Chemistry Paper 3 June 2024

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1. ASn2+ + 2I- -> Sn4+ + I2
2. BSn4+ + I2 -> Sn2+ + 2I-
3. CSn2+ + I2 -> Sn4+ + 2I-
4. DSn4+ + 2I- -> Sn2+ + I2

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1. AThe energy needed to remove one mole of electrons from one mole of gaseous atoms of an element.
2. BThe enthalpy change when one mole of a gaseous ionic compound condenses into its solid lattice at its melting point.
3. CThe enthalpy change when one mole of an ionic solid is formed from its constituent elements in their standard states.
4. DThe enthalpy change when one mole of an ionic solid is formed from its constituent gaseous ions.

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1. AThe second electron must be added to a higher, unoccupied electron shell than the first, which is inherently an endothermic process for any atom.
2. BX2- ions are always less stable than X- ions, so the second electron affinity is endothermic for every element without exception.
3. CAdding a second electron always requires breaking a covalent bond within the X- ion first, which costs more energy than is released.
4. DThe incoming electron is being added to an ion (X-) that is already negatively charged, so electrostatic repulsion between the electron and the ion must be overcome, which costs energy.

1. AThe energy needed to remove the outermost electron from a gaseous atom.
2. BThe tendency of an atom to lose electrons and form a positive ion in an ionic bond.
3. CThe ability of an atom, in a covalent bond, to attract the shared pair of bonding electrons towards itself.
4. DThe energy released when a gaseous atom gains one electron to form a gaseous negative ion.

1. AA bond becomes polar when the two bonded atoms have different electronegativities, so the shared electron pair sits closer to the more electronegative atom, giving the bond partial positive and negative charges.
2. BA bond becomes polar whenever it is formed between two atoms of the same element, since identical atoms always share electrons unevenly.
3. CA bond is polar only when it is broken heterolytically, giving one atom both bonding electrons and the other atom none.
4. DA bond is polar whenever it involves a transfer, rather than a sharing, of electrons between the two bonded atoms.

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1. ACS2 is non-polar overall: the two C=S bond dipoles point in exactly opposite directions in the linear, symmetric molecule and cancel out.
2. BCS2 is non-polar because carbon and sulfur have identical electronegativities, so neither C=S bond is polar in the first place.
3. CCS2 is polar only in the liquid state and non-polar only in the gas state, since its shape changes between the two states.
4. DCS2 is strongly polar overall: the two C=S bond dipoles point in the same direction and add together, giving a large net molecular dipole.

1. AThe enthalpy change when one mole of the compound dissolves completely in excess water under standard conditions.
2. BThe enthalpy change when one mole of the compound is completely burned in excess oxygen under standard conditions.
3. CThe enthalpy change when one mole of the compound's bonds are all broken into gaseous atoms under standard conditions.
4. DThe enthalpy change when one mole of the compound is formed from its elements in their standard states, under standard conditions.

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1. AUse it in a fume cupboard with good ventilation, and keep it away from naked flames and other ignition sources.
2. BStore it in an open container so that any vapour can escape freely into the room air.
3. CWarm it gently over a Bunsen burner flame before use to make it easier to handle.
4. DHandle it only in bright direct sunlight, since CS2 is unstable and decomposes in the dark.

1. AThe number of electrons per molecule increases down the group, giving stronger van der Waals (London dispersion) forces between molecules, which need more energy to overcome.
2. BThe halogen-halogen covalent bond gets stronger down the group, so more energy is needed to break the bond and boil the substance.
3. CThe molecules become more polar down the group, so stronger permanent dipole-dipole forces form between them.
4. DThe halogens become ionic rather than covalent down the group, so much stronger ionic bonds must be broken to boil them.

1. AThe halide ion X- becomes a stronger base down the group, so its conjugate acid HX becomes a weaker acid down the group.
2. BThe electronegativity difference between H and X increases down the group, making the H-X bond more polar and therefore more acidic.
3. CThe H-X bond gets weaker (longer, more easily broken) down the group, so the proton is released more readily -- this dominates over the smaller change in H-X electronegativity difference.
4. DHydrogen halides become less soluble in water down the group, and lower solubility always corresponds to stronger acidity.

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1. AA slow reaction occurs, forming potassium bromate and hydrogen gas, because bromine slowly oxidises the chloride ion over several hours.
2. BNo reaction occurs, because bromine is less reactive (a weaker oxidising agent) than chlorine and cannot displace chloride ions from solution.
3. CThe solution turns brown immediately, since bromine dissolves in the aqueous potassium chloride solution regardless of any reaction.
4. DA vigorous reaction occurs, forming potassium bromide and chlorine gas, because bromine is more reactive than chlorine.

1. ABoth KCl and I2 are highly soluble in water, since both are held together by weak intermolecular forces that water can easily overcome.
2. BKCl is insoluble in water because its lattice energy is too high for water molecules to overcome, while I2 dissolves freely as a non-polar solid.
3. CI2 is more soluble than KCl in water, because iodine's larger electron cloud interacts more strongly with water's polar O-H bonds than K+ or Cl- do.
4. DKCl is soluble (its ions are hydrated by polar water molecules), while I2 (a simple non-polar molecule) is only sparingly soluble, since there is no favourable ion-dipole interaction, only weak van der Waals forces.

1. ANeither KCl nor iodine can ever conduct electricity, in any state, because both are made of covalently bonded molecules.
2. BKCl conducts electricity when molten or dissolved in water (mobile ions are free to move), while solid iodine does not conduct in any state (it has no free ions or delocalised electrons).
3. CBoth KCl and iodine conduct electricity well in the solid state, since both contain delocalised electrons free to move through the structure.
4. DIodine conducts electricity in the solid state because it is a simple molecular solid, while KCl never conducts in any state.

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1. AGlycerol's molecules are held together only by weak van der Waals forces, but there are an extremely large number of them per molecule.
2. BGlycerol is ionic, so strong electrostatic (ionic) forces between its molecules must be broken before it can boil.
3. CGlycerol has three -OH groups per molecule, allowing extensive hydrogen bonding between its molecules, so more energy is needed to separate them.
4. DGlycerol has an unusually high relative molecular mass compared to other common organic solvents, and this alone explains its high boiling point.

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1. AN has the highest relative molecular mass of the three acids, and a higher relative molecular mass always makes a carboxylic acid more acidic.
2. BChlorine is electronegative and electron-withdrawing (inductive effect); N's two chlorine atoms withdraw more electron density from the O-H bond and better stabilise the negative charge on the resulting carboxylate anion than one chlorine (M) or none (L), making N's O-H bond easiest to break.
3. CN is the strongest acid only because it has the lowest melting point of the three, and a lower melting point always corresponds to greater acid strength.
4. DChlorine atoms release electron density towards the carboxyl group by an inductive effect, and N has the most chlorine atoms to release electron density, strengthening its O-H bond the least.

1. AAll three compounds hydrolyse at essentially the same rate, since each contains exactly one C-Cl bond and hydrolysis rate depends only on the number of C-Cl bonds present.
2. BP hydrolyses fastest of the three because haloalkanes are always more reactive towards water than any other class of chlorine-containing organic compound, including acyl chlorides.
3. CO hydrolyses fastest (even in cold water) since its carbonyl carbon is highly electrophilic and Cl- is a good leaving group; P hydrolyses slowly, needing heating with aqueous NaOH; Q strongly resists hydrolysis, since a chlorine lone pair delocalises into the ring, strengthening the C-Cl bond and the electron-rich ring repels nucleophiles.
4. DQ hydrolyses fastest because the aromatic ring makes the C-Cl bond in chlorobenzene especially weak and reactive; O hydrolyses slowest because acyl chlorides are chemically inert towards water.

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1. ANo reaction occurs, since -NH2 and -COOH are both unreactive towards dilute acids.
2. BThe ammonium salt, HOOC-C6H4-NH3+Br-, formed by the basic -NH2 group accepting a proton from the acid.
3. CA brominated ring product, formed by electrophilic substitution of the ring by Br+.
4. DAn amide, HOOC-C6H4-NHBr, formed by substitution of one N-H hydrogen by bromine.

1. ANo reaction occurs at all, since -COOH deactivates the ring completely towards any electrophilic attack.
2. BThe ring, strongly activated by -NH2, undergoes electrophilic substitution readily (no catalyst needed), brominating at position(s) available on the ring.
3. CThe -NH2 group is oxidised by bromine water to a nitro group, -NO2, leaving the ring itself untouched.
4. DBromine adds across the ring's delocalised pi system in an addition reaction, destroying the aromatic ring.

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1. A(1) acid-base (salt formation); (2) electrophilic substitution; (3) nucleophilic substitution/acylation (condensation).
2. B(1) addition; (2) nucleophilic substitution; (3) acid-base (salt formation).
3. C(1) electrophilic substitution; (2) acid-base (salt formation); (3) addition.
4. D(1) nucleophilic substitution; (2) acid-base (salt formation); (3) electrophilic substitution.

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1. ABy filtration: nonane is a solid at room temperature while 2,4-dimethylheptane is a liquid, so nonane can simply be filtered out.
2. BBy adding water and separating layers: nonane dissolves in water while the branched isomer does not, so they can be separated using a separating funnel.
3. CBy adding aqueous sodium hydroxide: it reacts with and dissolves only the branched isomer, leaving nonane behind unreacted.
4. DBy fractional distillation: the unbranched nonane has a higher boiling point than the more compact, branched 2,4-dimethylheptane, since branching reduces the surface contact (and so the van der Waals forces) between molecules.