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CAPE Chemistry Unit 1 study materials
CAPE Chemistry Unit 1 study resources

CAPE Chemistry Unit 1 practice questions

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Question 1Chemistry (CAPE Unit 1 content domains not enumerated in source)

An element has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁴. In which group and period of the periodic table does it appear?

  1. Group 6, Period 3
  2. Group 16, Period 4
  3. Group 14, Period 3
  4. Group 16, Period 3 Correct answer.

Explanation

The configuration ends in 3s² 3p⁴, so the highest occupied shell is n = 3, placing the element in Period 3. The valence-shell count is 2 + 4 = 6 electrons, and for p-block elements this equals the group number offset by ten, giving Group 16. The element is sulfur.

Question 2Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which statement correctly describes the trend in first ionisation energy across Period 3 from sodium to argon?

  1. It falls steadily because atomic radius increases
  2. It rises overall, with a dip at aluminium and at sulfur Correct answer.
  3. It rises steadily with no exceptions
  4. It stays constant because nuclear charge is shielded completely

Explanation

Across a period the nuclear charge increases while electrons enter the same shell, so the general trend is upward. Two dips occur: at aluminium, where the electron removed comes from the higher-energy 3p orbital, and at sulfur, where pairing in a 3p orbital increases electron–electron repulsion.

Question 3Chemistry (CAPE Unit 1 content domains not enumerated in source)

A gaseous ion X²⁻ has the electron configuration 1s² 2s² 2p⁶. What is the proton number of element X?

  1. 10
  2. 6
  3. 8 Correct answer.
  4. 12

Explanation

The configuration contains 10 electrons in total. The ion carries a 2− charge, so it has two electrons more than the neutral atom. The neutral atom therefore has 8 electrons and, being neutral, 8 protons, which identifies oxygen.

Question 4Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which pair of species is isoelectronic?

  1. Na⁺ and Ne
  2. F⁻ and O
  3. Mg²⁺ and Ar
  4. K⁺ and Cl⁻ Correct answer.

Explanation

Isoelectronic species contain the same total number of electrons. K⁺ has 18 electrons and Cl⁻ has 18 electrons, so they are isoelectronic; both match the argon configuration.

Question 5Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which molecule has a bond angle closest to 104.5°?

  1. CO₂
  2. NH₃
  3. CH₄
  4. H₂O Correct answer.

Explanation

Water has two bonding pairs and two lone pairs on oxygen, giving a bent shape with a bond angle of about 104.5°. The lone pairs repel more strongly than bonding pairs, compressing the tetrahedral angle.

Question 6Chemistry (CAPE Unit 1 content domains not enumerated in source)

According to VSEPR theory, what shape and bond angle are predicted for the SF₄ molecule?

  1. See-saw, with angles near 89° and 117° Correct answer.
  2. Trigonal bipyramidal, with angles of 90° and 120°
  3. Square planar, with angles of 90°
  4. Tetrahedral, with angles of 109.5°

Explanation

Sulfur in SF₄ has five electron pairs: four bonding pairs and one lone pair. The lone pair occupies an equatorial position of the trigonal bipyramid, so the molecular shape is a see-saw with F–S–F angles of roughly 89° and 117°.

Question 7Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which molecule is polar overall despite containing polar bonds?

  1. BF₃
  2. NH₃ Correct answer.
  3. CCl₄
  4. CO₂

Explanation

A molecule is polar only when its bond dipoles do not cancel. Ammonia is trigonal pyramidal, so the three N–H dipoles add to give a net dipole along the lone-pair axis.

Question 8Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which intermolecular force is chiefly responsible for the relatively high boiling point of ethanol compared with ethane of similar molar mass?

  1. Hydrogen bonding Correct answer.
  2. London dispersion forces
  3. Ion–dipole attraction
  4. Metallic bonding

Explanation

Ethanol contains an O–H group, so its molecules form hydrogen bonds through the hydrogen attached to the electronegative oxygen. Breaking these stronger attractions requires more energy, raising the boiling point above that of ethane.

Question 9Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which structure is the correct displayed formula for propan-2-ol?

  1. CH₃CH₂CH₂OH
  2. CH₃CH₂OH
  3. CH₃CH(OH)CH₃ Correct answer.
  4. CH₃OCH₂CH₃

Explanation

Propan-2-ol is a secondary alcohol in which the hydroxyl group sits on the middle carbon of a three-carbon chain. Its condensed formula is CH₃CH(OH)CH₃, with the OH attached to carbon-2.

Question 10Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which pair of compounds are functional group isomers of each other?

  1. Propanal and propanone Correct answer.
  2. Ethanol and ethanal
  3. Propene and propane
  4. Butane and 2-methylpropane

Explanation

Functional group isomers have the same molecular formula but different functional groups. Propanal (an aldehyde) and propanone (a ketone) both have the formula C₃H₆O but differ in the position of the carbonyl group within the functional class.

Question 11Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which reagent and condition are used to convert propan-1-ol into propanoic acid?

  1. Acidified potassium dichromate(VI), distilled immediately
  2. Sodium metal at room temperature
  3. Acidified potassium dichromate(VI), heated under reflux Correct answer.
  4. Concentrated sulfuric acid at 170 °C

Explanation

A primary alcohol is oxidised through the aldehyde to the carboxylic acid by acidified dichromate(VI). Heating under reflux keeps the aldehyde in contact with the oxidising agent so that further oxidation to the acid occurs.

Question 12Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which test distinguishes an aldehyde from a ketone?

  1. Adding sodium carbonate solution, which fizzes with the aldehyde
  2. Warming with Fehling's solution, which gives a red precipitate with the aldehyde Correct answer.
  3. Adding 2,4-dinitrophenylhydrazine, which gives a yellow precipitate only with the aldehyde
  4. Shaking with bromine water, which decolourises with the ketone

Explanation

Aldehydes are readily oxidised, so they reduce the copper(II) complex in Fehling's solution to a brick-red precipitate of copper(I) oxide. Ketones are not oxidised under these mild conditions and give no precipitate.

Question 13Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which product forms when hydrogen bromide reacts with propene by electrophilic addition?

  1. 2-bromopropane as the major product Correct answer.
  2. 1,2-dibromopropane
  3. Propan-2-ol
  4. 1-bromopropane as the major product

Explanation

The addition follows Markovnikov's rule: the electrophile, H⁺, adds to the carbon with more hydrogen atoms to give the more stable secondary carbocation. Bromide then attacks that carbocation, giving 2-bromopropane as the major product.

Question 14Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which reagent and condition bring about the hydration of ethene to ethanol?

  1. Bromine water at room temperature
  2. Acidified dichromate(VI) under reflux
  3. Steam with a phosphoric acid catalyst at high temperature and pressure Correct answer.
  4. Cold dilute sulfuric acid alone

Explanation

Industrial hydration passes ethene and steam over a phosphoric acid catalyst at around 300 °C and 60 atm. The acid provides protons that protonate the alkene, generating a carbocation that water attacks.

Question 15Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which statement about the mechanism of the reaction between bromine and ethene is correct?

  1. The bromine molecule is polarised as it approaches the alkene, generating a bromonium ion intermediate Correct answer.
  2. A carbocation forms first and is then attacked by a bromide ion
  3. A bromine radical attacks the alkene directly
  4. The alkene acts as an electrophile throughout

Explanation

The π electrons of ethene repel the electrons in the approaching Br₂ molecule, inducing a dipole. The nearer bromine becomes electrophilic and bonds to both carbons to give a cyclic bromonium ion, which is then opened by bromide ion.

Question 16Chemistry (CAPE Unit 1 content domains not enumerated in source)

What is the empirical formula of a compound containing 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass?

  1. CH₂O Correct answer.
  2. C₃H₆O₃
  3. C₂H₄O₂
  4. CHO

Explanation

Dividing each percentage by the atomic mass gives 3.33 mol C, 6.7 mol H and 3.33 mol O. Dividing by the smallest value gives the ratio 1 : 2 : 1, so the empirical formula is CH₂O.

Question 17Chemistry (CAPE Unit 1 content domains not enumerated in source)

A gas has a density of 1.96 g dm⁻³ at 273 K and 101 kPa. What is its molar mass?

  1. 22 g mol⁻¹
  2. 88 g mol⁻¹
  3. 32 g mol⁻¹
  4. 44 g mol⁻¹ Correct answer.

Explanation

At 273 K and 101 kPa, one mole of gas occupies about 22.4 dm³. Multiplying the density by this molar volume gives 1.96 × 22.4 ≈ 44 g mol⁻¹, consistent with carbon dioxide.

Question 18Chemistry (CAPE Unit 1 content domains not enumerated in source)

In the reaction 2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 5Cl₂ + 8H₂O, which element is oxidised and which is reduced?

  1. Chlorine is oxidised and manganese is reduced Correct answer.
  2. Potassium is oxidised and oxygen is reduced
  3. Manganese is oxidised and chlorine is reduced
  4. Hydrogen is oxidised and chlorine is reduced

Explanation

Chloride ions in HCl lose electrons to form chlorine molecules, so chlorine is oxidised. Manganese in permanganate goes from +7 to +2, gaining electrons, so it is reduced.

Question 19Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which quantity of electrons is transferred when 0.50 mol of copper(II) ions is reduced to copper metal?

  1. 0.50 mol of electrons
  2. 6.0 × 10²³ mol of electrons
  3. 1.0 mol of electrons Correct answer.
  4. 2.0 mol of electrons

Explanation

Each copper(II) ion gains two electrons, so one mole of Cu²⁺ requires two moles of electrons. For 0.50 mol of ions the transfer is 0.50 × 2 = 1.0 mol of electrons.

Question 20Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which statement about the lattice enthalpy of an ionic compound is correct?

  1. It is unaffected by ionic radius
  2. It becomes more endothermic as ionic charge increases
  3. It depends only on the number of electrons in the anion
  4. It becomes more exothermic as ionic radius decreases and ionic charge increases Correct answer.

Explanation

Lattice enthalpy depends on the electrostatic attraction between ions, which grows as the charges increase and as the ions get closer together. Smaller ions and higher charges therefore give more exothermic lattice enthalpies.

Question 21Chemistry (CAPE Unit 1 content domains not enumerated in source)

Using the bond enthalpies H–H = 436, Cl–Cl = 243 and H–Cl = 431 kJ mol⁻¹, what is the enthalpy change for H₂ + Cl₂ → 2HCl?

  1. −183 kJ mol⁻¹ Correct answer.
  2. +183 kJ mol⁻¹
  3. −1210 kJ mol⁻¹
  4. −431 kJ mol⁻¹

Explanation

Bonds broken cost 436 + 243 = 679 kJ mol⁻¹, and bonds formed release 2 × 431 = 862 kJ mol⁻¹. The net change is 679 − 862 = −183 kJ mol⁻¹, so the reaction is exothermic.

Question 22Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which statement explains why the second ionisation energy of magnesium is much larger than the first?

  1. The second electron experiences greater shielding from the nucleus
  2. The second electron is removed from a p orbital rather than an s orbital
  3. The second electron is removed from a more positive ion and from a lower-energy shell Correct answer.
  4. The second electron is removed from the same orbital as the first

Explanation

After the first electron is lost the ion has a 2+ charge, so the remaining electrons are held more strongly. The second electron also comes from the 2p subshell, which is closer to the nucleus and lower in energy than the 3s electron removed first.

Question 23Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which observation shows that a sample of ethanoic acid is a weak acid?

  1. Its solution has a pH of about 3 at 0.1 mol dm⁻³ Correct answer.
  2. It conducts electricity
  3. It reacts with magnesium to give hydrogen
  4. Its solution turns blue litmus red

Explanation

A strong monobasic acid at 0.1 mol dm⁻³ would have a pH near 1. A pH of about 3 shows that only a small fraction of the acid molecules has dissociated, which is the defining behaviour of a weak acid.

Question 24Chemistry (CAPE Unit 1 content domains not enumerated in source)

A buffer solution is prepared from ethanoic acid and sodium ethanoate. Which statement best explains how it resists a change in pH on addition of a small amount of acid?

  1. Sodium ions remove the added hydrogen ions by forming sodium hydride
  2. Hydroxide ions neutralise the added hydrogen ions directly
  3. Ethanoate ions combine with the added hydrogen ions to form more ethanoic acid Correct answer.
  4. Ethanoic acid molecules dissociate further to release hydrogen ions

Explanation

A buffer contains a reservoir of conjugate base. When acid is added, the ethanoate ions accept the extra protons to form weakly ionised ethanoic acid, so the hydrogen ion concentration changes very little.

Question 25Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which technique is most suitable for separating a mixture of two miscible liquids with boiling points of 78 °C and 140 °C?

  1. Fractional distillation Correct answer.
  2. Paper chromatography
  3. Filtration
  4. Crystallisation

Explanation

Miscible liquids with a large difference in boiling point are separated by fractional distillation, in which repeated vaporisation and condensation in the fractionating column enriches the vapour in the more volatile component.

Question 26Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which subatomic particle determines the identity of an element in the periodic table?

  1. The number of neutrons in the nucleus
  2. The number of protons in the nucleus Correct answer.
  3. The number of valence electrons present
  4. The number of electron shells occupied

Explanation

The atomic number, equal to the number of protons, is the property that defines which element an atom belongs to. Changing the proton count converts the atom into a different element entirely, whereas neutrons and electron arrangements can vary without changing identity.

Question 27Chemistry (CAPE Unit 1 content domains not enumerated in source)

An atom of an element has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁴. In which block and group of the periodic table does the element lie?

  1. d block, group 16
  2. s block, group 14
  3. p block, group 14
  4. p block, group 16 Correct answer.

Explanation

The highest-energy electrons occupy 3p orbitals, so the element belongs to the p block. Counting the two 3s and four 3p electrons gives six valence electrons, which places it in group 16 (oxygen family), consistent with sulfur.

Question 28Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which species has the same electron configuration as the neon atom?

  1. Ar
  2. Mg²⁺ Correct answer.
  3. Cl⁻
  4. Na⁺

Explanation

A magnesium atom has 12 electrons; losing two gives Mg²⁺ with 10 electrons, identical to neon's configuration. The isoelectronic series around neon includes Na⁺, F⁻ and O²⁻ but not species with 18 electrons.

Question 29Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which type of bonding is present in solid sodium chloride?

  1. Sharing of electron pairs between adjacent atoms
  2. Delocalised electrons in a lattice of positive ions
  3. Weak instantaneous dipoles between molecules
  4. Electrostatic attraction between oppositely charged ions Correct answer.

Explanation

Sodium chloride is an ionic lattice in which Na⁺ and Cl⁻ ions are held by strong electrostatic attractions in all directions. This explains its high melting point, brittleness and electrical conduction when molten or dissolved.

Question 30Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which molecule contains a bond angle of approximately 104.5°?

  1. CO₂
  2. BF₃
  3. CH₄
  4. H₂O Correct answer.

Explanation

Water has two bonding pairs and two lone pairs on oxygen, giving a bent shape with a bond angle of about 104.5° because lone-pair repulsion compresses the ideal tetrahedral angle. The other molecules have no lone pairs on the central atom and show larger angles.

Question 31Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which property is characteristic of a covalent network solid such as diamond?

  1. Electrical conduction when molten but not when solid
  2. Very high melting point and no electrical conduction in the solid state Correct answer.
  3. Formation of a lattice of ions that conducts when dissolved
  4. Low melting point and solubility in non-polar solvents

Explanation

Diamond consists of carbon atoms joined by an extensive network of strong covalent bonds, so melting requires breaking many bonds, giving an extremely high melting point. All valence electrons are localised in sigma bonds, so the solid does not conduct electricity.

Question 32Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which statement best explains why the boiling point of hydrogen fluoride is higher than that of hydrogen chloride?

  1. Hydrogen chloride is a non-polar molecule
  2. Hydrogen fluoride molecules form hydrogen bonds between them Correct answer.
  3. Hydrogen chloride has stronger London dispersion forces
  4. Hydrogen fluoride has a larger relative molecular mass

Explanation

Fluorine is highly electronegative and small, so the H–F bond is strongly polar and hydrogen bonding occurs between molecules. These additional strong intermolecular attractions require more energy to overcome than the dipole and dispersion forces in hydrogen chloride.

Question 33Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which pair of species are structural isomers?

  1. Propene and propane
  2. Propane and butane
  3. Ethanol and ethanal
  4. Butan-1-ol and butan-2-ol Correct answer.

Explanation

Structural isomers share the same molecular formula but differ in the arrangement of atoms. Butan-1-ol and butan-2-ol both have the formula C₄H₁₀O, but the hydroxyl group occupies a different carbon, giving different physical and chemical behaviour.

Question 34Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which reagent and condition are required to convert butan-2-one into butan-2-ol?

  1. Bromine water at room temperature
  2. Sodium tetrahydridoborate(III) in aqueous solution Correct answer.
  3. Acidified potassium dichromate(VI) under reflux
  4. Concentrated sulfuric acid at 170 °C

Explanation

Butan-2-one is a ketone, and reduction of the carbonyl group converts it to a secondary alcohol. Sodium tetrahydridoborate(III) (sodium borohydride) supplies hydride ions and is mild enough to reduce ketones without attacking other groups.

Question 35Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which mechanism describes the reaction of ethanal with hydrogen cyanide in the presence of a trace of alkali?

  1. Electrophilic addition
  2. Nucleophilic substitution
  3. Free-radical substitution
  4. Nucleophilic addition Correct answer.

Explanation

Cyanide ions act as nucleophiles and attack the electron-deficient carbonyl carbon, forming a tetrahedral intermediate that is protonated to give a hydroxynitrile. The carbonyl carbon is electrophilic because the C=O bond is polarised toward oxygen.

Question 36Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which test result confirms the presence of a carboxylic acid in an unknown liquid?

  1. Effervescence with aqueous sodium hydrogencarbonate Correct answer.
  2. Decolourisation of bromine water
  3. A yellow-orange precipitate with 2,4-dinitrophenylhydrazine
  4. A silver mirror with Tollens' reagent

Explanation

Carboxylic acids are acidic enough to react with sodium hydrogencarbonate, releasing carbon dioxide gas that gives visible effervescence. This distinguishes them from alcohols and most other common functional groups.

Question 37Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which product is formed when ethanoyl chloride reacts with methanol?

  1. Ethyl methanoate and hydrogen
  2. Ethanoic acid and chloromethane
  3. Methyl ethanoate and hydrogen chloride Correct answer.
  4. Methyl ethanoate and water

Explanation

Acyl chlorides react vigorously with alcohols in a nucleophilic acyl substitution, producing an ester and hydrogen chloride. The by-product is HCl rather than water, which is why the reaction is often preferred over acid-catalysed esterification.

Question 38Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which statement about free-radical substitution of methane by chlorine is correct?

  1. The reaction is an addition across the C–H bond
  2. Ultraviolet light provides energy to break the Cl–Cl bond homolytically Correct answer.
  3. The chlorine molecule acts as a nucleophile in the propagation step
  4. The reaction proceeds via a carbocation intermediate

Explanation

Initiation requires UV light to break Cl–Cl homolytically into two chlorine radicals. These radicals then abstract hydrogen from methane in a chain propagation sequence, giving methyl radicals and HCl.

Question 39Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which compound shows cis–trans isomerism?

  1. Propene
  2. 2-methylpropene
  3. But-1-ene
  4. But-2-ene Correct answer.

Explanation

Cis–trans isomerism requires restricted rotation about a C=C bond and two different groups on each carbon of the double bond. In but-2-ene, each alkene carbon bears a hydrogen and a methyl group, so two distinct arrangements exist.

Question 40Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which structural feature is required for a molecule to exhibit optical isomerism?

  1. A carbon–carbon double bond with restricted rotation
  2. Two identical groups on the same carbon atom
  3. A plane of symmetry through the molecule
  4. A chiral carbon bearing four different groups Correct answer.

Explanation

Optical isomerism arises when a molecule is non-superimposable on its mirror image, most commonly because a carbon atom is bonded to four different groups. Such a chiral centre gives two enantiomers that rotate plane-polarised light in opposite directions.

Question 41Chemistry (CAPE Unit 1 content domains not enumerated in source)

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

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

Explanation

Let the oxidation state of chromium be x. Oxygen is normally −2, so the total charge equation is 2x + 7(−2) = −2, giving 2x = 12 and x = +6.

Question 42Chemistry (CAPE Unit 1 content domains not enumerated in source)

In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, which species acts as the oxidising agent?

  1. Zn²⁺
  2. Zn
  3. Cu²⁺ Correct answer.
  4. SO₄²⁻

Explanation

The oxidising agent is the species that is reduced, accepting electrons. Copper(II) ions gain two electrons to form copper metal, so Cu²⁺ is reduced and therefore acts as the oxidising agent.

Question 43Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which equation represents the correct half-equation for the reduction of manganate(VII) ions in acidic solution?

  1. MnO₄⁻ + 8H⁺ → Mn²⁺ + 4H₂O + 5e⁻
  2. MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O Correct answer.
  3. MnO₄⁻ + e⁻ → MnO₄²⁻
  4. MnO₄⁻ + 4H⁺ + 3e⁻ → MnO₂ + 2H₂O

Explanation

In acid, manganate(VII) is reduced from +7 to +2, a change of five electrons, and the oxygen atoms are balanced with water while hydrogen is balanced with H⁺. The balanced half-equation is MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.

Question 44Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which statement about the reaction of magnesium with steam is correct?

  1. Magnesium oxide and hydrogen are produced
  2. Magnesium oxide and oxygen are produced
  3. Magnesium hydroxide and hydrogen are produced Correct answer.
  4. No reaction occurs because magnesium is below hydrogen in reactivity

Explanation

Magnesium reacts with steam to form magnesium oxide and hydrogen gas, with the equation Mg + H₂O → MgO + H₂. The oxide is the solid product, and hydrogen is evolved as a gas.

Question 45Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which property increases down group 2 from magnesium to barium?

  1. Reactivity with water decreases
  2. Atomic radius Correct answer.
  3. Electronegativity
  4. First ionisation energy

Explanation

Down group 2, an extra electron shell is added at each step, so the atomic radius increases despite the rising nuclear charge. The outer electrons are further from the nucleus and more shielded, which also explains the fall in ionisation energy and increase in reactivity.

Question 46Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which observation is expected when chlorine water is added to aqueous potassium bromide?

  1. A white precipitate of potassium chloride forms
  2. A gas is evolved and the solution turns colourless
  3. The solution turns orange-brown as bromine is displaced Correct answer.
  4. No change occurs because bromine is more reactive

Explanation

Chlorine is a stronger oxidising agent than bromine, so it oxidises bromide ions to bromine, which colours the solution orange-brown. The reaction is Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.

Question 47Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which statement explains why the thermal stability of group 17 hydrides decreases from HF to HI?

  1. The H–X bond becomes longer and weaker as the halogen gets larger Correct answer.
  2. The number of lone pairs on the halogen decreases
  3. Hydrogen bonding becomes stronger down the group
  4. The molecules become more polar as the halogen gets larger

Explanation

Down group 17, the halogen atom becomes larger and the overlap between its orbital and hydrogen's 1s orbital becomes poorer, so the H–X bond length increases and bond enthalpy falls. A weaker bond is broken more readily on heating, so thermal stability decreases.

Question 48Chemistry (CAPE Unit 1 content domains not enumerated in source)

A 25.0 cm³ sample of hydrochloric acid is neutralised exactly by 20.0 cm³ of 0.150 mol dm⁻³ sodium hydroxide. What is the concentration of the acid?

  1. 0.075 mol dm⁻³
  2. 0.187 mol dm⁻³
  3. 0.120 mol dm⁻³ Correct answer.
  4. 0.150 mol dm⁻³

Explanation

Moles of NaOH = 0.0200 dm³ × 0.150 mol dm⁻³ = 3.00 × 10⁻³ mol. Since HCl and NaOH react in a 1:1 ratio, the acid also contains 3.00 × 10⁻³ mol in 0.0250 dm³, giving 0.120 mol dm⁻³.

Question 49Chemistry (CAPE Unit 1 content domains not enumerated in source)

A sample of hydrated copper(II) sulfate, CuSO₄·xH₂O, has a molar mass of 249.6 g mol⁻¹. Given CuSO₄ = 159.6 g mol⁻¹ and H₂O = 18.0 g mol⁻¹, what is the value of x?

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

Explanation

The mass due to water is 249.6 − 159.6 = 90.0 g mol⁻¹. Dividing by the molar mass of water, 90.0 ÷ 18.0 = 5, so the formula is CuSO₄·5H₂O.

Question 50Chemistry (CAPE Unit 1 content domains not enumerated in source)

Which statement about the ideal gas equation pV = nRT is correct?

  1. The equation applies only to gases that do not react with each other
  2. Volume must be in cubic decimetres when R = 8.31 J K⁻¹ mol⁻¹
  3. Pressure must be in pascals and volume in cubic metres when R = 8.31 J K⁻¹ mol⁻¹ Correct answer.
  4. Temperature must always be in degrees Celsius

Explanation

The value R = 8.31 J K⁻¹ mol⁻¹ is expressed in SI units, so pressure must be in pascals and volume in cubic metres for consistency. Temperature must be converted to kelvin by adding 273 to the Celsius value.