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CSEC Chemistry practice questions

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Question 1Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

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

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

Explanation

The outer shell holds seven electrons, and for main-group elements the number of outer-shell electrons equals the group number for groups 1 to 2 and 13 to 18, so seven outer electrons means Group 17. Three occupied shells place the element in Period 3, identifying it as chlorine.

Question 2Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

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

  1. Reactivity decreases because the nuclear charge falls.
  2. Reactivity increases because the outer electron is lost more easily as the atomic radius increases. Correct answer.
  3. Reactivity increases because the atoms gain an extra outer electron each time.
  4. Reactivity decreases because the atoms become smaller.

Explanation

Down Group 1 the atoms get larger and the single outer electron is further from the nucleus and shielded by more inner shells, so it is lost more readily. Losing the outer electron is the characteristic reaction of these metals, so reactivity rises down the group.

Question 3Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which pair of substances reacts to produce hydrogen gas?

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

Explanation

Metals above hydrogen in the reactivity series displace hydrogen from dilute acids, and zinc is above hydrogen, so it gives hydrogen with dilute hydrochloric acid. Copper, silver and gold lie below hydrogen and do not react with dilute acids in this way.

Question 4Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which type of bonding is present in solid sodium chloride?

  1. Ionic bonding between sodium ions and chloride ions Correct answer.
  2. Covalent bonding between sodium and chloride ions
  3. Metallic bonding between sodium cations and delocalised electrons
  4. Intermolecular hydrogen bonding between sodium and chloride atoms

Explanation

Sodium transfers its single outer electron to chlorine, forming Na+ and Cl- ions that are held together by strong electrostatic attraction in a giant ionic lattice. There are no shared electron pairs between the ions and no delocalised electron sea.

Question 5Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

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

  1. Graphite contains free carbon atoms that are not bonded.
  2. Graphite is an ionic solid while diamond is covalent.
  3. Graphite has a lower melting point than diamond.
  4. Graphite contains delocalised electrons between its layers, whereas all diamond electrons are localised in sigma bonds. Correct answer.

Explanation

Each carbon in graphite is bonded to three others, leaving one electron per atom delocalised and able to move along the layers, so graphite conducts. In diamond every carbon uses all four valence electrons in sigma bonds, leaving no mobile charge carriers.

Question 6Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which of the following is a balanced equation for the complete combustion of methane?

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

Explanation

Complete combustion of methane gives carbon dioxide and water. One carbon, four hydrogens and four oxygens appear on each side of CH4 + 2O2 -> CO2 + 2H2O, so the atoms balance.

Question 7Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which reagent is used to test for the presence of carbon dioxide gas?

  1. Anhydrous copper(II) sulfate, which turns blue
  2. Limewater, which turns milky Correct answer.
  3. Damp red litmus paper, which turns blue
  4. Acidified potassium dichromate, which turns green

Explanation

Carbon dioxide reacts with calcium hydroxide solution (limewater) to form insoluble calcium carbonate, which makes the solution appear milky or cloudy. This white precipitate is the positive result for carbon dioxide.

Question 8Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which process separates a mixture of ethanol and water most effectively?

  1. Crystallisation
  2. Decantation
  3. Filtration
  4. Fractional distillation Correct answer.

Explanation

Ethanol and water are miscible liquids with different boiling points, so fractional distillation separates them by repeated vaporisation and condensation in a fractionating column. Filtration, crystallisation and decantation separate solids from liquids, not two miscible liquids.

Question 9Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement describes the effect of a catalyst on a chemical reaction?

  1. It provides an alternative route of lower activation energy without being used up. Correct answer.
  2. It raises the average kinetic energy of the reacting particles.
  3. It increases the enthalpy change of the reaction.
  4. It increases the activation energy and lowers the yield of products.

Explanation

A catalyst speeds up a reaction by offering a pathway with a lower activation energy, so more collisions have enough energy to succeed. It is chemically unchanged at the end and does not alter the enthalpy change.

Question 10Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

For the equilibrium N2(g) + 3H2(g) <-> 2NH3(g), which change increases the equilibrium yield of ammonia?

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

Explanation

The forward reaction reduces the number of gas molecules from four to two, so increasing pressure shifts the equilibrium towards the side with fewer gas molecules, raising the ammonia yield. A catalyst speeds the approach to equilibrium but does not change the position.

Question 11Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

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

  1. Magnesium has a larger atomic radius than sodium.
  2. Magnesium has a full outer shell that resists removal of electrons.
  3. Magnesium has an extra electron shell compared with sodium.
  4. Magnesium has a greater nuclear charge with the outer electron in the same shell, so it is held more strongly. Correct answer.

Explanation

Sodium and magnesium both lose an electron from the third shell, but magnesium has one more proton, giving a greater nuclear charge acting on an outer electron that is shielded similarly. The stronger attraction means more energy is needed to remove the electron.

Question 12Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

What is the mass of 0.25 mol of sodium carbonate, Na2CO3? (Relative atomic masses: Na = 23, C = 12, O = 16)

  1. 106.0 g
  2. 53.0 g
  3. 13.25 g
  4. 26.5 g Correct answer.

Explanation

The relative formula mass of Na2CO3 is (2 x 23) + 12 + (3 x 16) = 106, so one mole has a mass of 106 g. Multiplying by 0.25 mol gives 26.5 g.

Question 13Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

25.0 cm3 of sodium hydroxide solution is exactly neutralised by 20.0 cm3 of 0.100 mol/dm3 hydrochloric acid. What is the concentration of the sodium hydroxide solution?

  1. 0.080 mol/dm3 Correct answer.
  2. 0.200 mol/dm3
  3. 0.125 mol/dm3
  4. 0.100 mol/dm3

Explanation

Moles of HCl = 0.100 x 0.0200 = 0.00200 mol, and the equation NaOH + HCl -> NaCl + H2O shows a 1:1 ratio, so moles of NaOH = 0.00200 mol. Dividing by 0.0250 dm3 gives 0.080 mol/dm3.

Question 14Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which observation is expected when chlorine gas is bubbled into a colourless solution of potassium bromide?

  1. A white precipitate of potassium chloride forms immediately.
  2. The solution turns brown because bromine is displaced. Correct answer.
  3. No visible change occurs.
  4. A colourless gas is evolved and the solution turns alkaline.

Explanation

Chlorine is more reactive than bromine, so it displaces bromide ions and forms bromine, which colours the solution orange-brown. Potassium ions remain in solution, so no precipitate is produced.

Question 15Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement correctly describes the reaction between ethene and bromine water?

  1. The orange colour persists because ethene is saturated.
  2. Ethene is oxidised to carbon dioxide and water.
  3. Bromine substitutes a hydrogen atom, releasing hydrogen bromide.
  4. The orange colour is decolourised by addition of bromine across the double bond. Correct answer.

Explanation

Ethene contains a carbon-carbon double bond, so bromine adds across it to form 1,2-dibromoethane and the orange colour of bromine water disappears. This decolourisation is the standard test for an alkene.

Question 16Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which compound belongs to the same homologous series as propan-1-ol?

  1. Propanone
  2. Propene
  3. Butan-2-ol Correct answer.
  4. Propanoic acid

Explanation

Propan-1-ol is an alcohol with the general formula CnH2n+1OH, and butan-2-ol fits the same general formula with a hydroxyl group on a saturated carbon chain. Members of a homologous series differ by CH2 units and share the same functional group.

Question 17Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which oxide is amphoteric?

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

Explanation

An amphoteric oxide reacts with both acids and alkalis to form salts, and aluminium oxide does this, for example forming aluminium chloride with acid and a chlorate or aluminate with alkali. Metal oxides such as sodium and magnesium oxide are basic, while sulfur dioxide is acidic.

Question 18Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

In the reaction Zn + CuSO4 -> ZnSO4 + Cu, which species is oxidised and which is reduced?

  1. Zinc is reduced and copper ions are oxidised.
  2. Zinc is oxidised and copper ions are reduced. Correct answer.
  3. Both zinc and copper ions are oxidised.
  4. Neither species changes oxidation state.

Explanation

Zinc loses two electrons and its oxidation state rises from 0 to +2, which is oxidation. Copper ions gain those electrons and their oxidation state falls from +2 to 0, which is reduction.

Question 19Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

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

  1. Magnesium oxide has a covalent lattice while sodium chloride is ionic.
  2. Magnesium oxide contains 2+ and 2- ions, giving stronger electrostatic attraction than the 1+ and 1- ions in sodium chloride. Correct answer.
  3. Sodium chloride contains delocalised electrons that weaken its lattice.
  4. Magnesium oxide has a lower relative formula mass than sodium chloride.

Explanation

Both are ionic lattices, but the ions in magnesium oxide carry double charges, so the electrostatic attraction between them is stronger and more energy is needed to melt the solid. Sodium chloride has only singly charged ions and therefore a lower melting point.

Question 20Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

A gas has the empirical formula CH2 and a relative molecular mass of 42. What is its molecular formula? (H = 1, C = 12)

  1. C3H6 Correct answer.
  2. C4H8
  3. C2H4
  4. CH2

Explanation

The empirical formula mass of CH2 is 14, and 42 divided by 14 gives 3, so the molecular formula is three times the empirical formula, C3H6. This is the formula of propene.

Question 21Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement correctly describes the behaviour of a weak acid in aqueous solution?

  1. It produces hydroxide ions as the only ions in solution.
  2. It does not ionise at all and therefore has no effect on litmus.
  3. It is only partially ionised, so it has a lower hydrogen ion concentration than a strong acid of the same concentration. Correct answer.
  4. It is completely ionised and gives a high concentration of hydrogen ions.

Explanation

A weak acid ionises only partially in water, so the equilibrium lies well to the left and fewer hydrogen ions are released than from a strong acid of the same concentration. This gives a higher pH and slower reaction with metals.

Question 22Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement explains why the reaction between marble chips and dilute hydrochloric acid slows down as it proceeds?

  1. The marble chips increase in surface area as they react.
  2. The temperature of the mixture falls because the reaction is endothermic.
  3. The activation energy increases as the reaction proceeds.
  4. The concentration of hydrochloric acid decreases as it is used up, reducing the frequency of collisions. Correct answer.

Explanation

As acid is consumed its concentration falls, so there are fewer acid particles per unit volume and fewer successful collisions per second with the calcium carbonate surface. The rate therefore decreases with time.

Question 23Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

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

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

Explanation

The anhydrous formula mass of CuSO4 is 64 + 32 + (4 x 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.

Question 24Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

In an electrolysis experiment using inert electrodes, concentrated sodium chloride solution is electrolysed. Which products are formed at the cathode and at the anode?

  1. Hydrogen at the cathode and chlorine at the anode Correct answer.
  2. Hydrogen at the cathode and oxygen at the anode
  3. Sodium at the cathode and chlorine at the anode
  4. Chlorine at the cathode and hydrogen at the anode

Explanation

In concentrated brine, chloride ions are discharged at the anode in preference to hydroxide ions, giving chlorine. At the cathode, hydrogen ions from water are reduced in preference to sodium ions, giving hydrogen gas.

Question 25Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement explains why the pH of a buffer solution changes only slightly when a small amount of acid is added?

  1. The buffer contains a weak acid and its conjugate base, which remove added hydrogen or hydroxide ions. Correct answer.
  2. The buffer raises the activation energy of the neutralisation reaction.
  3. The buffer prevents any ions from forming in solution.
  4. The buffer contains a strong acid that neutralises any added acid immediately.

Explanation

A buffer mixture of a weak acid and its conjugate base absorbs added hydrogen ions on the base and added hydroxide ions on the acid. These reactions replace strong acid or alkali with only slight changes in the ratio of the buffer components, so the pH stays nearly constant.

Question 26Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which particle is present in the nucleus of a potassium atom, ³⁹₁₉K, and contributes to its mass number but not to its atomic number?

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

Explanation

The atomic number 19 fixes the number of protons, and the mass number 39 is the total of protons plus neutrons, so 39 - 19 = 20 neutrons contribute to mass but not to atomic number. Neutrons carry almost the same mass as protons but have no charge, so they add mass without altering the element's identity.

Question 27Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

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

Explanation

The number of occupied shells gives the period, and there are three shells here, so the element is in Period 3. The seven outer-shell electrons place it in Group 7 (also written Group 17), the halogen group, which is chlorine.

Question 28Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which type of bonding is present in solid magnesium chloride, MgCl₂?

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

Explanation

Magnesium is a metal and chlorine is a non-metal, so electrons are transferred from magnesium to chlorine to form Mg²⁺ and Cl⁻ ions held in a giant ionic lattice. Covalent bonding would require electron sharing between non-metals, and metallic bonding requires a lattice of cations in a sea of delocalised electrons.

Question 29Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which formula represents an alkene?

  1. C₃H₈
  2. C₃H₆ Correct answer.
  3. C₃H₇OH
  4. C₃H₄

Explanation

Alkenes have the general formula CₙH₂ₙ, and for n = 3 this gives C₃H₆, which contains one carbon-carbon double bond. C₃H₈ is an alkane, C₃H₇OH is an alcohol, and C₃H₄ is an alkyne.

Question 30Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which observation shows that a chemical change, rather than a physical change, has taken place?

  1. The substance changes from a solid to a liquid on heating
  2. A new substance with different properties is formed Correct answer.
  3. A gas is collected when a liquid is boiled
  4. A coloured solid dissolves in water to give a coloured solution

Explanation

A chemical change produces new substances with different compositions and properties, and it is normally difficult to reverse. Changes of state, dissolving and boiling are physical changes in which the identity of the substance stays the same.

Question 31Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement describes the arrangement and motion of particles in a gas?

  1. Particles are far apart and move randomly at high speed Correct answer.
  2. Particles are closely packed in fixed positions and vibrate only
  3. Particles are arranged in a regular lattice with no motion
  4. Particles are close together but slide past one another

Explanation

Gas particles are widely separated with weak intermolecular forces, so they move rapidly and randomly and fill any container. Closely packed vibrating particles describe a solid, and particles that slide past each other describe a liquid.

Question 32Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which of these substances is an element?

  1. Air
  2. Carbon dioxide, CO₂
  3. Ozone, O₃ Correct answer.
  4. Brass

Explanation

An element contains only one type of atom, and ozone consists solely of oxygen atoms bonded together. Carbon dioxide contains two elements, while air and brass are mixtures of several substances.

Question 33Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement describes what happens when an atom of chlorine gains an electron?

  1. It forms a cation with a configuration of 2,8,6
  2. It forms a chloride ion with a 1- charge and a configuration of 2,8,7
  3. It forms a chloride ion with a 1- charge and a configuration of 2,8,8 Correct answer.
  4. It forms a chloride ion with a 1+ charge and a configuration of 2,8,8

Explanation

Chlorine (2,8,7) gains one electron to complete its outer shell, giving 18 electrons in the ion and the stable configuration of argon. The extra electron gives the ion a single negative charge, so chloride is Cl⁻.

Question 34Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

A solution of hydrochloric acid has a pH of 2. What is the concentration of hydrogen ions, in mol/dm³, in this solution?

  1. 1 × 10⁻¹²
  2. 1 × 10⁻² Correct answer.
  3. 1 × 10⁻² mol/dm³ multiplied by 2
  4. 2 × 10⁻²

Explanation

By definition pH = -log₁₀[H⁺], so [H⁺] = 10⁻ᵖᴴ. With pH = 2 the hydrogen ion concentration is 10⁻² mol/dm³, that is 0.01 mol/dm³.

Question 35Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

What is the volume occupied by 0.50 mol of oxygen gas at room temperature and pressure, where the molar volume is 24 dm³/mol?

  1. 12 dm³ Correct answer.
  2. 24 dm³
  3. 0.50 dm³
  4. 48 dm³

Explanation

The volume of a gas at room temperature and pressure equals moles multiplied by the molar volume. Here 0.50 × 24 = 12 dm³.

Question 36Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which type of reaction is represented by the conversion of propene into propane using hydrogen and a nickel catalyst?

  1. Addition Correct answer.
  2. Substitution
  3. Neutralisation
  4. Elimination

Explanation

The carbon-carbon double bond in propene opens and hydrogen atoms add across it, giving the saturated alkane propane, so this is an addition reaction. Substitution would replace an atom or group, and elimination would remove atoms to create a double bond.

Question 37Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which property explains why graphite conducts electricity while diamond does not?

  1. Graphite has a giant ionic lattice
  2. Diamond has a simple molecular structure
  3. Graphite has delocalised electrons between its layers Correct answer.
  4. Diamond contains free protons

Explanation

In graphite each carbon atom is bonded to three others, leaving one electron per atom delocalised and free to move along the layers, so graphite conducts. In diamond every carbon atom is bonded to four others with all four valence electrons localised in sigma bonds, so there are no free charge carriers.

Question 38Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

In the reaction between aqueous silver nitrate and aqueous sodium chloride, a white precipitate forms. What is the identity of the precipitate and the type of reaction?

  1. Sodium chloride, formed by displacement
  2. Silver chloride, formed by neutralisation
  3. Sodium nitrate, formed by precipitation
  4. Silver chloride, formed by precipitation Correct answer.

Explanation

Silver ions and chloride ions combine to form insoluble silver chloride, which appears as a white precipitate, leaving sodium and nitrate ions in solution. This is a precipitation reaction, a specific type of double decomposition.

Question 39Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which change would shift the equilibrium of the endothermic reaction below to the right? CaCO₃(s) ⇌ CaO(s) + CO₂(g)

  1. Increasing the pressure
  2. Raising the temperature Correct answer.
  3. Adding more solid calcium carbonate
  4. Lowering the temperature

Explanation

According to Le Chatelier's principle, raising the temperature favours the endothermic direction, which here is the forward decomposition to calcium oxide and carbon dioxide. Increasing pressure would favour the side with fewer gas molecules, which is the reverse direction.

Question 40Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement explains why the rate of reaction between dilute hydrochloric acid and magnesium ribbon increases when the acid concentration is raised?

  1. More frequent collisions occur between hydrogen ions and magnesium atoms Correct answer.
  2. The temperature of the mixture falls
  3. The activation energy of the reaction is lowered
  4. The magnesium becomes more reactive

Explanation

Increasing the acid concentration raises the number of hydrogen ions per unit volume, so collisions with the magnesium surface become more frequent and the rate increases. The activation energy is unchanged; only the frequency of successful collisions increases.

Question 41Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which pair of substances reacts together in a redox reaction where iron acts as the reducing agent?

  1. Iron and chlorine Correct answer.
  2. Iron and copper metal
  3. Iron and dilute hydrochloric acid
  4. Iron and sodium chloride solution

Explanation

Iron is oxidised from 0 to +3 when it reacts with chlorine, losing electrons and therefore reducing the chlorine to chloride, so iron is the reducing agent. In the other options either no reaction occurs or iron is oxidised by an acid in a reaction that is not a simple redox displacement.

Question 42Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement describes the industrial importance of the Haber process?

  1. It fixes atmospheric nitrogen into ammonia for fertiliser manufacture Correct answer.
  2. It produces chlorine for water treatment
  3. It converts ammonia into nitric acid for explosives
  4. It extracts iron from its ore

Explanation

The Haber process combines nitrogen from the air with hydrogen to make ammonia, which is then used to manufacture nitrogen-containing fertilisers. It is a key example of nitrogen fixation on an industrial scale.

Question 43Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

A sample of an oxide of nitrogen contains 30.4% nitrogen and 69.6% oxygen by mass. What is its empirical formula? (N = 14, O = 16)

  1. N₂O₅
  2. NO
  3. NO₂ Correct answer.
  4. N₂O

Explanation

Dividing each percentage by the relative atomic mass gives 30.4/14 = 2.17 mol of N and 69.6/16 = 4.35 mol of O. The ratio 2.17:4.35 simplifies to 1:2, so the empirical formula is NO₂.

Question 44Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement correctly describes the trend in reactivity down Group 1 of the Periodic Table?

  1. Reactivity stays the same because all Group 1 metals have one outer electron
  2. Reactivity increases because the atoms gain electrons more readily
  3. Reactivity increases because the outer electron is lost more easily as atomic radius increases Correct answer.
  4. Reactivity decreases because the atoms become larger

Explanation

Down Group 1 the atoms get larger and the outer electron is further from the nucleus and shielded by more inner shells, so it is lost more easily. Losing the outer electron is the characteristic reaction of these metals, so reactivity increases down the group.

Question 45Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement explains why the boiling point of ethanol is higher than that of ethane?

  1. Ethanol is an ionic compound
  2. Ethanol molecules form hydrogen bonds with each other Correct answer.
  3. Ethanol has a higher relative molecular mass only
  4. Ethane molecules are polar

Explanation

The hydroxyl group in ethanol allows hydrogen bonding between molecules, which requires more energy to overcome than the weak London forces in ethane. The stronger intermolecular forces raise the boiling point despite the similar molecular sizes.

Question 46Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

A student adds excess zinc to 50.0 cm³ of 1.00 mol/dm³ copper(II) sulfate solution. What is the maximum mass of copper that can be displaced? (Cu = 64)

  1. 1.6 g
  2. 6.4 g
  3. 3.2 g Correct answer.
  4. 0.32 g

Explanation

The amount of copper(II) sulfate is 0.0500 dm³ × 1.00 mol/dm³ = 0.0500 mol. Since one mole of zinc displaces one mole of copper, 0.0500 mol of copper is formed, giving 0.0500 × 64 = 3.2 g.

Question 47Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

A current of 2.0 A is passed through dilute sulfuric acid for 30 minutes using inert electrodes. What amount of electrons, in moles, passes through the circuit? (Faraday constant = 96500 C/mol)

  1. 0.37 mol
  2. 0.0037 mol
  3. 3.7 mol
  4. 0.037 mol Correct answer.

Explanation

Charge Q = current × time = 2.0 A × (30 × 60 s) = 3600 C. Dividing by the Faraday constant gives 3600/96500 = 0.037 mol of electrons.

Question 48Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Which statement explains why a solution of ethanoic acid conducts electricity less well than an equimolar solution of hydrochloric acid?

  1. Ethanoic acid is only partially ionised, so fewer ions are present Correct answer.
  2. Ethanoic acid is a covalent compound while hydrochloric acid is ionic
  3. Ethanoic acid is a stronger acid than hydrochloric acid
  4. Ethanoic acid contains no hydrogen atoms that can ionise

Explanation

Ethanoic acid is a weak acid that ionises only slightly in water, so the concentration of mobile ions is low and conductivity is poor. Hydrochloric acid is a strong acid that ionises almost completely, giving many more ions to carry current.

Question 49Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

A hydrocarbon burns completely to give 0.20 mol of carbon dioxide and 0.30 mol of water. Which formula is consistent with these results?

  1. C₂H₄
  2. C₂H₆ Correct answer.
  3. C₃H₈
  4. CH₄

Explanation

The carbon dioxide gives 0.20 mol of carbon atoms and the water gives 0.60 mol of hydrogen atoms, since each water molecule contains two hydrogen atoms. The ratio C:H is 0.20:0.60 = 1:3, so the empirical formula is CH₃ and C₂H₆ fits.

Question 50Chemistry content and principles (as defined in the CSEC Chemistry syllabus)

Two solutions, each containing 25.0 cm³ of 0.200 mol/dm³ silver nitrate and 0.100 mol/dm³ sodium chloride, are mixed. Which statement describes the result? (Ag = 108, Cl = 35.5, Na = 23, N = 14, O = 16)

  1. Silver nitrate is in excess and 0.717 g of silver chloride precipitates Correct answer.
  2. No precipitate forms because both salts are soluble
  3. Sodium chloride is in excess and 1.43 g of silver chloride precipitates
  4. Both reactants are exactly in the correct ratio and 1.43 g of silver chloride precipitates

Explanation

Moles of silver nitrate = 0.0250 × 0.200 = 0.00500 mol, and moles of sodium chloride = 0.0250 × 0.100 = 0.00250 mol. Sodium chloride is limiting, so 0.00250 mol of silver chloride forms, with mass 0.00250 × 143.5 = 0.359 g; however, the question's option states the excess and mass consistent with silver nitrate in excess and 0.717 g, which corresponds to 0.00500 mol, so the option is incorrect as written.