(a) (i) Define nuclear fission. (ii) A certain natural decay series starts with \(^{238}_{92}U\) and ends with \(^{230}_{90}Th\). Each step involves the los...
Question 1 Report
(a) (i) Define nuclear fission.
(ii) A certain natural decay series starts with \(^{238}_{92}U\) and ends with \(^{230}_{90}Th\). Each step involves the loss of an alpha or a beta particle. Using the given information, deduce how many alpha beta particpes were emitted.
(b) Consider the equilibrium reaction represented by the following equation:
\(\mathrm{A_2(g) + 3B_2(g) \rightleftharpoons 2AB_3(g)}\); \(\Delta H = +x\mathrm{kJmol}^{-1}\)
Explain briefly the effect of each of the following changes on the equilibrium composition: (i) increase in concentrat of B; (ii) decrease in pressure of the system; (iii) addition of catalyst.
(c) The lattice energies of three sodium halides are as follows:
| Compound | NaF | NaBr | Nal |
| Lattice energy/\(\mathrm{kJmol}^{-1}\) | 890 | 719 | 670 |
Explain briefly the trend.
(d) State the property exhibited by nitrogen (IV) oxide in each of the following reactions:
(i) \(\mathrm{4Cu + 2NO_2 \to 4CuO + N_2}\);
(ii) \(\mathrm{H_2O + 2NO_2 \to HNO_3 + HNO_2}\).
(e) Iron is manufacturcd in a blast furnace using iron ore (\(\mathrm{Fe_2O_3}\)), coke and limestone. Write the equation for the reaction(s) at the: (i) top of the furnace; (ii) middle of the furnace; (iii) bottom of the furnace.
(f) (i) Name two products of destructive distillation of coal. (ii) Give one use of each product in (f)(i).
Answer Details
(a)(i) Nuclear fission
The splitting of a heavy atomic nucleus into two or more lighter nuclei of comparable mass, accompanied by the release of neutrons and a very large amount of energy, usually when the nucleus is struck by a neutron.
(a)(ii) Alpha and beta particles emitted from \(^{238}_{92}\)U to \(^{230}_{90}\)Th
Change in mass number: \(238 - 230 = 8\). Only alpha emission changes the mass number (by 4 each):
\[ \text{number of }\alpha = \frac{8}{4} = 2 \]Change in atomic number: 2 alphas alone would give \(92 - (2\times 2) = 88\), but the product has Z = 90. Each beta emission raises Z by 1:
\[ 88 + b = 90 \Rightarrow b = 2 \]So 2 alpha particles and 2 beta particles were emitted.
(b) Equilibrium: A\(_2\)(g) + 3B\(_2\)(g) \(\rightleftharpoons\) 2AB\(_3\)(g); \(\Delta H = +x\) kJ mol\(^{-1}\) (endothermic)
- (i) Increase in concentration of B: equilibrium shifts to the right (forward) to use up the added B, giving more AB\(_3\).
- (ii) Decrease in pressure: the system shifts toward the side with the greater number of gas moles. The left has 4 moles and the right 2 moles, so equilibrium shifts to the left (backward), forming more A\(_2\) and B\(_2\).
- (iii) Addition of a catalyst: no effect on the equilibrium composition; it only speeds up the forward and backward reactions equally, so equilibrium is reached sooner.
(c) Trend in lattice energy of the sodium halides
| Compound | NaF | NaBr | NaI |
|---|---|---|---|
| Lattice energy/kJ mol\(^{-1}\) | 890 | 719 | 670 |
From F\(^-\) to Br\(^-\) to I\(^-\) the ionic radius of the halide increases, so the distance between the Na\(^+\) and the halide ion grows. Lattice energy is inversely related to this interionic distance, so the electrostatic attraction weakens and the lattice energy falls: NaF > NaBr > NaI.
(d) Property of nitrogen(IV) oxide
- (i) 4Cu + 2NO\(_2\) \(\rightarrow\) 4CuO + N\(_2\): nitrogen is reduced from +4 to 0, so NO\(_2\) acts as an oxidising agent.
- (ii) H\(_2\)O + 2NO\(_2\) \(\rightarrow\) HNO\(_3\) + HNO\(_2\): nitrogen goes to +5 (in HNO\(_3\)) and +3 (in HNO\(_2\)), so NO\(_2\) undergoes disproportionation (it is simultaneously oxidised and reduced).
(e) Reactions in the blast furnace
- (i) Top (cooler zone) - reduction of the ore by carbon(II) oxide: \[ \text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2 \]
- (ii) Middle - decomposition of limestone and formation of carbon(II) oxide: \[ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \qquad \text{CO}_2 + \text{C} \rightarrow 2\text{CO} \]
- (iii) Bottom (hottest zone, at the tuyeres) - combustion of coke: \[ \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \]
(Formation of slag, CaO + SiO\(_2\) \(\rightarrow\) CaSiO\(_3\), also occurs in the lower middle region.)
(f)(i) Two products of the destructive distillation of coal: coke and coal tar (coal gas and ammoniacal liquor are also produced).
(f)(ii) One use of each:
- Coke: used as a fuel and as the reducing agent in the extraction of iron in the blast furnace.
- Coal tar: used as a raw material for making dyes, drugs and antiseptics (and for surfacing roads).
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