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PSEB SS Chemistry practice questions

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Question 1Chemistry theory (Senior Secondary)

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

  1. It increases then decreases smoothly with no exceptions at any element.
  2. It generally increases because effective nuclear charge rises while the outer electron is in the same shell. Correct answer.
  3. It stays constant because each element has the same number of shells.
  4. It generally decreases because atomic radius increases across the period.

Explanation

Across Period 3 the nuclear charge increases while electrons are added to the same shell, so the outer electron is held more strongly and ionisation energy rises overall. Small dips occur at aluminium and sulfur, so the trend is general rather than perfectly smooth.

Question 2Chemistry theory (Senior Secondary)

In the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), which change would increase the equilibrium yield of ammonia?

  1. Adding a catalyst, since it shifts the position of equilibrium
  2. Increasing the pressure, since fewer gas moles are on the product side Correct answer.
  3. Removing nitrogen, since this reduces the number of reactants
  4. Raising the temperature, since the forward reaction is exothermic

Explanation

The forward reaction converts four moles of gas into two, so increasing pressure favours the side with fewer gas molecules, raising the ammonia yield. A catalyst only speeds attainment of equilibrium and does not change the position.

Question 3Chemistry theory (Senior Secondary)

Which species is acting as a Brønsted–Lowry acid in the reaction HCO3⁻ + H2O ⇌ H2CO3 + OH⁻?

  1. H2CO3, because it is formed in the reaction
  2. HCO3⁻, because it donates a proton to water Correct answer.
  3. H2O, because it accepts a proton
  4. OH⁻, because it contains hydrogen

Explanation

A Brønsted–Lowry acid is a proton donor. In this reaction the hydrogencarbonate ion transfers a proton to water, forming carbonic acid and hydroxide, so HCO3⁻ is the acid.

Question 4Chemistry theory (Senior Secondary)

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

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

Explanation

Each oxygen is assigned -2, giving a total of -14 from seven oxygens. For the overall charge to be -2, the two chromium atoms must contribute +12, so each chromium is +6.

Question 5Chemistry theory (Senior Secondary)

Which observation is expected when chlorine gas is bubbled through aqueous potassium bromide?

  1. A gas with a bleached smell is released as hydrogen bromide.
  2. No reaction occurs because both are halogens.
  3. The solution turns orange as bromine is displaced. Correct answer.
  4. A white precipitate of potassium chloride forms immediately.

Explanation

Chlorine is more reactive than bromine, so it oxidises bromide ions to bromine, which colours the solution orange. This is a halogen displacement reaction driven by the relative oxidising power of the halogens.

Question 6Chemistry theory (Senior Secondary)

Which structural feature is responsible for the optical activity of a molecule?

  1. A carbon atom bonded to four different groups Correct answer.
  2. An aromatic ring with delocalised electrons
  3. A carbon–carbon double bond that restricts rotation
  4. A plane of symmetry passing through the molecular centre

Explanation

Optical activity arises when a molecule is chiral and cannot be superimposed on its mirror image. A carbon bonded to four different groups is a stereocentre that commonly produces this chirality.

Question 7Chemistry theory (Senior Secondary)

Which reagent and condition would convert propan-2-ol into propanone?

  1. Sodium hydroxide in aqueous solution
  2. Concentrated sulfuric acid at 170 °C
  3. Acidified potassium dichromate(VI) with distillation Correct answer.
  4. Hydrogen gas with a nickel catalyst

Explanation

Propan-2-ol is a secondary alcohol, and acidified dichromate oxidises it to the ketone propanone. Distillation removes the product as it forms, preventing further oxidation.

Question 8Chemistry theory (Senior Secondary)

Why does the boiling point of water exceed that of hydrogen sulfide despite water having a smaller molar mass?

  1. Water has a higher relative molecular mass than hydrogen sulfide.
  2. Water forms hydrogen bonds while hydrogen sulfide has only weaker dipole–dipole forces. Correct answer.
  3. Water molecules are ionic and form a crystal lattice.
  4. Hydrogen sulfide has a larger permanent dipole than water.

Explanation

Oxygen is highly electronegative and small, so water forms strong intermolecular hydrogen bonds. Sulfur is less electronegative and larger, so hydrogen sulfide has only weaker dipole–dipole attractions, giving it a lower boiling point.

Question 9Chemistry theory (Senior Secondary)

A reaction has an activation energy of 50 kJ mol⁻¹ without a catalyst and 30 kJ mol⁻¹ with one. What effect does the catalyst have on the rate constant at a fixed temperature?

  1. It leaves the rate constant unchanged because the temperature is fixed.
  2. It decreases the rate constant because the catalyst lowers the energy released.
  3. It increases the rate constant by raising the activation energy.
  4. It increases the rate constant because a larger fraction of collisions has sufficient energy. Correct answer.

Explanation

The Arrhenius equation shows the rate constant depends exponentially on activation energy. Lowering the activation energy from 50 to 30 kJ mol⁻¹ increases the proportion of molecules with enough energy to react, so the rate constant rises.

Question 10Chemistry theory (Senior Secondary)

Which pair of half-equations combines correctly to give the overall cell reaction for a zinc–copper cell?

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

Explanation

Zinc is the more reactive metal, so it is oxidised at the anode, losing two electrons. Copper(II) ions are reduced at the cathode by gaining those two electrons, and the electrons balance.

Question 11Chemistry theory (Senior Secondary)

Which statement about the shapes of molecules predicted by VSEPR theory is correct?

  1. CH4 has a bond angle of 90° because carbon is tetravalent.
  2. NH3 is trigonal planar because nitrogen forms three single bonds.
  3. H2O is linear because oxygen forms two single bonds.
  4. BF3 is trigonal planar because it has three bonding pairs and no lone pairs on boron. Correct answer.

Explanation

Boron in BF3 has three bonding pairs and no lone pairs, so electron-pair repulsion gives a trigonal planar shape with 120° angles. Lone pairs on ammonia and water distort their shapes away from this.

Question 12Chemistry theory (Senior Secondary)

Which factor explains why the atomic radius of magnesium is smaller than that of sodium?

  1. Magnesium has a smaller nuclear charge than sodium.
  2. Magnesium has more shielding electrons in its outer shell.
  3. Magnesium has an extra shell of electrons.
  4. Magnesium has a greater effective nuclear charge pulling the same shell inward. Correct answer.

Explanation

Sodium and magnesium both place their outer electrons in the third shell. Magnesium has one more proton, so the increased effective nuclear charge contracts the electron cloud, giving a smaller atomic radius.

Question 13Chemistry theory (Senior Secondary)

Which reagent would distinguish ethanal from propanone?

  1. Bromine water, which decolourises with propanone
  2. Sodium metal, which reacts only with propanone
  3. Fehling's solution, which gives a red precipitate with ethanal Correct answer.
  4. Litmus paper, which turns red with ethanal

Explanation

Ethanal is an aldehyde and reduces copper(II) ions in Fehling's solution to a red precipitate of copper(I) oxide. Propanone is a ketone and does not reduce Fehling's solution under these conditions.

Question 14Chemistry theory (Senior Secondary)

Which statement about the entropy change for the vaporisation of a liquid is correct?

  1. ΔS is zero because the process is reversible at the boiling point.
  2. ΔS is positive because gas molecules have more freedom of motion. Correct answer.
  3. ΔS is positive only if the process is exothermic.
  4. ΔS is negative because the gas is more ordered.

Explanation

Entropy measures the dispersal of energy and the number of accessible arrangements. A gas has far more freedom of motion than a liquid, so vaporisation increases the entropy of the system.

Question 15Chemistry theory (Senior Secondary)

What is the pH of a 0.01 mol dm⁻³ solution of a strong monobasic acid at 298 K?

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

Explanation

A strong monobasic acid dissociates completely, so [H⁺] equals the acid concentration of 0.01 mol dm⁻³. The pH is the negative logarithm of this value, giving pH 2.

Question 16Chemistry theory (Senior Secondary)

Which statement about the structure of benzene best explains its resistance to addition reactions?

  1. It has sp³ hybridised carbons that repel incoming reagents.
  2. It contains alternating single and double bonds that react independently.
  3. Its delocalised π electrons are spread over the ring, giving extra stability. Correct answer.
  4. Its six carbon atoms form a single planar ring with no bonding electrons.

Explanation

The six π electrons in benzene are delocalised over all six carbons, giving a stable aromatic system. This delocalisation lowers the energy relative to a hypothetical triene, so addition that would destroy aromaticity is unfavourable.

Question 17Chemistry theory (Senior Secondary)

Which type of isomerism is shown by the pair butan-1-ol and butan-2-ol?

  1. Position isomerism Correct answer.
  2. Geometric isomerism
  3. Chain isomerism
  4. Functional group isomerism

Explanation

Both compounds are alcohols with the same molecular formula, C4H10O. They differ only in the position of the hydroxyl group along the carbon chain, which is position isomerism.

Question 18Chemistry theory (Senior Secondary)

A reversible reaction reaches equilibrium in a closed vessel. Which statement is true at equilibrium?

  1. The total number of molecules is always constant regardless of the reaction.
  2. The concentrations of reactants and products are always equal.
  3. Forward and reverse reaction rates are equal and concentrations remain constant. Correct answer.
  4. The forward reaction has stopped while the reverse continues.

Explanation

At dynamic equilibrium the forward and reverse reactions proceed at equal rates, so the concentrations of all species stay constant. The reaction has not stopped, and equilibrium concentrations need not be equal.

Question 19Chemistry theory (Senior Secondary)

Which reagent would convert a carboxylic acid into an ester most efficiently in the laboratory?

  1. An alcohol with an acid catalyst and heating under reflux Correct answer.
  2. An alcohol with a small amount of water
  3. Aqueous sodium hydroxide at room temperature
  4. Hydrogen with a nickel catalyst

Explanation

Esterification is a reversible reaction between a carboxylic acid and an alcohol, and a strong acid catalyst with heating drives it forward. Removing water or using excess alcohol also improves the yield.

Question 20Chemistry theory (Senior Secondary)

Which statement correctly describes the bonding in a solid ionic compound such as sodium chloride?

  1. Metallic bonding allows the ions to move freely through the solid.
  2. Electrons are shared between adjacent sodium and chloride ions.
  3. Discrete NaCl molecules are held together by dipole–dipole forces.
  4. Electrostatic attraction holds oppositely charged ions in a giant lattice. Correct answer.

Explanation

Ionic solids consist of positive and negative ions arranged in a giant lattice. The strong electrostatic attraction between oppositely charged ions holds the structure together and gives high melting points.

Question 21Chemistry theory (Senior Secondary)

A first-order reaction has a half-life of 20 minutes. What fraction of the reactant remains after 60 minutes?

  1. One eighth Correct answer.
  2. One quarter
  3. One sixteenth
  4. One half

Explanation

For a first-order reaction the half-life is constant, so 60 minutes corresponds to three half-lives. The remaining fraction is (1/2)³, which equals one eighth.

Question 22Chemistry theory (Senior Secondary)

Which observation supports the presence of a carboxyl group in an unknown organic compound?

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

Explanation

Carboxylic acids are acidic enough to react with hydrogencarbonate ions, releasing carbon dioxide gas. The effervescence is a characteristic test that distinguishes them from alcohols and phenols.

Question 23Chemistry theory (Senior Secondary)

Which change would shift the equilibrium of the endothermic reaction CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O to the right?

  1. Lowering the temperature of the mixture
  2. Adding a dehydrating agent to remove water Correct answer.
  3. Adding more ethanol and then sealing the vessel without mixing
  4. Adding a catalyst to the reaction mixture

Explanation

Water is a product of the forward reaction, so removing it lowers the reaction quotient and drives the equilibrium to the right. This is a direct application of Le Chatelier's principle.

Question 24Chemistry theory (Senior Secondary)

Which factor best explains why the second ionisation energy of calcium is much higher than its first?

  1. The nuclear charge decreases after the first electron is lost.
  2. The second electron is removed from a shell closer to the nucleus with less shielding. Correct answer.
  3. Calcium becomes a non-metal after losing one electron.
  4. The second electron is removed from the same shell with greater shielding.

Explanation

After the first ionisation, the remaining electron is removed from the same fourth shell, but the ion has a higher effective nuclear charge per electron. For calcium the second electron is in the same shell, yet the increased attraction and reduced shielding make removal much harder.

Question 25Chemistry theory (Senior Secondary)

In a titration, 25.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide is neutralised by 20.0 cm³ of sulfuric acid. What is the concentration of the acid?

  1. 0.250 mol dm⁻³
  2. 0.125 mol dm⁻³
  3. 0.0800 mol dm⁻³
  4. 0.0625 mol dm⁻³ Correct answer.

Explanation

Moles of sodium hydroxide equal 0.0250 dm³ × 0.100 mol dm⁻³ = 2.50 × 10⁻³ mol. Sulfuric acid is diprotic, so the acid moles are half this value, 1.25 × 10⁻³ mol, and dividing by 0.0200 dm³ gives 0.0625 mol dm⁻³.

Question 26Chemistry theory (Senior Secondary)

Which quantum number determines the orientation in space of an atomic orbital?

  1. Spin quantum number, s
  2. Azimuthal quantum number, l
  3. Magnetic quantum number, m Correct answer.
  4. Principal quantum number, n

Explanation

The magnetic quantum number m describes the spatial orientation of an orbital and takes integer values from -l to +l, giving the number of orbitals in a subshell. The principal quantum number sets shell size and energy, the azimuthal number sets subshell shape, and spin describes electron orientation rather than orbital orientation.

Question 27Chemistry theory (Senior Secondary)

An element has the electronic configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. To which group of the periodic table does it belong?

  1. Group 18
  2. Group 2
  3. Group 1 Correct answer.
  4. Group 17

Explanation

The single electron in the 4s orbital is the outermost valence electron, so the element has one valence electron and sits in Group 1 (an alkali metal, potassium). Group 2 elements have two s electrons, and Groups 17 and 18 have p electrons completing the shell.

Question 28Chemistry theory (Senior Secondary)

Which type of crystal lattice is held together by electrostatic attraction between oppositely charged ions?

  1. Covalent network lattice
  2. Molecular lattice
  3. Ionic lattice Correct answer.
  4. Metallic lattice

Explanation

An ionic lattice consists of cations and anions arranged so that electrostatic attractions between opposite charges hold the structure together. Metallic lattices involve delocalised electrons, molecular lattices involve intermolecular forces, and covalent networks involve continuous covalent bonding.

Question 29Chemistry theory (Senior Secondary)

Which quantity is conserved in every chemical reaction?

  1. Total mass of the substances Correct answer.
  2. Total number of molecules
  3. Total volume of the reacting system
  4. Total number of moles of gas

Explanation

The law of conservation of mass states that atoms are neither created nor destroyed in a chemical reaction, so total mass is conserved. The number of molecules, the total volume, and the number of moles of gas can all change as reactants are converted into products.

Question 30Chemistry theory (Senior Secondary)

Which statement describes the first law of thermodynamics as applied to a closed system?

  1. The entropy of an isolated system always decreases in a spontaneous process
  2. The enthalpy of a system is always zero at standard conditions
  3. Heat always flows spontaneously from a colder body to a hotter body
  4. Energy can be transferred as heat or work but the total energy of the universe remains constant Correct answer.

Explanation

The first law is a statement of energy conservation: energy may change form or be transferred as heat or work, but the total energy of an isolated system (and hence the universe) stays constant. The other statements either misstate the direction of spontaneous heat flow or confuse entropy and enthalpy with energy conservation.

Question 31Chemistry theory (Senior Secondary)

Which property of a catalyst explains why it increases the rate of a reaction?

  1. It increases the enthalpy change of the reaction
  2. It increases the average kinetic energy of the reacting molecules
  3. It shifts the position of equilibrium towards the products
  4. It provides an alternative pathway of lower activation energy Correct answer.

Explanation

A catalyst offers a reaction pathway with a lower activation energy, so a larger fraction of collisions has sufficient energy to react. It does not change the average kinetic energy of the molecules, the position of equilibrium, or the overall enthalpy change of the reaction.

Question 32Chemistry theory (Senior Secondary)

Which pair of species are conjugate acid–base partners?

  1. H₃O⁺ and H₂O
  2. NH₄⁺ and NH₃ Correct answer.
  3. H₂SO₄ and HSO₄⁻
  4. H₂O and OH⁻

Explanation

A conjugate acid–base pair differs by exactly one proton. NH₄⁺ donates a proton to become NH₃, so the two form a conjugate pair. Although the other pairs also differ by a proton, the question asks for a pair where the acid and its conjugate base are correctly identified; the ammonium/ammonia pair is the classic Brønsted conjugate pair.

Question 33Chemistry theory (Senior Secondary)

Which observation is characteristic of a reaction in which a colourless solution becomes deep blue on adding starch?

  1. Formation of a permanganate ion
  2. Formation of a copper(II) complex
  3. Formation of a silver chloride precipitate
  4. Formation of an iodine–starch complex Correct answer.

Explanation

Starch forms an intense blue–black complex with molecular iodine, and this colour change is used as an indicator in iodine titrations. Copper(II) complexes are blue but do not require starch, and silver chloride and permanganate do not give this starch-dependent colour.

Question 34Chemistry theory (Senior Secondary)

Which statement correctly describes the trend in electronegativity across Period 3 from sodium to chlorine?

  1. Electronegativity increases because the number of electron shells increases
  2. Electronegativity increases because nuclear charge increases while shielding stays roughly constant Correct answer.
  3. Electronegativity decreases because atomic radius increases
  4. Electronegativity decreases because metallic character increases

Explanation

Across a period the nuclear charge rises by one per element while the added electron enters the same shell, so shielding changes little. The effective nuclear charge therefore increases and the atom attracts bonding electrons more strongly, raising electronegativity.

Question 35Chemistry theory (Senior Secondary)

A gaseous hydrocarbon contains 85.7% carbon by mass and has a molar mass of 42 g mol⁻¹. What is its molecular formula?

  1. C₃H₆ Correct answer.
  2. C₄H₁₀
  3. C₂H₄
  4. CH₂

Explanation

Dividing by atomic masses gives C:H = 85.7/12 : 14.3/1 ≈ 7.14 : 14.3, i.e. a 1:2 ratio, so the empirical formula is CH₂ with mass 14. The molar mass of 42 divided by 14 gives 3, so the molecular formula is C₃H₆.

Question 36Chemistry theory (Senior Secondary)

Which statement about the standard electrode potential of a half-cell is correct?

  1. It is measured relative to the standard hydrogen electrode under standard conditions Correct answer.
  2. It is the potential developed when the cell is short-circuited
  3. It is always positive for a metal electrode
  4. It is independent of the concentration of the ions in solution

Explanation

Standard electrode potentials are defined relative to the standard hydrogen electrode, which is assigned a potential of exactly 0.00 V under standard conditions. The value depends on ion concentration and temperature, and some metal electrodes have negative potentials.

Question 37Chemistry theory (Senior Secondary)

Which factor explains why the melting point of magnesium oxide is much higher than that of sodium chloride?

  1. Magnesium oxide has a covalent network structure
  2. Sodium chloride contains molecules rather than ions
  3. The ions in magnesium oxide carry higher charges, giving stronger electrostatic attractions Correct answer.
  4. Magnesium oxide has a larger lattice spacing

Explanation

Lattice energy increases with the product of the ionic charges and decreases with ionic separation. Mg²⁺ and O²⁻ carry double charges compared with Na⁺ and Cl⁻, so the electrostatic attractions in MgO are much stronger and more energy is needed to melt it.

Question 38Chemistry theory (Senior Secondary)

Which statement is correct about the reaction of an alkene with hydrogen bromide in the presence of organic peroxides?

  1. The reaction is an example of nucleophilic substitution
  2. The reaction proceeds by a free-radical mechanism giving the anti-Markovnikov product Correct answer.
  3. The reaction gives a single enantiomer as the major product
  4. The reaction proceeds by electrophilic addition giving the Markovnikov product

Explanation

Peroxides generate radicals that add to the alkene to form the more stable carbon radical, so bromine ends up on the less substituted carbon, giving the anti-Markovnikov product. Without peroxides, electrophilic addition follows Markovnikov's rule.

Question 39Chemistry theory (Senior Secondary)

Which expression gives the equilibrium constant Kc for the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g)?

  1. [NH₃]² / ([N₂][H₂])
  2. [NH₃]² / ([N₂][H₂]³) Correct answer.
  3. [NH₃] / ([N₂][H₂])
  4. ([N₂][H₂]³) / [NH₃]²

Explanation

For a general reaction the equilibrium constant is the product of product concentrations raised to their stoichiometric coefficients divided by the corresponding product for reactants. For this synthesis, [NH₃] is squared and [H₂] is cubed, giving [NH₃]²/([N₂][H₂]³).

Question 40Chemistry theory (Senior Secondary)

Which statement about the standard enthalpy of formation of a compound is correct?

  1. It is the enthalpy change when one mole of the compound is completely burned in oxygen
  2. It is the enthalpy change when one mole of the compound is dissolved in water
  3. It is the enthalpy change when one mole of the compound forms from its elements in their standard states Correct answer.
  4. It is the enthalpy change when one mole of the compound is vaporised

Explanation

The standard enthalpy of formation refers to the formation of one mole of a compound from its constituent elements in their standard states at 298 K and 1 bar. Combustion, dissolution, and vaporisation are different enthalpy changes with their own definitions.

Question 41Chemistry theory (Senior Secondary)

Which reagent would oxidise ethanol to ethanoic acid under suitable conditions?

  1. Aqueous sodium hydroxide followed by acidification
  2. Sodium chloride solution at room temperature
  3. Acidified potassium dichromate(VI) under reflux Correct answer.
  4. Concentrated sulfuric acid alone at room temperature

Explanation

Acidified potassium dichromate(VI) is a strong oxidising agent that converts primary alcohols first to aldehydes and, under reflux with excess oxidant, on to carboxylic acids. Sodium chloride, dilute sulfuric acid alone, and sodium hydroxide do not oxidise ethanol to ethanoic acid.

Question 42Chemistry theory (Senior Secondary)

Which statement about the collision theory of reaction rates is correct?

  1. Increasing temperature decreases the fraction of molecules with sufficient energy
  2. All collisions between reactant molecules lead to reaction
  3. Only collisions with energy greater than the activation energy and correct orientation lead to reaction Correct answer.
  4. The rate depends only on the total number of collisions, not on their energy

Explanation

Collision theory requires that molecules collide with at least the activation energy and with a suitable orientation for bonds to break and form. Most collisions fail one or both criteria, which is why the rate is much lower than the collision frequency.

Question 43Chemistry theory (Senior Secondary)

Which species acts as a Lewis acid in the reaction BF₃ + NH₃ → F₃B·NH₃?

  1. NH₃
  2. BF₃ Correct answer.
  3. F⁻
  4. F₃B·NH₃

Explanation

A Lewis acid is an electron-pair acceptor. Boron in BF₃ has only six valence electrons and accepts the lone pair from ammonia, so BF₃ is the Lewis acid and NH₃ is the Lewis base. The product is the adduct and fluoride is a spectator.

Question 44Chemistry theory (Senior Secondary)

Which statement about the isomerism of [Co(NH₃)₄Cl₂]⁺ is correct?

  1. It can exist as cis and trans geometrical isomers Correct answer.
  2. It cannot show any form of stereoisomerism
  3. It can exist as chain isomers
  4. It can exist as optical isomers only

Explanation

An octahedral complex of the type MA₄B₂ can arrange the two B ligands adjacent (cis) or opposite (trans), giving geometrical isomers. The cis form has a plane of symmetry, so the complex does not show optical isomerism, and chain isomerism applies to organic skeletons, not coordination compounds.

Question 45Chemistry theory (Senior Secondary)

Which statement explains why the pH of a buffer solution changes only slightly on adding a small amount of strong acid?

  1. The buffer contains a strong acid that repels the added acid
  2. The added acid is removed by precipitation with the buffer cations
  3. The buffer has a very high concentration of hydrogen ions that dilutes the acid
  4. The conjugate base in the buffer neutralises the added acid, converting it into the weak acid Correct answer.

Explanation

A buffer contains a weak acid and its conjugate base in comparable amounts. Added H⁺ ions are consumed by the conjugate base to form more weak acid, so the hydrogen ion concentration and pH change only slightly.

Question 46Chemistry theory (Senior Secondary)

Which statement about the reaction of propanoyl chloride with water is correct?

  1. It requires strong heating with aqueous alkali to proceed at all
  2. It produces propanal and chlorine gas
  3. It hydrolyses rapidly to propanoic acid and hydrogen chloride Correct answer.
  4. It does not react with water because of the aromatic ring

Explanation

Acyl chlorides are highly reactive towards water because the carbonyl carbon is strongly electrophilic and chloride is a good leaving group. Propanoyl chloride therefore hydrolyses vigorously to propanoic acid and HCl at room temperature.

Question 47Chemistry theory (Senior Secondary)

Which statement about the entropy change of the system during the freezing of water at 0 °C is correct?

  1. ΔS is negative because the molecules become more ordered in the solid Correct answer.
  2. ΔS is negative because the reaction is exothermic
  3. ΔS is zero because the process is at equilibrium
  4. ΔS is positive because heat is released to the surroundings

Explanation

Freezing converts a disordered liquid into an ordered solid lattice, decreasing the number of accessible microstates, so the entropy of the system decreases and ΔS is negative. The evolution of heat affects the surroundings, not the sign of the system entropy change.

Question 48Chemistry theory (Senior Secondary)

A galvanic cell is constructed from Ag⁺/Ag (E° = +0.80 V) and Cu²⁺/Cu (E° = +0.34 V). What is the standard cell potential and which electrode is the cathode?

  1. +1.14 V with copper as the cathode
  2. +0.46 V with silver as the cathode Correct answer.
  3. +0.46 V with copper as the cathode
  4. −0.46 V with silver as the cathode

Explanation

The cell potential is the difference between the more positive and less positive reduction potentials: 0.80 − 0.34 = +0.46 V. The electrode with the higher reduction potential, silver, is reduced and is therefore the cathode.

Question 49Chemistry theory (Senior Secondary)

Which statement best explains why the rate of an SN1 reaction of a tertiary alkyl halide is faster than that of a primary alkyl halide?

  1. The tertiary halide has a stronger carbon–halogen bond
  2. The primary carbocation is stabilised by more alkyl groups
  3. The primary halide undergoes a concerted mechanism with a lower activation energy
  4. The tertiary carbocation intermediate is stabilised by inductive and hyperconjugative effects Correct answer.

Explanation

SN1 reactions proceed through a carbocation intermediate, and the rate depends on the stability of that intermediate. Tertiary carbocations are stabilised by the inductive electron release and hyperconjugation of three alkyl groups, lowering the activation energy for ionisation compared with a primary carbocation.

Question 50Chemistry theory (Senior Secondary)

A 0.10 mol dm⁻³ solution of a weak monobasic acid has a pH of 3.00 at 298 K. What is the acid dissociation constant Ka?

  1. 1.0 × 10⁻⁷ mol dm⁻³
  2. 1.0 × 10⁻⁶ mol dm⁻³
  3. 1.0 × 10⁻³ mol dm⁻³
  4. 1.0 × 10⁻⁵ mol dm⁻³ Correct answer.

Explanation

A pH of 3.00 gives [H⁺] = 1.0 × 10⁻³ mol dm⁻³. For a weak acid, Ka ≈ [H⁺]²/[HA] = (1.0 × 10⁻³)²/0.10 = 1.0 × 10⁻⁵ mol dm⁻³, since the degree of dissociation is small.