Brass is an alloy of zinc.
Explain, in terms of particles, why brass is harder than pure zinc.
▶️ Answer/Explanation
(a) The ore is called zinc blende (or sphalerite).
(b) (i) The chemical equation is: \[ \text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO} \] or \[ 2\text{ZnO} + \text{C} \rightarrow 2\text{Zn} + \text{CO}_2 \]
(b) (ii) – Chemical change: Reduction. – Explanation: Zinc oxide loses oxygen (ZnO → Zn), so it is reduced. Carbon acts as the reducing agent.
(b) (iii) Aluminium is more reactive than carbon, so carbon cannot displace aluminium from its oxide (Al₂O₃).
(b) (iv) An alternative method is electrolysis (of molten zinc oxide or zinc sulfate solution).
(c) Brass is harder than pure zinc because: 1. It contains different-sized atoms (zinc and copper). 2. These atoms distort the layers in the metallic structure. 3. This makes it harder for layers to slide over each other, increasing hardness.