When magnesium nitrate is heated strongly, magnesium oxide is formed.
(a) The equation for this reaction is shown.
\[ 2Mg(NO_3)_2 \rightarrow 2MgO + 4NO_2 + O_2 \]
(i) State the change in oxidation number of nitrogen, N, in this reaction.
(ii) Identify the element which is oxidised in this reaction.
(iii) Calculate the volume of NO2 gas, at r.t.p., formed when 7.40 g of Mg(NO3)2 is heated.
Use the following steps.
- Calculate the Mr of Mg(NO3)2.
- Calculate the number of moles of Mg(NO3)2 used.
- Determine the number of moles of NO2 formed.
- Calculate the volume of NO2 gas, in cm3, at r.t.p.
(b) Magnesium oxide, MgO, is an ionic compound.
Complete the dot-and-cross diagram in Fig. 4.1 of the ions in magnesium oxide.
Give the charges on each of the ions.
(c) Oxygen is a covalent molecule.
Complete the dot-and-cross diagram in Fig. 4.2 of a molecule of oxygen. The inner shells have been drawn.
▶️ Answer/Explanation
(a)(i) from +5 to +4
In Mg(NO3)2, nitrogen has an oxidation number of +5 (each oxygen is -2, and the three oxygens make -6, so nitrogen must be +5 to balance). In NO2, nitrogen has an oxidation number of +4 (each oxygen is -2, two oxygens make -4, so nitrogen is +4).
(a)(ii) oxygen
The oxidation number of oxygen changes from -2 in NO3– to 0 in O2, indicating oxidation.
(a)(iii)
1. Mr of Mg(NO3)2 = 24 + (14 + 48)×2 = 148
2. Moles of Mg(NO3)2 = 7.40 g / 148 g/mol = 0.0500 mol
3. From the equation, 2 moles Mg(NO3)2 produce 4 moles NO2, so 0.0500 mol produces 0.100 mol NO2
4. Volume of NO2 = 0.100 mol × 24,000 cm3/mol = 2,400 cm3
(b)
1. Mg ion should show 8 electrons (2 in inner shell, 8 in outer shell) with a 2+ charge
2. O ion should show 8 electrons (2 in inner shell, 8 in outer shell) with a 2- charge
3. The diagram should show Mg2+ and O2- ions with their respective electron configurations
(c)
1. Oxygen molecule should show a double bond between the two oxygen atoms
2. Each oxygen atom should have 4 additional electrons (2 lone pairs)
3. The diagram should show O=O with 4 non-bonding electrons on each oxygen
Nitrogen dioxide, \(NO_2\), 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, \((NH_2)_2CO\), 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.
\((NH_2)_2CO → NH_3 + \)
(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.
\(NO_2 + NH_3 → N_2 + 12H_2O\)
(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, \((NH_2)_2CO\), is stored in the tank of a car. Calculate the mass, in kg, of the stored \((NH_2)_2CO\).
(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 (\(NO_2\)) forms in car engines when nitrogen (\(N_2\)) and oxygen (\(O_2\)) from the air react at high temperatures (combustion conditions).
(b) Another adverse effect is acid rain, as \(NO_2\) dissolves in rainwater to form nitric acid (\(HNO_3\)).
(c) (i) The reaction is thermal decomposition (breaking down by heat).
(ii) The balanced equation is: \((NH_2)_2CO → NH_3 + HCNO\) (isocyanic acid).
(iii) Test for ammonia:
● Test: Damp red litmus paper.
● Observation: Turns blue (ammonia is alkaline).
(d) (i) The balanced equation is: \(6NO_2 + 8NH_3 → 7N_2 + 12H_2O\).
(ii) Nitrogen in \(NO_2\) is reduced because its oxidation state decreases from +4 (in \(NO_2\)) to 0 (in \(N_2\)).
(iii) Redox means simultaneous reduction (gain of electrons) and oxidation (loss of electrons).
(e)
● Molar mass of urea (\((NH_2)_2CO\)) = 60 g/mol.
● Mass = moles × molar mass = \(135 \times 60 = 8100 \, \text{g} = 8.1 \, \text{kg}\).
(f) (i) The gas formed by incomplete combustion is carbon monoxide (CO).
(ii) The two non-toxic products are carbon dioxide (CO2) and nitrogen (N2).