(i) Nuclear fission; (ii) Nuclear fusion.
(b) Alpha particle emission by \(^{293}_{25}U\) proceduces an element A. Beta particle emission by the particle A produces another element B. Element B also undergoes alpha particle emission to produce \(^{227}_{89}AC\). Write balanced equations to represent the above statement.
(c) The models below represent the filling of orbitals in an atom.
State which rule(s) is/are violated or obeyed by each model.
(d) Explain why the boiling point of H\(_2\)S with relative molecular mass of 34 is lower than that of H\(_2\)O with relative molecular mass of 18.
(ii) methylbenzene. I. State the effect of each solution on blue litmus paper II. Compare the electrical conductivities of the two solutions.
(f) Zinc dust is added to copper (II) tetraoxosulphate (VI) solution. State;
(i) what is observed; (ii) the type of reaction that occurs.
(a) Definitions and one use of each
(i) Nuclear fission: the splitting of a heavy, unstable nucleus into two lighter nuclei of comparable mass, together with the release of neutrons and a very large amount of energy, usually after the nucleus is struck by a slow neutron. Use: generation of electricity in nuclear power reactors (also used in the atomic bomb).
(ii) Nuclear fusion: the combination (joining) of two light nuclei to form a single heavier nucleus, accompanied by the release of an enormous amount of energy. Use: it is the source of energy of the sun and stars (also used in the hydrogen bomb).
(b) Balanced nuclear equations
The parent nuclide is uranium-235, \(^{235}_{92}\text{U}\) (the printed \(^{293}_{25}\text{U}\) is a typographical error). Working through each emission:
Alpha emission by uranium gives A: an alpha particle is \(^{4}_{2}\text{He}\); mass number falls by 4, atomic number by 2.
\[^{235}_{92}\text{U} \;\rightarrow\; ^{231}_{90}\text{Th} \;+\; ^{4}_{2}\text{He} \qquad (\text{A} = {}^{231}_{90}\text{Th})\]
Beta emission by A gives B: a beta particle is \(^{0}_{-1}e\); mass number is unchanged, atomic number rises by 1.
\[^{231}_{90}\text{Th} \;\rightarrow\; ^{231}_{91}\text{Pa} \;+\; ^{0}_{-1}e \qquad (\text{B} = {}^{231}_{91}\text{Pa})\]
Alpha emission by B gives actinium-227:
\[^{231}_{91}\text{Pa} \;\rightarrow\; ^{227}_{89}\text{Ac} \;+\; ^{4}_{2}\text{He}\]
The mass numbers balance (231 = 227 + 4) and the atomic numbers balance (91 = 89 + 2), confirming the scheme.
(c) The orbital-filling models
Two rules govern the filling shown: the Pauli exclusion principle (an orbital holds at most two electrons and they must have opposite spins) and Hund's rule of maximum multiplicity (each orbital in a sub-shell is singly filled with parallel spins before any is doubly filled).
Model I: the \(s\) orbital is correctly paired (up and down), but the \(P_x\) orbital contains two electrons drawn with the same (parallel) spin (both arrows up) while \(P_y\) and \(P_z\) each carry one. Two electrons in the same orbital with parallel spins is forbidden, so Model I violates the Pauli exclusion principle.
Model II: the \(s\) orbital is paired, and in the \(p\) sub-shell \(P_x\) holds two electrons of opposite spin (up and down) while \(P_y\) and \(P_z\) are each singly occupied. For four \(p\)-electrons this is exactly the correct ground-state arrangement, so Model II obeys both the Pauli exclusion principle and Hund's rule.
(d) Why H2S boils lower than H2O
In water the molecules are held together by hydrogen bonds, which form because oxygen is small and very electronegative, drawing the shared electrons strongly and leaving the hydrogen atoms highly positive. These hydrogen bonds are strong intermolecular forces that require much energy to break, giving water a high boiling point. In H2S, sulphur is much less electronegative and larger, so it cannot form hydrogen bonds; the molecules are held only by weak van der Waals (dipole-dipole) forces. Far less energy is needed to separate H2S molecules, so its boiling point is lower even though its relative molecular mass is greater.
(e) HCl in different solvents
I. Effect on blue litmus paper:
- In water: HCl ionizes completely to give \(\text{H}^+\) (hydroxonium) and \(\text{Cl}^-\) ions, forming an acidic solution that turns blue litmus red.
- In methylbenzene: HCl dissolves as molecules and does not ionize in this non-polar solvent, so no hydrogen ions are produced; blue litmus remains blue (no effect).
II. Comparison of electrical conductivities: the aqueous solution conducts electricity because it contains free mobile ions (\(\text{H}^+\) and \(\text{Cl}^-\)), whereas the solution in methylbenzene does not conduct because HCl remains molecular and provides no ions.
(f) Zinc dust added to copper(II) tetraoxosulphate(VI) solution
(i) Observations: the blue colour of the solution gradually fades and finally becomes colourless; a reddish-brown (pink) deposit of copper is formed; the zinc dust dissolves and the mixture becomes warm.
(ii) Type of reaction: a displacement reaction (a redox reaction): zinc, being higher in the reactivity (electrochemical) series than copper, displaces copper from its salt.
\[\text{Zn(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{Cu(s)}\]
