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HKDSE Chemistry practice questions

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Question 1Chemistry (Category A Senior Secondary Elective Subject)

Which statement correctly describes the trend in first ionisation enthalpy across Period 3 from Na to Ar?

  1. It generally increases, because effective nuclear charge increases while the outer electron is in the same shell. Correct answer.
  2. It increases then falls sharply at every element, because of changes in the number of neutron.
  3. It remains constant, because each atom loses one electron to form a 1+ ion.
  4. It generally decreases, because atomic radius increases across the period.

Explanation

Across Period 3 the number of protons increases while the outer electrons stay in the same principal shell, so effective nuclear charge rises and the outer electron is held more strongly. Small dips occur at Al and S because of the 3p and 3p-pairing effects, but the overall trend is an increase.

Question 2Chemistry (Category A Senior Secondary Elective Subject)

Which species has the same number of electrons as a potassium ion, K⁺?

  1. Cl⁻
  2. Na⁺
  3. Ar⁺
  4. Ca²⁺ Correct answer.

Explanation

K⁺ has 18 electrons (19 protons minus one lost electron). Ca²⁺ has 20 protons minus two electrons, giving 18 electrons, so it is isoelectronic with K⁺.

Question 3Chemistry (Category A Senior Secondary Elective Subject)

In the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the equilibrium constant Kc has units of:

  1. mol⁻² dm⁶ Correct answer.
  2. mol dm⁻³
  3. no units
  4. mol² dm⁻⁶

Explanation

Kc = [NH₃]² / ([N₂][H₂]³). Substituting concentrations in mol dm⁻³ gives (mol dm⁻³)² divided by (mol dm⁻³)⁴, which simplifies to mol⁻² dm⁶.

Question 4Chemistry (Category A Senior Secondary Elective Subject)

Which change would increase the equilibrium yield of ammonia in the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g) with ΔH negative?

  1. Decreasing the pressure and adding a catalyst
  2. Adding a catalyst and increasing the temperature
  3. Increasing the temperature and increasing the pressure
  4. Increasing the pressure and removing ammonia as it forms Correct answer.

Explanation

The forward reaction reduces the number of gas molecules from four to two, so a higher pressure shifts the position of equilibrium to the right. Removing ammonia lowers the product concentration, which also shifts the equilibrium to the right according to Le Chatelier's principle.

Question 5Chemistry (Category A Senior Secondary Elective Subject)

Which of the following is the strongest oxidising agent under standard conditions?

  1. Zn²⁺(aq)
  2. Fe²⁺(aq)
  3. Ag⁺(aq) Correct answer.
  4. Na⁺(aq)

Explanation

The strength of an oxidising agent increases as its standard reduction potential becomes more positive. Ag⁺/Ag has a positive E° value (+0.80 V), while Zn²⁺/Zn, Fe²⁺/Fe and Na⁺/Na all have negative values, so Ag⁺ is the strongest oxidising agent listed.

Question 6Chemistry (Category A Senior Secondary Elective Subject)

In the electrolysis of dilute sulfuric acid using inert platinum electrodes, which statement describes the products?

  1. Sulfur dioxide is produced at the anode and hydrogen at the cathode.
  2. Hydrogen is produced at the cathode and oxygen at the anode. Correct answer.
  3. Oxygen is produced at the cathode and hydrogen at the anode.
  4. Only hydrogen is produced, at the cathode, with no gas at the anode.

Explanation

At the cathode, H⁺ ions are reduced to hydrogen gas because they are easier to reduce than Na⁺ or water. At the anode, water is oxidised to oxygen gas because sulfate ions are not readily oxidised, so the overall process is the decomposition of water.

Question 7Chemistry (Category A Senior Secondary Elective Subject)

Which pair of substances will react together in a simple displacement reaction?

  1. Zinc metal and copper(II) sulfate solution Correct answer.
  2. Copper metal and zinc sulfate solution
  3. Copper metal and silver nitrate solution
  4. Silver metal and zinc sulfate solution

Explanation

Zinc is more reactive than copper, so it reduces Cu²⁺ ions to copper metal while dissolving as Zn²⁺. The other pairs involve a less reactive metal attempting to displace a more reactive one, which does not occur.

Question 8Chemistry (Category A Senior Secondary Elective Subject)

A hydrocarbon X burns completely to give carbon dioxide and water. When 0.10 mol of X is burned, 0.30 mol of CO₂ and 0.30 mol of H₂O are formed. What is the molecular formula of X?

  1. C₃H₆ Correct answer.
  2. C₃H₄
  3. C₂H₆
  4. C₃H₈

Explanation

The number of carbon atoms equals the moles of CO₂ per mole of X, giving 3 carbons. The number of hydrogen atoms is twice the moles of H₂O per mole of X, giving 6 hydrogens, so the formula is C₃H₆.

Question 9Chemistry (Category A Senior Secondary Elective Subject)

Which statement about the bonding in a molecule of carbon dioxide is correct?

  1. It contains two polar C=O bonds and is a polar molecule overall.
  2. It contains ionic bonds between carbon and oxygen.
  3. It contains two polar C=O bonds and is a non-polar molecule overall. Correct answer.
  4. It contains only non-polar bonds and is a non-polar molecule.

Explanation

Each C=O bond is polar because oxygen is more electronegative than carbon. Carbon dioxide is linear, so the two bond dipoles are equal and opposite and cancel out, giving a net dipole moment of zero and a non-polar molecule.

Question 10Chemistry (Category A Senior Secondary Elective Subject)

Which of the following gives the correct order of increasing boiling point?

  1. H₂O < CH₄ < NaCl
  2. NaCl < H₂O < CH₄
  3. CH₄ < H₂O < NaCl Correct answer.
  4. CH₄ < NaCl < H₂O

Explanation

Methane is a small non-polar molecule with only weak instantaneous dipole–induced dipole forces, so it boils lowest. Water has hydrogen bonding, giving a much higher boiling point, and sodium chloride has a giant ionic lattice with very strong electrostatic attractions, so it boils highest.

Question 11Chemistry (Category A Senior Secondary Elective Subject)

Which reagent would distinguish between propan-1-ol and propan-2-ol?

  1. Bromine water
  2. Acidified potassium dichromate(VI) solution Correct answer.
  3. Sodium metal
  4. Sodium carbonate solution

Explanation

Propan-1-ol is a primary alcohol and is oxidised to propanal and then propanoic acid, giving a colour change from orange to green with acidified dichromate. Propan-2-ol is a secondary alcohol and is oxidised to propanone, which also gives a colour change, so the products must be distinguished by further tests; however, the standard distinguishing reagent for the two classes is acidified dichromate, used together with product testing.

Question 12Chemistry (Category A Senior Secondary Elective Subject)

Which observation is expected when ethanoic acid is warmed with ethanol in the presence of concentrated sulfuric acid?

  1. A white precipitate of silver chloride forms.
  2. A gas is evolved and the mixture turns cloudy.
  3. A sweet fruity smell develops as ethyl ethanoate forms. Correct answer.
  4. The mixture turns from orange to green.

Explanation

Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters and water. The ester formed from ethanoic acid and ethanol is ethyl ethanoate, which has a characteristic sweet, fruity smell.

Question 13Chemistry (Category A Senior Secondary Elective Subject)

Which of the following is a correct statement about a homologous series?

  1. Members have the same general formula and similar chemical properties, with successive members differing by a CH₂ unit. Correct answer.
  2. Members all have the same molecular formula but different structural formulae.
  3. Members differ by one hydrogen atom in successive molecules.
  4. Members have identical physical properties because they share the same functional group.

Explanation

A homologous series is a family of compounds with the same general formula and the same functional group, so they show similar chemical behaviour. Successive members differ by a CH₂ unit, which gives a gradual change in physical properties such as boiling point.

Question 14Chemistry (Category A Senior Secondary Elective Subject)

Which of these pairs are functional group isomers of each other?

  1. Propane and butane
  2. Ethanol and ethanoic acid
  3. Ethanol and dimethyl ether Correct answer.
  4. But-1-ene and but-2-ene

Explanation

Ethanol (C₂H₆O) and dimethyl ether (CH₃OCH₃, also C₂H₆O) have the same molecular formula but different functional groups, namely an alcohol and an ether. They are therefore functional group isomers.

Question 15Chemistry (Category A Senior Secondary Elective Subject)

In an experiment, a student adds excess magnesium ribbon to 50.0 cm³ of 1.0 mol dm⁻³ hydrochloric acid and records the volume of hydrogen collected over time. Which change would increase the initial rate of reaction without changing the total volume of gas collected?

  1. Using the same mass of magnesium powder instead of ribbon Correct answer.
  2. Cooling the reaction mixture in an ice bath
  3. Adding water to dilute the acid
  4. Using 100 cm³ of 0.5 mol dm⁻³ hydrochloric acid

Explanation

Powdering the magnesium increases the surface area available for reaction, raising the collision frequency and the initial rate. The amount of acid, and therefore the moles of hydrogen produced, is unchanged, so the final gas volume stays the same.

Question 16Chemistry (Category A Senior Secondary Elective Subject)

Which statement explains why increasing temperature increases the rate of a chemical reaction?

  1. The particles become larger and collide more often.
  2. A greater proportion of collisions has energy greater than the activation energy. Correct answer.
  3. The equilibrium constant always increases with temperature.
  4. The activation energy of the reaction decreases as temperature rises.

Explanation

Raising the temperature increases the average kinetic energy of the particles, so a larger fraction of colliding molecules possesses energy equal to or greater than the activation energy. This greatly increases the frequency of successful collisions and hence the reaction rate.

Question 17Chemistry (Category A Senior Secondary Elective Subject)

Which statement about the pH of aqueous solutions is correct?

  1. A solution of pH 3 has a hydrogen ion concentration ten times that of a solution of pH 4. Correct answer.
  2. The pH of a solution increases when it is made more acidic.
  3. A solution of pH 3 is weakly alkaline.
  4. A solution of pH 3 has a hydrogen ion concentration three times that of a solution of pH 4.

Explanation

pH is defined as the negative logarithm of the hydrogen ion concentration, so a decrease of one pH unit corresponds to a tenfold increase in [H⁺]. A pH of 3 therefore has ten times the hydrogen ion concentration of a pH of 4.

Question 18Chemistry (Category A Senior Secondary Elective Subject)

25.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide solution is titrated with 0.0500 mol dm⁻³ sulfuric acid. What volume of the acid is required to reach the end point?

  1. 100 cm³
  2. 25.0 cm³ Correct answer.
  3. 50.0 cm³
  4. 12.5 cm³

Explanation

Moles of NaOH = 0.100 × 0.0250 = 2.50 × 10⁻³ mol. The equation is 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O, so moles of acid needed = 1.25 × 10⁻³ mol. Volume = 1.25 × 10⁻³ / 0.0500 = 0.0250 dm³ = 25.0 cm³.

Question 19Chemistry (Category A Senior Secondary Elective Subject)

In the reaction Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂, which statement is correct?

  1. Chlorine is reduced and acts as the oxidising agent. Correct answer.
  2. No change in oxidation state occurs.
  3. Chlorine is oxidised and acts as the reducing agent.
  4. Bromide ions are oxidised and act as the oxidising agent.

Explanation

Chlorine goes from oxidation state 0 to −1, so it gains electrons and is reduced; the species that is reduced acts as the oxidising agent. Bromide ions lose electrons and are oxidised to bromine.

Question 20Chemistry (Category A Senior Secondary Elective Subject)

Which statement correctly compares the thermal stability of the Group 2 carbonates?

  1. Thermal stability depends only on the mass of the carbonate ion.
  2. Thermal stability increases down the group as the cation becomes larger. Correct answer.
  3. Thermal stability decreases down the group as the cation becomes larger.
  4. All Group 2 carbonates decompose at the same temperature.

Explanation

Down Group 2 the cations become larger and their charge density decreases, so their polarising power on the large carbonate ion falls. This makes the carbonate less easily distorted and decomposed, so thermal stability increases from magnesium carbonate to barium carbonate.

Question 21Chemistry (Category A Senior Secondary Elective Subject)

Chlorine water is added to a solution containing bromide ions and then to a solution containing iodide ions. Which observation is correct?

  1. Both solutions turn brown or orange-brown as the halogens are displaced. Correct answer.
  2. Both solutions turn colourless because chloride ions are formed.
  3. Neither solution changes colour because chlorine is less reactive.
  4. Only the bromide solution changes colour; iodide is unaffected.

Explanation

Chlorine is more reactive than both bromine and iodine, so it displaces them from their halide solutions. The displaced halogens give a brown or orange-brown colour depending on concentration and halogen.

Question 22Chemistry (Category A Senior Secondary Elective Subject)

Which of the following is the correct IUPAC name for CH₃CH(CH₃)CH₂OH?

  1. 2-methylpropan-1-ol Correct answer.
  2. 2-methylpropan-2-ol
  3. 2-methylbutan-1-ol
  4. butan-1-ol

Explanation

The longest chain containing the OH group has three carbons, so the parent is propan-1-ol. A methyl group is attached to carbon 2, giving 2-methylpropan-1-ol.

Question 23Chemistry (Category A Senior Secondary Elective Subject)

A sample of a hydrated salt, MgSO₄·xH₂O, has a molar mass of 246 g mol⁻¹. Given that the molar mass of anhydrous MgSO₄ is 120 g mol⁻¹, what is the value of x?

  1. 6
  2. 10
  3. 5
  4. 7 Correct answer.

Explanation

The mass due to water is 246 − 120 = 126 g mol⁻¹. Dividing by the molar mass of water (18 g mol⁻¹) gives x = 126 / 18 = 7, so the salt is MgSO₄·7H₂O.

Question 24Chemistry (Category A Senior Secondary Elective Subject)

Which statement explains why diamond is extremely hard while graphite is soft and slippery?

  1. Diamond contains ionic bonds, whereas graphite contains only covalent bonds.
  2. Diamond contains free electrons that hold the atoms rigidly in place.
  3. Diamond has a rigid three-dimensional covalent network, whereas graphite has layers held by weak forces that slide. Correct answer.
  4. Graphite has a higher melting point than diamond, which makes it softer.

Explanation

In diamond each carbon atom is covalently bonded to four others in a rigid tetrahedral network, so the structure resists deformation. In graphite each carbon is bonded to three others in layers, and the layers are held by weak van der Waals forces that allow them to slide over one another.

Question 25Chemistry (Category A Senior Secondary Elective Subject)

A reaction has an activation energy of 50 kJ mol⁻¹ without a catalyst and 20 kJ mol⁻¹ with a catalyst. Which statement is correct?

  1. The catalyst increases the equilibrium constant of the reaction.
  2. The catalyst provides an alternative pathway of lower activation energy, increasing the fraction of successful collisions. Correct answer.
  3. The catalyst increases the activation energy, so more collisions are successful.
  4. The catalyst increases the enthalpy change of the reaction.

Explanation

A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions has sufficient energy to react. This increases the rate of both the forward and reverse reactions, so the position of equilibrium is unchanged.

Question 26Chemistry (Category A Senior Secondary Elective Subject)

Which particle is present in the nucleus of a ³⁵Cl atom but absent from the nucleus of a ³⁷Cl atom?

  1. An electron
  2. An alpha particle
  3. A proton
  4. A neutron Correct answer.

Explanation

³⁵Cl and ³⁷Cl are isotopes of chlorine, so both have 17 protons and 17 electrons. The mass number difference of 2 arises from neutrons: ³⁵Cl has 18 neutrons while ³⁷Cl has 20 neutrons. Therefore the neutron is the particle counted differently between the two nuclei.

Question 27Chemistry (Category A Senior Secondary Elective Subject)

An element has the electronic configuration 2,8,7. In which position of the periodic table does it appear?

  1. Group 7, Period 2
  2. Group 7, Period 3 Correct answer.
  3. Group 2, Period 7
  4. Group 17, Period 2

Explanation

The number of occupied electron shells gives the period, so three shells (2,8,7) mean Period 3. Seven outer-shell electrons place the element in Group 7, counting the s and p block convention used in the HKDSE syllabus. This identifies the element as chlorine.

Question 28Chemistry (Category A Senior Secondary Elective Subject)

Which type of bonding is present in solid sodium chloride?

  1. Ionic bonding Correct answer.
  2. Covalent bonding
  3. Hydrogen bonding
  4. Metallic bonding

Explanation

Sodium chloride is formed from Na⁺ and Cl⁻ ions held together by strong electrostatic attraction in a giant ionic lattice. Metal atoms transfer electrons to non-metal atoms, producing ions rather than shared electron pairs. Hydrogen bonding is an intermolecular force, not the bonding within an ionic solid.

Question 29Chemistry (Category A Senior Secondary Elective Subject)

Which change represents oxidation of the carbon-containing species?

  1. CH₄ being converted to CH₃Cl
  2. CH₃COOH being converted to CH₃CHO
  3. CH₃CH₂OH being converted to CH₃CHO
  4. CH₃CHO being converted to CH₃COOH Correct answer.

Explanation

Oxidation of an organic species can be tracked by the increase in the number of oxygen atoms or the decrease in the number of hydrogen atoms attached to carbon. Ethanal (CH₃CHO) gains an oxygen atom on conversion to ethanoic acid (CH₃COOH), which is oxidation. This reaction is typical of aldehydes with acidified dichromate.

Question 30Chemistry (Category A Senior Secondary Elective Subject)

A gas occupies 240 cm³ at room temperature and pressure. What amount of the gas is present, taking molar volume as 24 dm³ mol⁻¹ at r.t.p.?

  1. 0.10 mol
  2. 0.24 mol
  3. 1.0 mol
  4. 0.010 mol Correct answer.

Explanation

At r.t.p. one mole of any gas occupies about 24 dm³, which equals 24 000 cm³. Dividing the given volume by the molar volume gives 240 ÷ 24 000 = 0.010 mol. The units must be consistent before dividing.

Question 31Chemistry (Category A Senior Secondary Elective Subject)

Which statement describes the trend in reactivity of the Group 1 metals down the group?

  1. Reactivity increases because the outer electron is lost more easily as atomic size increases Correct answer.
  2. Reactivity increases because the number of outer electrons increases down the group
  3. Reactivity stays the same because all Group 1 metals have one outer electron
  4. Reactivity decreases because the atoms become larger and the outer electron is further from the nucleus

Explanation

Down Group 1 the atoms get larger and the outer electron is increasingly shielded from the nucleus, so less energy is needed to remove it. The ionisation energy falls and the metals react more vigorously with water and oxygen. This is why potassium is more reactive than sodium.

Question 32Chemistry (Category A Senior Secondary Elective Subject)

Which pair of half-equations can be combined to give the overall reaction Zn + Cu²⁺ → Zn²⁺ + Cu?

  1. Zn²⁺ + 2e⁻ → Zn and Cu → Cu²⁺ + 2e⁻
  2. Zn → Zn²⁺ + 2e⁻ and Cu → Cu²⁺ + 2e⁻
  3. Zn²⁺ + 2e⁻ → Zn and Cu²⁺ + 2e⁻ → Cu
  4. Zn → Zn²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu Correct answer.

Explanation

Zinc is oxidised, losing two electrons, and copper(II) ions are reduced by gaining those electrons. Adding the two half-equations cancels the electrons and gives the overall equation. The electron counts match directly because both changes involve two electrons.

Question 33Chemistry (Category A Senior Secondary Elective Subject)

Which statement about the reaction between an alkene and hydrogen in the presence of a nickel catalyst is correct?

  1. It is a substitution reaction and produces a halogenoalkane
  2. It is an addition reaction and converts the alkene into an alkane Correct answer.
  3. It is an oxidation reaction and produces a carboxylic acid
  4. It is an elimination reaction and produces a shorter alkene

Explanation

Alkenes contain a C=C double bond that can open to add atoms across it. Hydrogen adds across the double bond with a nickel catalyst, giving a saturated alkane, for example ethene to ethane. This is the characteristic addition reaction of unsaturated compounds.

Question 34Chemistry (Category A Senior Secondary Elective Subject)

Which reagent gives a yellow-orange precipitate when warmed with an aldehyde but gives no such precipitate with a ketone?

  1. Sodium carbonate solution
  2. Bromine water
  3. Fehling's solution Correct answer.
  4. 2,4-dinitrophenylhydrazine solution

Explanation

Fehling's solution contains copper(II) ions that are reduced to copper(I) oxide, a brick-red precipitate, by aldehydes on warming. Ketones are not oxidised in this way, so no precipitate forms. This test therefore distinguishes aldehydes from ketones.

Question 35Chemistry (Category A Senior Secondary Elective Subject)

Which statement about the rate of an enzyme-catalysed reaction as temperature rises from 10 °C to 60 °C is correct?

  1. The rate increases up to an optimum and then falls sharply as the enzyme is denatured Correct answer.
  2. The rate increases steadily and is highest at 60 °C
  3. The rate falls steadily because higher temperature reduces collisions
  4. The rate is unaffected by temperature because enzymes are catalysts

Explanation

Enzymes are proteins with a specific active-site shape. Raising temperature increases collision frequency so the rate rises, but beyond the optimum the protein structure is disrupted, the active site changes shape and the enzyme is denatured, so the rate falls sharply.

Question 36Chemistry (Category A Senior Secondary Elective Subject)

Which statement best explains why a reaction reaches dynamic equilibrium in a closed system?

  1. The forward reaction stops while the reverse reaction continues
  2. The activation energy of the forward reaction becomes zero
  3. The reactants are completely used up so no further change occurs
  4. The forward and reverse reactions proceed at equal rates so concentrations remain constant Correct answer.

Explanation

At dynamic equilibrium the forward and reverse reactions continue but at the same rate, so the concentrations of reactants and products stay constant. The system is closed so no substances escape. Both directions remain active at the molecular level.

Question 37Chemistry (Category A Senior Secondary Elective Subject)

Which change would increase the yield of ammonia in the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH negative?

  1. Decreasing the pressure and decreasing the temperature
  2. Increasing the temperature and decreasing the pressure
  3. Increasing the temperature and increasing the pressure
  4. Increasing the pressure and decreasing the temperature Correct answer.

Explanation

Four moles of gas react to give two moles of ammonia, so higher pressure shifts the equilibrium towards the side with fewer gas molecules, favouring ammonia. The forward reaction is exothermic, so lowering the temperature shifts the equilibrium in the exothermic direction, also increasing the ammonia yield.

Question 38Chemistry (Category A Senior Secondary Elective Subject)

Which property of water is mainly responsible for its unusually high boiling point compared with hydrogen sulfide?

  1. The presence of delocalised electrons in water
  2. The low molar mass of water
  3. The ionic nature of the O–H bond
  4. Extensive hydrogen bonding between water molecules Correct answer.

Explanation

Oxygen is highly electronegative and small, so water molecules form strong hydrogen bonds with each other. These intermolecular forces require considerable energy to overcome, raising the boiling point. Hydrogen sulfide molecules form only weak dipole–dipole and dispersion forces.

Question 39Chemistry (Category A Senior Secondary Elective Subject)

Which statement about the electrical conductivity of graphite is correct?

  1. Graphite does not conduct electricity because all four valence electrons are used in bonding
  2. Graphite conducts because it contains strong covalent bonds between layers
  3. Graphite conducts because each carbon atom has a delocalised electron in the layered structure Correct answer.
  4. Graphite conducts because its ions are free to move in the solid state

Explanation

In graphite each carbon is bonded to three others, leaving one electron per atom delocalised across the layer. These mobile electrons allow graphite to conduct electricity along the layers. The weak forces between layers do not provide charge carriers.

Question 40Chemistry (Category A Senior Secondary Elective Subject)

Which statement about the reaction of chlorine with cold dilute sodium hydroxide is correct?

  1. It produces only sodium chloride and hydrogen gas
  2. It produces sodium chlorate(V) and water
  3. It produces sodium chloride and oxygen gas
  4. It produces sodium chloride and sodium chlorate(I) through disproportionation Correct answer.

Explanation

Chlorine reacts with cold dilute alkali to give chloride ions and chlorate(I) ions. Chlorine is both oxidised and reduced in the same reaction, so this is a disproportionation. The reaction is used to make household bleach.

Question 41Chemistry (Category A Senior Secondary Elective Subject)

Which observation confirms that a colourless gas is carbon dioxide?

  1. It turns moist blue litmus paper red and then bleaches it
  2. It burns with a squeaky pop
  3. It relights a glowing splint
  4. It turns limewater milky and the milkiness disappears with excess gas Correct answer.

Explanation

Carbon dioxide reacts with calcium hydroxide solution to form insoluble calcium carbonate, which makes limewater milky. With excess carbon dioxide, soluble calcium hydrogencarbonate forms and the milkiness clears. This is the standard confirmatory test for carbon dioxide.

Question 42Chemistry (Category A Senior Secondary Elective Subject)

What is the oxidation number of chromium in the dichromate ion, Cr₂O₇²⁻?

  1. +3
  2. +6 Correct answer.
  3. +7
  4. +12

Explanation

The sum of oxidation numbers in an ion equals the charge on the ion. With seven oxygen atoms at −2 each, the total from oxygen is −14, and the ion has a charge of −2. Letting each chromium be x, 2x − 14 = −2, so x = +6.

Question 43Chemistry (Category A Senior Secondary Elective Subject)

Which statement about the molar enthalpy change of neutralisation of a strong acid by a strong alkali is correct?

  1. It is approximately constant for different strong acids and alkalis because the reaction is essentially H⁺ + OH⁻ → H₂O Correct answer.
  2. It is always exactly the same as the enthalpy of combustion of hydrogen
  3. It varies widely with the identity of the strong acid used
  4. It is zero because neutralisation is not exothermic

Explanation

In dilute solution strong acids and alkalis are fully ionised, so the only reaction occurring is the combination of hydrogen ions and hydroxide ions to form water. Because the same reaction happens in each case, the enthalpy change per mole of water formed is roughly constant. Weak acids give a smaller value because energy is used in ionising them.

Question 44Chemistry (Category A Senior Secondary Elective Subject)

Which statement about the use of a catalyst in an industrial process is correct?

  1. It increases the yield of the product at equilibrium
  2. It is used up completely during the reaction
  3. It provides an alternative pathway with lower activation energy and is not consumed overall Correct answer.
  4. It increases the enthalpy change of the reaction

Explanation

A catalyst lowers the activation energy by providing an alternative reaction pathway, so a greater fraction of collisions is successful and the rate increases. It is not used up overall and does not change the position of equilibrium or the enthalpy change.

Question 45Chemistry (Category A Senior Secondary Elective Subject)

Which structural feature is shared by all members of the homologous series of alcohols?

  1. A carboxyl group attached to a carbon chain
  2. A C=C double bond in the carbon chain
  3. A carbonyl group attached to a carbon chain
  4. A hydroxyl group attached to a saturated carbon chain Correct answer.

Explanation

Alcohols are characterised by an –OH group bonded to a carbon atom of a saturated hydrocarbon chain. Successive members differ by a CH₂ unit. The general formula for acyclic alcohols is CₙH₂ₙ₊₁OH.

Question 46Chemistry (Category A Senior Secondary Elective Subject)

Which method is suitable for separating ethanol from a mixture of ethanol and water?

  1. Crystallisation
  2. Fractional distillation Correct answer.
  3. Filtration
  4. Using a separating funnel

Explanation

Ethanol and water are miscible liquids with different boiling points, so fractional distillation can separate them. The more volatile ethanol distils over first. Filtration and crystallisation separate solids from liquids.

Question 47Chemistry (Category A Senior Secondary Elective Subject)

25.0 cm³ of ethanoic acid solution of concentration 0.200 mol dm⁻³ is exactly neutralised by 20.0 cm³ of sodium hydroxide solution. What is the concentration of the sodium hydroxide solution?

  1. 0.625 mol dm⁻³
  2. 0.160 mol dm⁻³
  3. 0.250 mol dm⁻³ Correct answer.
  4. 0.200 mol dm⁻³

Explanation

Ethanoic acid and sodium hydroxide react in a 1:1 mole ratio. Moles of acid = 0.200 × 0.0250 = 0.00500 mol, so 0.00500 mol of alkali is needed. Dividing by its volume, 0.00500 ÷ 0.0200 = 0.250 mol dm⁻³.

Question 48Chemistry (Category A Senior Secondary Elective Subject)

Which statement explains why the first ionisation energy of magnesium is higher than that of sodium?

  1. Magnesium has a full outer shell, so its electron is harder to remove
  2. Sodium has a larger nuclear charge than magnesium
  3. The magnesium nucleus has one more proton and the outer electron is in the same shell, so it is held more strongly Correct answer.
  4. Magnesium has more electron shells, so its outer electron is further from the nucleus

Explanation

Across Period 3 the outer electrons are in the same shell, so shielding is similar, but the nuclear charge increases from sodium to magnesium. The stronger attraction holds the outer electron more tightly, so more energy is needed to remove it.

Question 49Chemistry (Category A Senior Secondary Elective Subject)

Which statement about the polymer formed from ethene is correct?

  1. It is formed by joining monomers with the loss of a small molecule
  2. It is an unsaturated polymer formed by condensation with water as a by-product
  3. It contains ester linkages in its backbone
  4. It is a saturated addition polymer with the repeating unit –CH₂–CH₂– Correct answer.

Explanation

Ethene molecules add to one another across their C=C double bonds to form poly(ethene), a saturated addition polymer. No small molecule is eliminated during addition polymerisation. The repeating unit is –CH₂–CH₂–.

Question 50Chemistry (Category A Senior Secondary Elective Subject)

A student wants to prepare a pure dry sample of copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid. Which sequence of steps is most appropriate?

  1. Add excess copper(II) oxide to warm acid, filter off the excess solid, then crystallise the filtrate Correct answer.
  2. Add copper(II) oxide to acid and filter the mixture to collect the residue as the product
  3. Add copper(II) oxide to acid and distil the mixture to collect the crystals
  4. Add excess acid to copper(II) oxide, then evaporate to dryness without filtering

Explanation

Adding excess copper(II) oxide ensures all the acid reacts, and the unreacted solid is removed by filtration. The filtrate contains copper(II) sulfate, which can be crystallised by slow evaporation. This avoids contaminating the product with unreacted oxide or acid.