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HSC Chemistry practice questions

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Question 1Chemical fundamentals: structure, properties and reactions

Which particle arrangement best describes the structure of a solid ionic compound such as sodium chloride?

  1. A lattice of alternating cations and anions held by electrostatic attraction Correct answer.
  2. Discrete molecules of NaCl held together by covalent bonds
  3. A random mixture of neutral atoms with no regular order
  4. Layers of positive ions surrounded by a sea of delocalised electrons

Explanation

Ionic solids consist of a three-dimensional lattice in which each cation is surrounded by anions and vice versa, with the whole array held together by strong electrostatic forces between oppositely charged ions. There are no distinct molecules in the lattice. The regular repeating arrangement maximises attraction and minimises repulsion.

Question 2Chemical fundamentals: structure, properties and reactions

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

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

Explanation

The single electron in the 4s subshell is the outermost valence electron, and elements with one valence electron occupy Group 1 (the alkali metals). The fully filled inner shells do not contribute to group placement. This element is potassium.

Question 3Chemical fundamentals: structure, properties and reactions

Which type of bond forms when two atoms share a pair of electrons unequally because of a difference in electronegativity?

  1. An ionic bond
  2. A non-polar covalent bond
  3. A metallic bond
  4. A polar covalent bond Correct answer.

Explanation

When two non-metal atoms share electrons but differ in electronegativity, the shared pair is pulled toward the more electronegative atom, producing a polar covalent bond with partial charges. Equal sharing gives a non-polar bond. A large enough electronegativity difference leads to electron transfer and ionic bonding.

Question 4Chemical fundamentals: structure, properties and reactions

Which intermolecular force is primarily responsible for the relatively high boiling point of water compared with hydrogen sulfide?

  1. London dispersion forces
  2. Dipole–dipole attractions only
  3. Hydrogen bonding Correct answer.
  4. Ion–dipole attractions

Explanation

Water molecules form extensive hydrogen bonds because hydrogen is attached to a highly electronegative oxygen atom with lone pairs available. These strong intermolecular attractions require considerable energy to overcome, raising the boiling point. Hydrogen sulfide has weaker dipole and dispersion forces and boils at a much lower temperature.

Question 5Chemical fundamentals: structure, properties and reactions

Which observation provides direct evidence that a chemical reaction has occurred rather than a physical change?

  1. A solid is broken into smaller pieces
  2. A new gas is produced that was not present in either reactant Correct answer.
  3. A liquid is heated until it boils
  4. A coloured solution is diluted with water

Explanation

Formation of a new substance, such as a gas not present in the reactants, indicates that chemical bonds have been broken and new bonds formed. Physical changes alter only the state or size of a substance. Boiling, crushing and dilution leave the chemical identity unchanged.

Question 6Chemical fundamentals: structure, properties and reactions

Which equation correctly represents the balanced combustion of methane?

  1. 2CH₄ + 2O₂ → 2CO₂ + 4H₂O
  2. CH₄ + 3O₂ → CO₂ + 2H₂O
  3. CH₄ + 2O₂ → CO₂ + 2H₂O Correct answer.
  4. CH₄ + O₂ → CO₂ + H₂O

Explanation

Balancing requires equal numbers of each atom on both sides: one carbon, four hydrogen and four oxygen atoms appear on each side of CH₄ + 2O₂ → CO₂ + 2H₂O. The other equations violate conservation of atoms. Combustion of a hydrocarbon in sufficient oxygen yields carbon dioxide and water.

Question 7Chemical fundamentals: structure, properties and reactions

Which property increases down Group 1 of the periodic table from lithium to caesium?

  1. First ionisation energy
  2. Electronegativity
  3. Atomic radius Correct answer.
  4. Nuclear charge felt by the valence electron per unit shielding

Explanation

Each step down Group 1 adds a new electron shell, so the atomic radius increases despite the growing nuclear charge. The added inner shells shield the valence electron, so ionisation energy and electronegativity both decrease down the group. The effective nuclear charge experienced by the valence electron stays roughly similar because shielding grows alongside nuclear charge.

Question 8Chemical fundamentals: structure, properties and reactions

Which statement correctly describes an exothermic reaction?

  1. The activation energy is always zero
  2. The products always have greater total bond energy than the reactants
  3. The enthalpy of the products is lower than that of the reactants Correct answer.
  4. The reaction absorbs heat from the surroundings

Explanation

In an exothermic reaction the system releases energy to the surroundings, so the products have lower enthalpy than the reactants and the enthalpy change is negative. The energy released comes from stronger bonds in the products. Exothermic reactions still require activation energy to begin.

Question 9Chemical fundamentals: structure, properties and reactions

A sample of magnesium ribbon is burned in air and the white ash is added to water. Which product forms when the resulting solid is dissolved in water?

  1. A solution of sodium hydroxide, which is strongly alkaline
  2. A solution of magnesium chloride, which is neutral
  3. A solution of sulfuric acid, which is strongly acidic
  4. A solution of magnesium hydroxide, which is weakly alkaline Correct answer.

Explanation

Burning magnesium produces magnesium oxide, a basic oxide that reacts with water to form magnesium hydroxide, which is only slightly soluble and gives a weakly alkaline solution. No chlorine or sodium is present, so those products cannot form. Metal oxides of Group 2 are basic, not acidic.

Question 10Chemical fundamentals: structure, properties and reactions

A solution of silver nitrate is added to a solution of sodium chloride. A white precipitate forms. Which ionic equation best represents the reaction?

  1. AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(s)
  2. Ag⁺(aq) + Na⁺(aq) → AgNa(s)
  3. Na⁺(aq) + NO₃⁻(aq) → NaNO₃(s)
  4. Ag⁺(aq) + Cl⁻(aq) → AgCl(s) Correct answer.

Explanation

A precipitation reaction occurs when silver ions and chloride ions combine to form insoluble silver chloride, leaving sodium and nitrate ions as spectator ions in solution. The net ionic equation therefore shows only the ions that form the precipitate. Sodium nitrate remains soluble and does not appear.

Question 11Chemical fundamentals: structure, properties and reactions

Which change would increase the rate of reaction between marble chips and dilute hydrochloric acid without changing the amount of gas eventually produced?

  1. Using larger chips of the same total mass
  2. Using the same mass of marble in smaller chips Correct answer.
  3. Cooling the reaction mixture in an ice bath
  4. Adding water to dilute the acid further

Explanation

Smaller chips expose a greater surface area to the acid, so collisions between acid particles and the solid occur more frequently and the reaction is faster. The total amount of marble is unchanged, so the final volume of gas is the same. Diluting, cooling or using larger chips all slow the reaction.

Question 12Chemical fundamentals: structure, properties and reactions

In the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), which change shifts the equilibrium position to the right?

  1. Adding a catalyst
  2. Removing some nitrogen gas
  3. Increasing the temperature, given that the forward reaction is exothermic
  4. Increasing the total pressure by compressing the mixture Correct answer.

Explanation

The forward reaction reduces the number of gas molecules from four to two, so increasing pressure favours the side with fewer gas molecules, shifting equilibrium to the right. Temperature changes affect the equilibrium constant rather than simply shifting position in the direction stated. A catalyst speeds attainment of equilibrium without changing its position.

Question 13Chemical fundamentals: structure, properties and reactions

Which species acts as a Brønsted–Lowry acid in the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻?

  1. NH₃, because it accepts a proton from water
  2. OH⁻, because it carries a negative charge
  3. NH₄⁺, because it contains hydrogen atoms
  4. H₂O, because it donates a proton to ammonia Correct answer.

Explanation

A Brønsted–Lowry acid is a proton donor. In this reaction water donates a proton to ammonia, forming the ammonium ion and hydroxide ion, so water is the acid and ammonia is the base. The products are the conjugate acid and conjugate base respectively.

Question 14Chemical fundamentals: structure, properties and reactions

Which pair of substances would produce a salt and water when reacted together in a neutralisation reaction?

  1. Calcium carbonate and sodium chloride
  2. Sodium chloride and potassium nitrate
  3. Hydrochloric acid and potassium hydroxide Correct answer.
  4. Copper and dilute sulfuric acid

Explanation

Neutralisation occurs when an acid reacts with a base, producing a salt and water. Hydrochloric acid and potassium hydroxide form potassium chloride and water. The other pairs either contain no acid–base pair or involve substances that do not neutralise each other.

Question 15Chemical fundamentals: structure, properties and reactions

A sample of an unknown hydrocarbon is completely burned and produces 0.20 mol of carbon dioxide and 0.30 mol of water. What is the empirical formula of the hydrocarbon?

  1. C₃H₆
  2. C₂H₆ Correct answer.
  3. CH₄
  4. C₂H₄

Explanation

The carbon dioxide gives 0.20 mol of carbon atoms and the water gives 0.30 × 2 = 0.60 mol of hydrogen atoms. The mole ratio C:H is 0.20:0.60, which simplifies to 1:3, giving the empirical formula CH₃, and doubling gives C₂H₆. Checking combustion stoichiometry confirms ethane produces CO₂ and H₂O in a 2:3 ratio.

Question 16Chemical fundamentals: structure, properties and reactions

Which statement correctly explains why graphite conducts electricity while diamond does not?

  1. Graphite dissolves in water to release ions, whereas diamond is insoluble
  2. Graphite contains metal ions that carry charge, whereas diamond contains only non-metal atoms
  3. Diamond has a lower melting point, so its electrons cannot move freely
  4. Graphite has delocalised electrons between its layers, whereas diamond has all valence electrons localised in sigma bonds Correct answer.

Explanation

In graphite each carbon atom forms three sigma bonds, leaving one p electron delocalised across the hexagonal layers, and these electrons can move and carry current. In diamond every valence electron is used in a tetrahedral network of sigma bonds, so no mobile charge carriers exist. Neither substance relies on ions or metal atoms for conduction.

Question 17Chemical fundamentals: structure, properties and reactions

A galvanic cell is constructed with a zinc electrode in zinc sulfate solution and a copper electrode in copper sulfate solution. Which statement describes the spontaneous reaction?

  1. Copper is oxidised at the anode and zinc ions are reduced at the cathode
  2. Zinc is oxidised at the anode and copper ions are reduced at the cathode Correct answer.
  3. Both electrodes are oxidised and hydrogen gas is released at both
  4. No reaction occurs because both metals are unreactive

Explanation

Zinc is more reactive than copper, so it has a more negative standard reduction potential and is oxidised at the anode, releasing electrons. Those electrons travel through the external circuit to the copper cathode, where copper ions are reduced and deposited. The overall cell potential is positive, so the reaction is spontaneous.

Question 18Chemical fundamentals: structure, properties and reactions

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

  1. Geometric isomerism
  2. Position isomerism Correct answer.
  3. Optical isomerism
  4. Chain isomerism

Explanation

Both compounds share the molecular formula C₃H₈O and the same carbon skeleton, but the hydroxyl group occupies a different position on the chain. This is position isomerism. Chain isomerism would require different branching, and geometric and optical isomerism require different structural features that these molecules lack.

Question 19Chemical fundamentals: structure, properties and reactions

Which factor best explains why the ionic radius of Na⁺ is smaller than the atomic radius of Na?

  1. Sodium atoms have a full outer shell, which expands the atom
  2. The nucleus of Na⁺ contains fewer protons than that of Na
  3. Loss of an electron reduces electron–electron repulsion while the nuclear charge stays the same, pulling the remaining electrons closer Correct answer.
  4. Formation of a cation adds an extra electron shell

Explanation

When sodium loses its single 3s electron, the remaining electron configuration is that of neon, and the whole outer shell is removed. With the same nuclear charge but fewer electrons and less repulsion, the remaining electrons are drawn closer to the nucleus, so the ionic radius is smaller. The number of protons does not change.

Question 20Chemical fundamentals: structure, properties and reactions

A student adds excess zinc powder to 100 mL of 0.50 mol L⁻¹ copper(II) sulfate solution. What mass of copper is deposited, given that the reaction is Zn + CuSO₄ → ZnSO₄ + Cu and the molar mass of copper is 63.5 g mol⁻¹?

  1. 0.32 g
  2. 1.59 g
  3. 6.35 g
  4. 3.18 g Correct answer.

Explanation

The amount of copper sulfate is 0.100 L × 0.50 mol L⁻¹ = 0.050 mol, and the equation shows one mole of copper is deposited per mole of copper sulfate. The mass is therefore 0.050 mol × 63.5 g mol⁻¹ = 3.18 g. Excess zinc ensures the copper sulfate is completely displaced.

Question 21Chemical fundamentals: structure, properties and reactions

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

  1. Magnesium has one more proton, so the outer electron experiences a greater effective nuclear attraction across the same shell Correct answer.
  2. Magnesium has a full outer shell, making it a noble gas
  3. Magnesium has a larger atomic radius, which increases the energy required to remove an electron
  4. Magnesium's outer electron is in a lower shell than sodium's

Explanation

Sodium and magnesium lose an electron from the same 3s subshell, but magnesium has an additional proton and the same shielding, so the effective nuclear charge felt by the outer electron is greater. More energy is therefore required to remove it. Atomic radius actually decreases from sodium to magnesium, which reinforces the higher ionisation energy.

Question 22Chemical fundamentals: structure, properties and reactions

Ethanoic acid is a weak acid. Which statement best explains why a 0.10 mol L⁻¹ solution of ethanoic acid has a higher pH than a 0.10 mol L⁻¹ solution of hydrochloric acid?

  1. Ethanoic acid reacts with water to produce hydroxide ions, which neutralise the acid
  2. Ethanoic acid has a higher molar mass, so fewer moles are present in the same volume
  3. Ethanoic acid only partially ionises, so the concentration of hydrogen ions in solution is lower Correct answer.
  4. Hydrochloric acid is a weaker acid and therefore releases fewer hydrogen ions

Explanation

A weak acid ionises only partially in water, so at the same analytical concentration it produces a lower concentration of hydrogen ions than a strong acid that ionises completely. Since pH depends on the hydrogen ion concentration, the weaker acid gives the higher pH. Molar mass does not affect the number of moles when concentration and volume are fixed.

Question 23Chemical fundamentals: structure, properties and reactions

Consider the reaction between hydrogen and iodine to form hydrogen iodide: H₂(g) + I₂(g) ⇌ 2HI(g), with an equilibrium constant of 64 at a certain temperature. If 1.0 mol of H₂ and 1.0 mol of I₂ are sealed in a 1.0 L vessel, what is the equilibrium concentration of HI?

  1. 0.8 mol L⁻¹
  2. 3.2 mol L⁻¹
  3. 0.4 mol L⁻¹
  4. 1.6 mol L⁻¹ Correct answer.

Explanation

Let x be the amount of H₂ reacting, so equilibrium amounts are (1 − x) mol each of H₂ and I₂ and 2x mol of HI in 1.0 L. The expression 64 = (2x)² / (1 − x)² gives 8 = 2x / (1 − x), so x = 0.8 and the HI concentration is 2x = 1.6 mol L⁻¹. Taking the square root simplifies the calculation.

Question 24Chemical fundamentals: structure, properties and reactions

A compound of carbon, hydrogen and oxygen has a molar mass of 60 g mol⁻¹. Complete combustion of 0.10 mol of the compound produces 0.20 mol of CO₂ and 0.20 mol of H₂O. What is the molecular formula?

  1. C₂H₄O₂ Correct answer.
  2. C₃H₈O
  3. CH₂O
  4. C₂H₆O

Explanation

The carbon dioxide gives 0.20 mol of carbon and the water gives 0.40 mol of hydrogen per 0.10 mol of compound, so each molecule contains two carbon and four hydrogen atoms. The remaining mass is 60 − 24 − 4 = 32 g mol⁻¹, corresponding to two oxygen atoms, giving C₂H₄O₂. The empirical formula CH₂O also has the correct mass ratio.

Question 25Chemical fundamentals: structure, properties and reactions

A galvanic cell is set up with a silver electrode in silver nitrate solution and a nickel electrode in nickel(II) sulfate solution. Given the standard reduction potentials Ag⁺/Ag = +0.80 V and Ni²⁺/Ni = −0.25 V, which statement is correct?

  1. Nickel is oxidised, silver ions are reduced, and the standard cell potential is 1.05 V Correct answer.
  2. Silver is oxidised, nickel ions are reduced, and the standard cell potential is 0.55 V
  3. Nickel is oxidised, silver ions are reduced, and the standard cell potential is 0.55 V
  4. Silver is oxidised, nickel ions are reduced, and the standard cell potential is 1.05 V

Explanation

The more negative reduction potential belongs to nickel, so nickel is oxidised at the anode and silver ions are reduced at the cathode. The standard cell potential is the difference between the cathode and anode potentials: 0.80 − (−0.25) = 1.05 V. A positive value confirms the reaction is spontaneous.

Question 26Chemical fundamentals: structure, properties and reactions

Which particle is found in the nucleus of an atom?

  1. Photon
  2. Proton Correct answer.
  3. Electron
  4. Positron

Explanation

The nucleus contains protons and neutrons, which are collectively called nucleons. Electrons occupy the space around the nucleus. Photons and positrons are not constituents of the nucleus.

Question 27Chemical fundamentals: structure, properties and reactions

Which element has the electron configuration 1s² 2s² 2p⁶ 3s²?

  1. Magnesium Correct answer.
  2. Neon
  3. Aluminium
  4. Sodium

Explanation

The configuration shows 12 electrons in total, which corresponds to the atomic number of magnesium. Sodium has 11 electrons (1s² 2s² 2p⁶ 3s¹), aluminium has 13, and neon has 10.

Question 28Chemical fundamentals: structure, properties and reactions

Which type of bonding is present in solid sodium chloride?

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

Explanation

Sodium chloride is formed from sodium cations and chloride anions held together by strong electrostatic attractions in a lattice. This is the definition of ionic bonding. Covalent and metallic bonding involve sharing or delocalising electrons, and hydrogen bonding is a weak intermolecular force.

Question 29Chemical fundamentals: structure, properties and reactions

Which statement describes a physical change?

  1. Methane burning in oxygen
  2. Silver tarnishing in air
  3. Ice melting at 0 °C Correct answer.
  4. Iron rusting in moist air

Explanation

A physical change alters the state or form of a substance without changing its chemical identity. Melting ice changes solid water to liquid water, but the molecules remain H₂O. The other options involve formation of new substances.

Question 30Chemical fundamentals: structure, properties and reactions

Which property is characteristic of a homologous series?

  1. Members have the same boiling point
  2. Members have the same molar mass
  3. Members have the same general formula Correct answer.
  4. Members have identical physical properties

Explanation

A homologous series is a family of organic compounds with the same general formula and similar chemical properties, differing by CH₂ units. Physical properties change gradually along the series, and molar masses differ.

Question 31Chemical fundamentals: structure, properties and reactions

Which gas is produced when dilute hydrochloric acid reacts with calcium carbonate?

  1. Carbon dioxide Correct answer.
  2. Hydrogen
  3. Oxygen
  4. Chlorine

Explanation

Calcium carbonate reacts with an acid to form a salt, water, and carbon dioxide. The gas turns limewater milky. Hydrogen is produced when acids react with reactive metals, not with carbonates.

Question 32Chemical fundamentals: structure, properties and reactions

Which particle is emitted when a radioactive nucleus undergoes alpha decay?

  1. An electron
  2. A helium nucleus Correct answer.
  3. A neutron
  4. A gamma ray

Explanation

Alpha decay involves emission of an alpha particle, which is a helium nucleus consisting of two protons and two neutrons. Beta decay emits an electron, and gamma emission is electromagnetic radiation.

Question 33Chemical fundamentals: structure, properties and reactions

Which factor explains why the noble gases are chemically unreactive?

  1. They have full outer electron shells Correct answer.
  2. They are monatomic gases
  3. They have low molar masses
  4. They have weak dispersion forces

Explanation

Noble gases have a stable octet (or duet for helium) in their outermost shell, so they neither gain nor lose electrons readily. This full shell configuration makes them chemically inert under normal conditions.

Question 34Chemical fundamentals: structure, properties and reactions

Which compound is an isomer of butane, C₄H₁₀?

  1. Butan-1-ol
  2. Propane
  3. 2-methylpropane Correct answer.
  4. But-1-ene

Explanation

Isomers have the same molecular formula but different structural formulas. 2-methylpropane has the formula C₄H₁₀ and a branched structure, so it is an isomer of butane. The other compounds have different molecular formulas.

Question 35Chemical fundamentals: structure, properties and reactions

Which equation represents the complete combustion of ethanol?

  1. C₂H₅OH + 2O₂ → 2CO + 3H₂O
  2. C₂H₅OH + O₂ → 2CO₂ + 3H₂O
  3. C₂H₅OH → C₂H₄ + H₂O
  4. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O Correct answer.

Explanation

Complete combustion of an alcohol produces carbon dioxide and water. The equation must be balanced: one ethanol molecule requires three oxygen molecules to give two carbon dioxide and three water molecules. The other equations are unbalanced or represent incomplete combustion or dehydration.

Question 36Chemical fundamentals: structure, properties and reactions

A solution contains 0.20 mol of sodium hydroxide. What volume of 0.10 mol L⁻¹ hydrochloric acid is required to neutralise it?

  1. 0.10 L
  2. 0.020 L
  3. 2.0 L Correct answer.
  4. 0.50 L

Explanation

The neutralisation reaction is HCl + NaOH → NaCl + H₂O, so one mole of acid reacts with one mole of base. Moles of acid needed = 0.20 mol; volume = moles ÷ concentration = 0.20 ÷ 0.10 = 2.0 L.

Question 37Chemical fundamentals: structure, properties and reactions

Which change would shift the equilibrium of the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) to the right?

  1. Increasing the temperature
  2. Adding a catalyst
  3. Decreasing the pressure
  4. Increasing the concentration of O₂ Correct answer.

Explanation

According to Le Chatelier's principle, increasing the concentration of a reactant shifts the equilibrium towards the products to counteract the change. The forward reaction is exothermic, so increasing temperature shifts it left; decreasing pressure favours the side with more gas moles (left); a catalyst does not change the equilibrium position.

Question 38Chemical fundamentals: structure, properties and reactions

Which statement explains why the first ionisation energy of chlorine is greater than that of bromine?

  1. Bromine has a higher nuclear charge
  2. Chlorine has a larger atomic radius
  3. Bromine has more valence electrons
  4. Chlorine has fewer electron shells Correct answer.

Explanation

Ionisation energy increases across a period and decreases down a group. Chlorine is above bromine in Group 17, so its valence electrons are in a shell closer to the nucleus, with less shielding. The greater effective nuclear attraction makes the electron harder to remove.

Question 39Chemical fundamentals: structure, properties and reactions

Which reagent would distinguish between ethane and ethene?

  1. Distilled water
  2. Universal indicator
  3. Sodium chloride solution
  4. Bromine water Correct answer.

Explanation

Ethene contains a carbon–carbon double bond and decolourises bromine water by addition, whereas ethane is saturated and does not react under these conditions. The other reagents do not react selectively with alkenes.

Question 40Chemical fundamentals: structure, properties and reactions

Which statement describes the role of a catalyst in a chemical reaction?

  1. It provides an alternative pathway with lower activation energy Correct answer.
  2. It increases the activation energy
  3. It is consumed during the reaction
  4. It changes the enthalpy change of the reaction

Explanation

A catalyst speeds up a reaction by providing a pathway with a lower activation energy. It is not consumed overall, and it does not alter the enthalpy change (ΔH) of the reaction.

Question 41Chemical fundamentals: structure, properties and reactions

Which pair of substances reacts to produce hydrogen gas?

  1. Magnesium and dilute hydrochloric acid Correct answer.
  2. Silver and water
  3. Copper and dilute hydrochloric acid
  4. Gold and dilute sulfuric acid

Explanation

Metals above hydrogen in the reactivity series, such as magnesium, react with dilute acids to produce hydrogen gas and a salt. Copper, silver, and gold are below hydrogen and do not react with dilute acids under normal conditions.

Question 42Chemical fundamentals: structure, properties and reactions

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

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

Explanation

The sum of oxidation states equals the charge on the ion. Let the oxidation state of chromium be x: 2x + 7(−2) = −2, so 2x − 14 = −2, giving 2x = 12 and x = +6.

Question 43Chemical fundamentals: structure, properties and reactions

Which type of crystal lattice is held together by electrostatic attractions between positive and negative ions?

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

Explanation

An ionic lattice consists of alternating cations and anions held by strong electrostatic forces. Molecular solids are held by intermolecular forces, metallic solids by delocalised electrons, and covalent network solids by covalent bonds.

Question 44Chemical fundamentals: structure, properties and reactions

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

  1. It decreases because effective nuclear charge increases Correct answer.
  2. It remains constant because the number of shells is constant
  3. It increases because nuclear charge increases
  4. It increases because shielding increases

Explanation

Across a period, protons are added to the nucleus while electrons enter the same shell. The increasing effective nuclear charge pulls the electron cloud closer, so atomic radius decreases from left to right.

Question 45Chemical fundamentals: structure, properties and reactions

Which reaction is an example of an addition reaction?

  1. Ethene reacting with hydrogen to form ethane Correct answer.
  2. Methane reacting with chlorine in sunlight
  3. Propane burning in oxygen
  4. Ethanol reacting with ethanoic acid to form an ester

Explanation

Addition reactions involve two molecules combining to form a single product without loss of any atoms. Ethene and hydrogen add across the double bond to give ethane. Esterification and chlorination are substitution reactions, and combustion is an oxidation reaction.

Question 46Chemical fundamentals: structure, properties and reactions

A galvanic cell has a standard cell potential of +1.10 V. Which statement is correct?

  1. The reaction is non-spontaneous and ΔG is positive
  2. The reaction is spontaneous and ΔG is negative Correct answer.
  3. The reaction is spontaneous and ΔG is positive
  4. The reaction is at equilibrium and ΔG is zero

Explanation

A positive cell potential indicates that the cell reaction is spontaneous under standard conditions. The relationship ΔG = −nFE shows that a positive E gives a negative ΔG, which is the thermodynamic criterion for spontaneity.

Question 47Chemical fundamentals: structure, properties and reactions

Which statement best explains why the boiling point of ethanol is higher than that of dimethyl ether, CH₃OCH₃, despite both having the formula C₂H₆O?

  1. Dimethyl ether has a more polar structure
  2. Ethanol has a higher molar mass
  3. Ethanol molecules can form hydrogen bonds with each other Correct answer.
  4. Dimethyl ether has stronger dispersion forces

Explanation

Ethanol contains an −OH group, allowing intermolecular hydrogen bonding, which requires more energy to overcome. Dimethyl ether lacks an O−H bond and cannot form hydrogen bonds between its molecules, so its boiling point is lower.

Question 48Chemical fundamentals: structure, properties and reactions

A 0.10 mol L⁻¹ solution of a weak monoprotic acid has a pH of 3.00. What is the acid dissociation constant, Ka?

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

Explanation

For a weak acid, [H⁺] = 10⁻³ mol L⁻¹. The concentration of undissociated acid is approximately 0.10 − 0.001 ≈ 0.10 mol L⁻¹. Ka = [H⁺]² / [HA] = (10⁻³)² / 0.10 = 1.0 × 10⁻⁵.

Question 49Chemical fundamentals: structure, properties and reactions

Which statement explains why the second ionisation energy of calcium is higher than its first ionisation energy?

  1. The second electron is removed from a more stable, filled shell
  2. The second electron is further from the nucleus
  3. The second electron experiences more shielding
  4. The second electron is removed from a positively charged ion, increasing electrostatic attraction Correct answer.

Explanation

After the first electron is removed, the resulting Ca⁺ ion has a net positive charge, so the remaining electrons are held more tightly. Removing a second electron therefore requires more energy. The other options would suggest a lower ionisation energy.

Question 50Chemical fundamentals: structure, properties and reactions

In a titration, 25.0 mL of 0.100 mol L⁻¹ ethanoic acid is titrated with 0.100 mol L⁻¹ sodium hydroxide. What is the pH at the equivalence point? (Ka for ethanoic acid = 1.8 × 10⁻⁵)

  1. 5.28
  2. 9.25
  3. 7.00
  4. 8.72 Correct answer.

Explanation

At the equivalence point, all ethanoic acid has been converted to ethanoate ion, which is a weak base. The concentration of ethanoate is 0.0500 mol L⁻¹ (since the volume has doubled). Kb = Kw / Ka = 5.6 × 10⁻¹⁰; [OH⁻] = √(Kb × 0.0500) ≈ 5.3 × 10⁻⁶ mol L⁻¹; pOH ≈ 5.28, so pH ≈ 8.72.