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MATSEC SEC Chemistry study materials
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MATSEC SEC Chemistry practice questions

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Question 1Chemistry (SEC syllabus content)

Which statement correctly describes the arrangement of particles in a solid?

  1. Particles are far apart and move rapidly in random directions with negligible forces between them.
  2. Particles are closely packed but move freely past one another throughout the whole volume.
  3. Particles are closely packed in fixed positions and vibrate about those positions. Correct answer.
  4. Particles are far apart but held in a regular lattice by strong electrostatic attractions.

Explanation

In a solid the particles are held close together in a regular arrangement by strong forces, so each particle can only vibrate about a fixed mean position. The particles cannot slide past one another, which is why a solid keeps its own shape.

Question 2Chemistry (SEC syllabus content)

An atom of an element has the symbol shown as ³⁵₁₇X. How many neutrons are present in this atom?

  1. 18 Correct answer.
  2. 35
  3. 17
  4. 52

Explanation

The lower number (17) is the proton number, and the upper number (35) is the mass number, which counts protons plus neutrons. Neutrons = 35 − 17 = 18.

Question 3Chemistry (SEC syllabus content)

Which formula is correct for aluminium sulfate?

  1. AlSO₄
  2. Al₂(SO₄)₃ Correct answer.
  3. Al(SO₄)₃
  4. Al₃(SO₄)₂

Explanation

Aluminium ions carry a 3+ charge and sulfate ions carry a 2− charge, so the formula must balance charge. The lowest whole-number ratio is two Al³⁺ to three SO₄²⁻, giving Al₂(SO₄)₃.

Question 4Chemistry (SEC syllabus content)

Which observation shows that a chemical change has taken place when dilute hydrochloric acid is added to calcium carbonate?

  1. The calcium carbonate settles at the bottom of the tube.
  2. Effervescence occurs and a gas turns limewater milky. Correct answer.
  3. The mixture becomes warm.
  4. The solid gradually disappears.

Explanation

A chemical change produces a new substance. Here carbon dioxide is formed, and its ability to turn limewater milky is a test for that new gas, so this observation indicates a reaction.

Question 5Chemistry (SEC syllabus content)

What is the relative molecular mass of ethanoic acid, CH₃COOH? (Aᵣ: H = 1, C = 12, O = 16)

  1. 44
  2. 32
  3. 46
  4. 60 Correct answer.

Explanation

CH₃COOH contains 2 carbon atoms, 4 hydrogen atoms and 2 oxygen atoms. The total is (2 × 12) + (4 × 1) + (2 × 16) = 24 + 4 + 32 = 60.

Question 6Chemistry (SEC syllabus content)

Which volume of 0.50 mol/dm³ sodium hydroxide solution exactly neutralises 25.0 cm³ of 0.20 mol/dm³ hydrochloric acid?

  1. 62.5 cm³
  2. 100 cm³
  3. 10.0 cm³ Correct answer.
  4. 25.0 cm³

Explanation

HCl and NaOH react in a 1:1 mole ratio. Moles of acid = 0.20 × 0.025 = 0.0050 mol, so 0.0050 mol of alkali is needed. Volume = 0.0050 ÷ 0.50 = 0.010 dm³ = 10.0 cm³.

Question 7Chemistry (SEC syllabus content)

Which statement explains why the noble gases are chemically unreactive?

  1. They have a full outer electron shell. Correct answer.
  2. They exist as single atoms.
  3. They are present in air in small amounts.
  4. They have very high melting points.

Explanation

Chemical reactions involving electron transfer or sharing are driven by the tendency to gain a stable outer shell. Noble gases already have a complete outer shell, so they neither lose, gain nor share electrons readily.

Question 8Chemistry (SEC syllabus content)

Chlorine gas is bubbled through colourless potassium bromide solution. Which observation and equation are both correct?

  1. The solution turns orange-brown; Cl₂ + 2KBr → 2KCl + Br₂. Correct answer.
  2. The solution turns violet; Cl₂ + 2KBr → 2KCl + Br₂.
  3. A white precipitate forms; KBr + Cl₂ → KCl + BrCl.
  4. The solution stays colourless; no reaction occurs.

Explanation

Chlorine is more reactive than bromine, so it displaces bromide ions from solution, producing orange-brown bromine. The balanced equation is Cl₂ + 2KBr → 2KCl + Br₂.

Question 9Chemistry (SEC syllabus content)

Which type of bonding is present in solid magnesium chloride?

  1. Ionic bonding between magnesium ions and chloride ions. Correct answer.
  2. Hydrogen bonding between magnesium and chlorine.
  3. Metallic bonding between magnesium cations and delocalised electrons.
  4. Covalent bonding between magnesium and chlorine atoms.

Explanation

Magnesium is a metal and chlorine is a non-metal, so electrons are transferred from magnesium to chlorine. The resulting Mg²⁺ and Cl⁻ ions are held in a giant ionic lattice by strong electrostatic attraction.

Question 10Chemistry (SEC syllabus content)

Why does graphite conduct electricity while diamond does not?

  1. Each carbon atom in graphite is bonded to three others, leaving one delocalised electron per atom. Correct answer.
  2. Graphite has a higher melting point than diamond.
  3. Diamond contains only covalent bonds whereas graphite contains ionic bonds.
  4. Graphite contains free carbon ions that carry the current.

Explanation

In graphite each carbon atom forms three covalent bonds, and the fourth outer electron becomes delocalised and can move along the layers. In diamond all four outer electrons are used in bonding, so there are no mobile charge carriers.

Question 11Chemistry (SEC syllabus content)

Which change would shift the equilibrium of the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH negative, towards the products?

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

Explanation

There are four moles of gas on the left and two on the right, so increasing the pressure favours the side with fewer gas molecules, which is the ammonia side. The forward reaction is also exothermic, so low temperature favours products.

Question 12Chemistry (SEC syllabus content)

Which statement about the rate of a reaction between marble chips and dilute hydrochloric acid is correct?

  1. Adding water increases the rate by diluting the acid.
  2. Cooling the mixture increases the rate because particles collide more often.
  3. Powdering the marble increases the rate because the activation energy is lowered.
  4. Powdering the marble increases the rate because the surface area is greater. Correct answer.

Explanation

Reaction occurs at the surface of the solid, so breaking the marble into powder exposes more surface area. More particles are available for collision with acid particles per unit time, so the rate increases.

Question 13Chemistry (SEC syllabus content)

Which oxide is amphoteric?

  1. Aluminium oxide Correct answer.
  2. Calcium oxide
  3. Sodium oxide
  4. Sulfur dioxide

Explanation

An amphoteric oxide reacts with both acids and alkalis to form salts. Aluminium oxide reacts with hydrochloric acid to give aluminium chloride and with sodium hydroxide to give a sodium aluminate.

Question 14Chemistry (SEC syllabus content)

When copper(II) sulfate solution is electrolysed using inert electrodes, what forms at the cathode and at the anode?

  1. Oxygen at the cathode and copper at the anode.
  2. Copper at the cathode and oxygen at the anode. Correct answer.
  3. Copper at the cathode and hydrogen at the anode.
  4. Hydrogen at the cathode and oxygen at the anode.

Explanation

Copper is below hydrogen in the reactivity series, so Cu²⁺ ions are preferentially discharged at the cathode to give copper metal. At the anode, hydroxide ions are discharged in preference to sulfate ions, releasing oxygen.

Question 15Chemistry (SEC syllabus content)

Which pair of substances reacts to produce a salt and water only?

  1. Copper(II) oxide and dilute sulfuric acid Correct answer.
  2. Zinc and dilute hydrochloric acid
  3. Sodium chloride and dilute nitric acid
  4. Sodium carbonate and dilute hydrochloric acid

Explanation

A metal oxide reacting with an acid is a neutralisation that gives a salt and water only. Copper(II) oxide plus sulfuric acid gives copper(II) sulfate and water.

Question 16Chemistry (SEC syllabus content)

Which reagent is used to distinguish an aldehyde from a ketone?

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

Explanation

Aldehydes reduce Fehling's solution on warming to give a brick-red precipitate of copper(I) oxide, while ketones do not. This makes Fehling's solution a test for the aldehyde group.

Question 17Chemistry (SEC syllabus content)

Which compound is an isomer of butane, C₄H₁₀?

  1. But-1-ene
  2. 2-methylpropane Correct answer.
  3. Propan-2-ol
  4. Butanoic acid

Explanation

Isomers have the same molecular formula but different structures. 2-methylpropane has the formula C₄H₁₀, the same as butane, but a branched chain, so it is an isomer.

Question 18Chemistry (SEC syllabus content)

Which statement describes a homologous series?

  1. A set of elements with the same number of electron shells.
  2. A mixture of compounds that can be separated by fractional distillation.
  3. A family of compounds with the same general formula and similar chemical properties. Correct answer.
  4. A group of compounds with the same molecular formula but different structures.

Explanation

Members of a homologous series share a general formula and functional group, and each member differs from the next by a CH₂ unit. Because the functional group is the same, their chemical properties are similar.

Question 19Chemistry (SEC syllabus content)

Which observation is expected when ethanol is warmed with acidified potassium dichromate(VI)?

  1. The orange solution turns green. Correct answer.
  2. The purple solution turns colourless.
  3. A white precipitate forms.
  4. A brick-red precipitate forms.

Explanation

Acidified potassium dichromate(VI) is orange and is reduced to chromium(III) ions, which are green, as it oxidises ethanol to ethanal or ethanoic acid. The colour change from orange to green is the key observation.

Question 20Chemistry (SEC syllabus content)

Which reagent and condition are needed to convert propene into propane?

  1. Acidified potassium dichromate(VI) with heat
  2. Steam and a phosphoric acid catalyst
  3. Hydrogen with a nickel catalyst Correct answer.
  4. Bromine water at room temperature

Explanation

Converting an alkene to an alkane requires addition of hydrogen across the double bond. This hydrogenation is carried out with hydrogen gas and a nickel catalyst, usually with gentle warming.

Question 21Chemistry (SEC syllabus content)

A gaseous hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. What is its empirical formula? (Aᵣ: C = 12, H = 1)

  1. CH₂ Correct answer.
  2. C₂H₄
  3. C₂H₆
  4. CH₃

Explanation

Divide each percentage by the relative atomic mass: carbon 85.7 ÷ 12 = 7.14, hydrogen 14.3 ÷ 1 = 14.3. Dividing by the smaller value gives a ratio of 1:2, so the empirical formula is CH₂.

Question 22Chemistry (SEC syllabus content)

Which statement about the trend in reactivity down Group I (the alkali metals) is correct?

  1. Reactivity stays the same because all the atoms have one outer electron.
  2. Reactivity increases because the atoms gain an extra outer electron.
  3. Reactivity increases because the outer electron is lost more easily as the atoms get larger. Correct answer.
  4. Reactivity decreases because the atoms become smaller.

Explanation

Going down Group I the atoms get larger and the outer electron is further from the nucleus and shielded by more inner shells. Less energy is needed to remove it, so the metals react more vigorously.

Question 23Chemistry (SEC syllabus content)

Which statement explains why the melting point of magnesium is higher than that of sodium?

  1. Magnesium has a lower first ionisation energy than sodium.
  2. Magnesium atoms are larger than sodium atoms.
  3. Magnesium contributes two delocalised electrons per atom, giving stronger metallic bonding. Correct answer.
  4. Magnesium has a higher relative atomic mass than sodium.

Explanation

Both are metals with a lattice of cations in a sea of delocalised electrons. Magnesium forms 2+ ions and releases two electrons per atom, so the electrostatic attraction is stronger and more energy is needed to melt the metal.

Question 24Chemistry (SEC syllabus content)

A reaction has an activation energy of 50 kJ/mol with no catalyst. Which effect does adding a catalyst have?

  1. It lowers the activation energy so more collisions have sufficient energy. Correct answer.
  2. It increases the yield of product at equilibrium.
  3. It increases the temperature of the reaction mixture.
  4. It increases the activation energy so fewer collisions succeed.

Explanation

A catalyst provides an alternative reaction pathway of lower activation energy. With a lower energy barrier, a larger fraction of collisions has enough energy to react, so the rate increases.

Question 25Chemistry (SEC syllabus content)

Which statement about the reaction between ethene and steam over a phosphoric acid catalyst is correct?

  1. The product is ethane and the reaction is a reduction.
  2. The product is ethanoic acid and the reaction is an oxidation.
  3. The product is ethyl ethanoate and the reaction is an esterification.
  4. The product is ethanol and the reaction is an addition. Correct answer.

Explanation

Water adds across the carbon-carbon double bond of ethene in an addition reaction, giving ethanol. The phosphoric acid catalyst is used industrially at high temperature and pressure.

Question 26Chemistry (SEC syllabus content)

An atom of an element has the electron configuration 2,8,7. In which group and period of the Periodic Table is the element found?

  1. Group 7, Period 3 Correct answer.
  2. Group 15, Period 3
  3. Group 17, Period 2
  4. Group 7, Period 7

Explanation

The number of occupied shells gives the period, so three shells place the element in Period 3. The seven outer electrons place it in Group VII (also written Group 17) of the main block.

Question 27Chemistry (SEC syllabus content)

Which statement about the particles in an atom is correct?

  1. The proton number equals the number of neutrons in every atom
  2. The mass number is the total number of protons and electrons
  3. An ion always contains more protons than electrons
  4. Isotopes of an element have different numbers of neutrons but the same number of protons Correct answer.

Explanation

Isotopes are atoms of the same element, so they share the same proton number, but they differ in neutron number and therefore in mass number. The mass number counts protons plus neutrons, and ions may be positive or negative.

Question 28Chemistry (SEC syllabus content)

What is the relative molecular mass of ethanoic acid, CH₃COOH? (Aᵣ: C = 12, H = 1, O = 16)

  1. 44
  2. 46
  3. 32
  4. 60 Correct answer.

Explanation

The formula contains two carbon atoms, four hydrogen atoms and two oxygen atoms. Adding (2 × 12) + (4 × 1) + (2 × 16) gives 24 + 4 + 32 = 60.

Question 29Chemistry (SEC syllabus content)

Which equation is correctly balanced?

  1. Mg + HCl → MgCl₂ + H₂
  2. H₂ + O₂ → H₂O
  3. Fe + O₂ → Fe₂O₃
  4. 2Na + 2H₂O → 2NaOH + H₂ Correct answer.

Explanation

In 2Na + 2H₂O → 2NaOH + H₂, both sides contain two sodium, four hydrogen and two oxygen atoms, so the equation is balanced. The other equations fail to conserve atoms on both sides.

Question 30Chemistry (SEC syllabus content)

Which of these substances is an ionic compound?

  1. Methane, CH₄
  2. Hydrogen chloride gas, HCl
  3. Carbon dioxide, CO₂
  4. Sodium chloride, NaCl Correct answer.

Explanation

Sodium chloride is formed by transfer of an electron from sodium to chlorine, producing Na⁺ and Cl⁻ ions held in a giant ionic lattice. The other substances are held together by covalent bonds.

Question 31Chemistry (SEC syllabus content)

Which property is typical of a simple molecular substance such as iodine?

  1. It has a high melting point because of strong covalent bonds
  2. It dissolves readily in water to give a conducting solution
  3. It conducts electricity when solid
  4. It has a low melting point because only weak intermolecular forces are overcome Correct answer.

Explanation

In a simple molecular solid, the strong covalent bonds are within molecules, but melting only requires overcoming the weak forces between molecules. This explains the low melting points of substances such as iodine.

Question 32Chemistry (SEC syllabus content)

Which statement about an exothermic reaction is correct?

  1. The activation energy is always zero
  2. The reaction always needs a catalyst to proceed
  3. The products have more energy than the reactants
  4. The temperature of the surroundings increases as energy is released Correct answer.

Explanation

In an exothermic reaction, energy is released to the surroundings, so the surroundings become warmer and the products have less energy than the reactants. A catalyst is not always required and the activation energy is never zero.

Question 33Chemistry (SEC syllabus content)

Which reagent gives a white precipitate with dilute hydrochloric acid to test for sulfate ions?

  1. Silver nitrate solution
  2. Sodium hydroxide solution
  3. Barium chloride solution Correct answer.
  4. Limewater

Explanation

Barium ions react with sulfate ions to form insoluble barium sulfate, which appears as a white precipitate. The test is carried out with dilute hydrochloric acid to remove carbonate ions that would also give a white precipitate.

Question 34Chemistry (SEC syllabus content)

25.0 cm³ of 0.100 mol/dm³ sodium hydroxide solution is titrated with hydrochloric acid of concentration 0.125 mol/dm³. What volume of acid is required to reach the end point?

  1. 20.0 cm³ Correct answer.
  2. 50.0 cm³
  3. 31.3 cm³
  4. 12.5 cm³

Explanation

The reaction is NaOH + HCl → NaCl + H₂O, so moles of acid equal moles of alkali. Moles of NaOH = 0.100 × 0.0250 = 2.50 × 10⁻³ mol; volume of acid = 2.50 × 10⁻³ / 0.125 = 0.0200 dm³ = 20.0 cm³.

Question 35Chemistry (SEC syllabus content)

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

  1. The outer electron of magnesium is in a shell closer to the nucleus
  2. Magnesium has more electron shells than sodium
  3. Magnesium atoms have a larger atomic radius than sodium atoms
  4. Magnesium has a greater nuclear charge with the outer electron in the same shell Correct answer.

Explanation

Both elements lose an electron from the same third shell, but magnesium has one more proton, giving a greater nuclear charge and stronger attraction for the outer electron. This raises the first ionisation energy.

Question 36Chemistry (SEC syllabus content)

Which statement about the halogens is correct?

  1. Their oxidising power increases down the group
  2. Their colour becomes lighter down the group
  3. Their oxidising power decreases down the group Correct answer.
  4. Their melting points decrease down the group

Explanation

Down Group 17, the atoms become larger and the outer shell is further from the nucleus, so the ability to attract an electron decreases. This is why chlorine displaces bromide but bromine cannot displace chloride.

Question 37Chemistry (SEC syllabus content)

Which change would increase the rate of reaction between zinc granules and dilute sulfuric acid?

  1. Diluting the acid with an equal volume of water
  2. Cooling the reaction mixture in an ice bath
  3. Using the same mass of zinc powder instead of granules Correct answer.
  4. Using larger pieces of zinc

Explanation

Powder has a much larger surface area than granules for the same mass, so more acid particles can collide with the zinc per unit time, increasing the rate. Diluting or cooling the mixture decreases the rate.

Question 38Chemistry (SEC syllabus content)

For the equilibrium 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH negative, which change would increase the equilibrium yield of sulfur trioxide?

  1. Removing some oxygen
  2. Raising the temperature
  3. Adding a catalyst
  4. Increasing the total pressure Correct answer.

Explanation

The forward reaction reduces the number of gas molecules from three to two, so increasing pressure shifts the equilibrium towards the products. Lowering the temperature would also favour the products, since the forward reaction is exothermic.

Question 39Chemistry (SEC syllabus content)

Which statement about strong and weak acids of the same concentration is correct?

  1. Strong acids do not conduct electricity, while weak acids do
  2. A weak acid reacts faster with magnesium than a strong acid
  3. A weak acid has a lower pH than a strong acid
  4. A strong acid is completely ionised in water, while a weak acid is only partially ionised Correct answer.

Explanation

A strong acid, such as hydrochloric acid, is almost completely ionised in water, giving a high concentration of hydrogen ions and a low pH. A weak acid, such as ethanoic acid, is only partially ionised, so it has a higher pH and reacts more slowly.

Question 40Chemistry (SEC syllabus content)

Which method is most suitable for preparing a pure sample of a soluble salt such as copper(II) sulfate?

  1. Titrate copper(II) sulfate with sodium hydroxide and evaporate the mixture
  2. React copper(II) oxide with excess dilute sulfuric acid, filter, then crystallise the filtrate Correct answer.
  3. React copper with dilute sulfuric acid and collect the gas produced
  4. React copper(II) oxide with dilute sulfuric acid and evaporate the mixture to dryness

Explanation

An insoluble base is added in excess to ensure all the acid is used up, the excess solid is filtered off, and the filtrate is evaporated slowly to crystallise the salt. Evaporating to dryness would leave impurities and could decompose the salt.

Question 41Chemistry (SEC syllabus content)

Which statement about the electrolysis of molten lead(II) bromide is correct?

  1. Lead metal is produced at the anode
  2. Bromide ions are oxidised at the anode to form bromine Correct answer.
  3. Hydrogen gas is produced at the cathode
  4. Bromine gas is produced at the cathode

Explanation

In molten lead(II) bromide, bromide ions migrate to the anode where they lose electrons and are oxidised to bromine. Lead ions move to the cathode and are reduced to lead metal.

Question 42Chemistry (SEC syllabus content)

Ethanol can be converted to ethene. Which reagent and condition are suitable for this reaction?

  1. Dilute sulfuric acid at room temperature
  2. Concentrated sulfuric acid at about 170 °C Correct answer.
  3. Sodium metal at room temperature
  4. Acidified potassium dichromate(VI) at about 60 °C

Explanation

Heating ethanol with concentrated sulfuric acid at about 170 °C causes dehydration, removing water to form ethene. Acidified dichromate would oxidise ethanol to ethanoic acid instead.

Question 43Chemistry (SEC syllabus content)

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

  1. Propene and propane
  2. Propan-1-ol and propan-2-ol Correct answer.
  3. Ethanal and propanal
  4. Ethanol and ethanoic acid

Explanation

Propan-1-ol and propan-2-ol both have the molecular formula C₃H₈O and the same alcohol functional group, but the position of the –OH group differs, making them positional isomers. The other pairs differ in molecular formula or functional group.

Question 44Chemistry (SEC syllabus content)

Which statement about the polymer formed from ethene is correct?

  1. It is an unsaturated polymer with a C=C double bond in the chain
  2. It is formed by joining ethene molecules with glycosidic bonds
  3. It is a condensation polymer formed with the loss of water
  4. It is an addition polymer with the repeating unit –CH₂–CH₂– Correct answer.

Explanation

Ethene undergoes addition polymerisation: the C=C double bonds open and the molecules join without loss of any other substance. The repeating unit of poly(ethene) is –CH₂–CH₂–.

Question 45Chemistry (SEC syllabus content)

A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. What is its empirical formula? (Aᵣ: C = 12, H = 1, O = 16)

  1. CH₂O Correct answer.
  2. C₂H₄O
  3. CHO
  4. C₂H₂O

Explanation

Dividing each percentage by the relative atomic mass gives 40.0/12 = 3.33 mol C, 6.7/1 = 6.7 mol H and 53.3/16 = 3.33 mol O. Dividing by the smallest value gives the ratio 1:2:1, so the empirical formula is CH₂O.

Question 46Chemistry (SEC syllabus content)

Which statement about the reaction of alkenes with bromine water is correct?

  1. The orange-brown colour is decolourised, showing that an addition reaction has occurred Correct answer.
  2. The colour changes from orange to green, showing that an oxidation reaction has occurred
  3. A white precipitate of silver bromide forms, showing that a substitution reaction has occurred
  4. The solution turns milky, showing that carbon dioxide has been produced

Explanation

Alkenes decolourise bromine water because bromine adds across the C=C double bond to form a colourless dibromo compound. This is a standard test for unsaturation.

Question 47Chemistry (SEC syllabus content)

A student adds excess magnesium ribbon to 50.0 cm³ of 0.200 mol/dm³ hydrochloric acid and collects the hydrogen produced. What is the maximum volume of hydrogen, measured at room temperature and pressure, that can be collected? (molar volume = 24.0 dm³/mol)

  1. 240 cm³ Correct answer.
  2. 480 cm³
  3. 60 cm³
  4. 120 cm³

Explanation

The equation is Mg + 2HCl → MgCl₂ + H₂, so one mole of hydrogen forms from two moles of acid. Moles of HCl = 0.200 × 0.0500 = 1.00 × 10⁻² mol, giving 5.00 × 10⁻³ mol H₂; volume = 5.00 × 10⁻³ × 24.0 = 0.120 dm³ = 120 cm³.

Question 48Chemistry (SEC syllabus content)

Which statement explains why the melting point of aluminium is higher than that of magnesium?

  1. Aluminium has more delocalised electrons per atom contributing to metallic bonding Correct answer.
  2. Magnesium has a giant covalent structure, while aluminium is metallic
  3. Aluminium atoms are larger than magnesium atoms, so the ions attract more strongly
  4. Aluminium has a lower charge on its ions, so the bonding is stronger

Explanation

Both metals have giant metallic lattices, but aluminium contributes three delocalised electrons per atom while magnesium contributes two. The greater electron density gives stronger electrostatic attraction between the ions and the sea of electrons, raising the melting point.

Question 49Chemistry (SEC syllabus content)

In the reaction between aqueous iron(III) ions and iodide ions, 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂, which statement correctly identifies the oxidation and reduction processes?

  1. Iron(III) ions are oxidised because they gain electrons
  2. Iodide ions are reduced because they lose electrons
  3. Neither species changes its oxidation state because the charges balance
  4. Iron(III) ions are reduced because they gain electrons, and iodide ions are oxidised because they lose electrons Correct answer.

Explanation

Iron(III) ions gain one electron each to become iron(II) ions, which is reduction, and their oxidation state falls from +3 to +2. Iodide ions lose electrons to form iodine, which is oxidation, and their oxidation state rises from −1 to 0.

Question 50Chemistry (SEC syllabus content)

A sample of hydrated copper(II) sulfate, CuSO₄·xH₂O, has a relative formula mass of 250. What is the value of x? (Aᵣ: Cu = 64, S = 32, O = 16, H = 1)

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

Explanation

The mass of the anhydrous salt CuSO₄ is 64 + 32 + (4 × 16) = 160. The water of crystallisation accounts for 250 − 160 = 90, and since each water molecule has a mass of 18, x = 90/18 = 5.