Edexcel A-Level Chemistry AS Paper 1, June 2024: Question 8

14 marks · Medium difficulty · Short Open Response

Explain the solubility of calcium chloride and methanol in water, and compare the boiling temperatures of various covalent and molecular substances based on their intermolecular forces and structures.

Practise this question

Question

Exam question about physical properties containing four parts. Part (a)(i) asks to explain using labelled diagrams why water is a good solvent for calcium chloride (4 marks). Part (a)(ii) asks to explain why methanol dissolves in water with a labelled diagram (3 marks). Part (b)(i) gives a table of boiling temperatures for 2,2-dimethylpropane (283 K) and pentane (309 K) and asks to explain the difference (3 marks). Part (b)(ii) gives a table of boiling temperatures for silicon(IV) oxide (2503 K) and silicon tetrachloride (331 K) and asks to explain the difference in boiling temperatures (4 marks).
Question text

8 This question is about the physical properties of some substances.

(a) Water is able to dissolve many compounds.

(i) Explain, using suitable labelled diagrams, why water is a good solvent for

calcium chloride.

(4)

(ii) Explain why methanol dissolves in water.

Include a suitable labelled diagram.

(3)

(b) The boiling temperature of a compound is dependent on the intermolecular*P76893A01924*

forces present and the shape of the molecule.

(i) Data about two isomeric alkanes are shown.

Compound Formula Boiling temperature/K

2,2‑dimethylpropane C(CH3)4 283

pentane CH3(CH2)3CH3 309

Explain why pentane has a higher boiling temperature than

2,2‑dimethylpropane.

(3)

(ii) Data about two silicon compounds are shown.

Name of compound Formula Boiling temperature/K

silicon(IV) oxide SiO2 2503

silicon tetrachloride SiCl4 331

Explain why these two covalently bonded substances have very different

boiling temperatures.

(4)

… *P76893A02024*

(Total for Question 8 = 14 marks)

Mark scheme

Show the mark scheme Mark scheme providing detailed marking points and diagrams for all four parts of question 8, including acceptable diagrams for hydration shells of Ca2+ and Cl- ions, hydrogen bonding between methanol and water, surface area explanations for alkane boiling points, and giant covalent versus simple molecular structures for silicon compounds.

Question

Acceptable Answer Additional Guidance Mark

Number

8(a)(i) An explanation that makes reference to the following points: δ+ (4)

δ+

2+ δ−

• diagram of Ca surrounded by water molecules (any

number > 1) (1)

− δ−

• diagram of Cl surrounded by water molecules (any δ+

number > 1) (1)

δ+

Penalise one water molecule on each ion once

only

Penalise incorrect ion / ion charge once only

• dipoles shown on at least one water molecule in each case (1) Do not award if water molecule shown as ions

• energy / strength of new interactions between solute and Energy from making bonds / bond strength

solvent is approximately the same as (the sum of) the between water and ions compensates for / is

energy / strength of the interactions between the solute greater than the energy needed to break bonds in

particles and solvent particles (1) water / solvent and calcium chloride / solute /

lattice

Question

Acceptable Answer Additional Guidance Mark

Number

8(a)(ii) An explanation that makes reference to the following (3)

points:

• hydrogen bonding occurs between methanol and

water molecules. (1)

• labelled diagram of hydrogen bonding including

a least one lone pair on the relevant oxygen (max (1)

2 lone pairs on any O atom) Allow either or both diagrams.

minimum = 3 dashes/dots

• labelled diagram of hydrogen bonding including (1)

bond angle O−H−O = approximately180o (visual Hydrogen bond must be identified for M2 and M3.

assessment is adequate) Penalise once only

Penalise incorrect structure of methanol / water once only

for M2/M3

If two or more hydrogen bonds are shown then both or all

must be correct to score M2 and M3.

Question

Acceptable Answer Additional Guidance Mark

Number

8(b)(i) An explanation that makes reference to the following points: (3)

Allow reverse argument throughout

e.g. pentane has no branches for M1

• 2,2-dimethylpropane has two branches (1) Allow is branched

Allow pentane has a longer chain length

• Contact/surface area for 2,2-dimethylpropane is much

smaller than for pentane (1)

• London/van der Waals/ instantaneous dipole/temporary (1) Do not award if reference to different number of

dipole/fluctuating dipole forces/ dispersion forces etc electrons

are smaller/weaker in 2,2-dimethylpropane (and Do not award if reference to covalent bonds

therefore a lower boiling temperature) breaking

Question

Acceptable Answer Additional Guidance Mark

Number

8(b)(ii) An explanation that makes reference to the following points: (4)

• identification of structure of silicon ((IV)) oxide / SiO2 (1) Silicon ((IV)) oxide is a giant (covalent) structure /

lattice (of atoms)

Allow reference to silicon dioxide

Do not award reference to silicon ((IV)) oxide

molecules or double bonds

• identification of structure of silicon tetrachloride / SiCl4 (1) Silicon tetrachloride is simple molecular

• SiO2 has strong covalent bonds which have to be (1) Penalise lack of amount of energy needed once only

broken therefore require high amounts of energy in M3 and M4

• SiCl4 only has to break weak London forces therefore Allow van der Waals’/dispersion forces /

lower amounts of energy (1) instantaneous dipole – induced dipole for London

forces

(Total for Question 8 = 14 marks)

How to answer it

Physical Properties of Substances

What this question tests

This 14-mark structured question assesses your understanding of chemical bonding, intermolecular forces, and the physical properties of substances. You will be tested on hydration of ions (ionic solutions), hydrogen bonding in polar organic solvents, the effect of molecular branching on London forces and boiling points, and contrasting giant covalent lattices with simple molecular structures.

Question 8 (a) (i) — Ionic Dissolution & Hydration

Explaining why calcium chloride dissolves in water

✅ Correct Answer / Mark Scheme

  • Diagram showing Ca²⁺ surrounded by water molecules (oxygen pointing towards the positive ion).
  • Diagram showing Cl⁻ surrounded by water molecules (hydrogens pointing towards the negative ion).
  • Correct partial charges ( δ⁺ on H, δ⁻ on O) shown on at least one water molecule in each hydration shell.
  • Energy released making new solute-solvent interactions compensates for (or exceeds) the energy needed to break the ionic lattice and intermolecular bonds in water.

💡 Key Knowledge

Ionic compounds dissolve when the hydration enthalpy (energy released when ions form ion-dipole bonds with water) is large enough to overcome the rigid electrostatic attraction of the ionic lattice.

🧠 Exam Technique & Guidance

When drawing hydrated ions, orient the dipoles accurately: negative oxygen ( δ⁻ ) must face the calcium cation ( Ca²⁺ ), and positive hydrogens ( δ⁺ ) must face the chloride anion ( Cl⁻ ).

❌ Common Errors

  • Drawing water molecules backwards (e.g., hydrogens pointing at Ca²⁺ ).
  • Omitting partial charges ( δ⁺ / δ⁻ ).
  • Vaguely stating "bonds break" without specifying that ion-dipole attractions form between the ions and water.
Question 8 (a) (ii) — Hydrogen Bonding in Solution

Explaining why methanol dissolves in water

✅ Correct Answer / Mark Scheme

  • State that hydrogen bonding occurs between methanol ( CH₃OH ) and water ( H₂O ) molecules.
  • Include a labelled diagram showing a hydrogen bond (represented by 3 dashes or dots) between a lone pair on an oxygen atom and a δ⁺ hydrogen attached to O.
  • Show a linear (or approximately 180°) bond angle for the O—H···O linkage.

💡 Key Knowledge

Methanol is fully miscible with water because its polar hydroxyl ( —OH ) group can form strong intermolecular hydrogen bonds with water molecules, substituting solvent-solvent and solute-solute interactions with stable solute-solvent hydrogen bonds.

🧠 Exam Technique & Guidance

Ensure your hydrogen bond dashed line goes directly from the lone pair on oxygen to the hydrogen atom covalently bonded to the other electronegative atom. Clearly label the "hydrogen bond".

❌ Common Errors

  • Drawing the hydrogen bond between a hydrogen attached to carbon ( C—H ) instead of the —OH hydrogen.
  • Failing to show lone pairs on the oxygen atoms.
Question 8 (b) (i) — Intermolecular Forces & Molecular Shape

Comparing boiling temperatures of pentane and 2,2-dimethylpropane

✅ Correct Answer / Mark Scheme

  • Identify that 2,2-dimethylpropane is branched (has more branches) whereas pentane is a straight-chain alkane (longer chain / unbranched).
  • State that 2,2-dimethylpropane has a smaller surface area of contact between molecules compared to pentane.
  • Conclude that London forces (instantaneous dipole-induced dipole forces) are weaker in 2,2-dimethylpropane, requiring less energy to overcome, resulting in a lower boiling temperature.

💡 Key Knowledge

Isomers have the same molecular formula but different structures. Branching reduces molecular surface contact area, packing efficiency, and consequently the cumulative strength of temporary London dispersion forces.

🧠 Exam Technique & Guidance

Always link molecular shape directly to surface area of contact and subsequently to the strength of London forces. Avoid loose phrasing like "molecules are closer together" when discussing branching.

❌ Common Errors

  • Incorrectly stating that covalent bonds within the molecules are broken during boiling.
  • Attributing boiling point differences to a differing number of electrons (isomers have the exact same number of electrons!).
Question 8 (b) (ii) — Bonding Types & Boiling Temperatures

Explaining the extreme boiling point difference between SiO₂ and SiCl₄

✅ Correct Answer / Mark Scheme

  • Identify silicon(IV) oxide ( SiO₂ ) as having a giant covalent structure / lattice.
  • Identify silicon tetrachloride ( SiCl₄ ) as having a simple molecular structure.
  • State that SiO₂ has strong covalent bonds throughout the lattice that require large amounts of energy to break.
  • State that SiCl₄ only has weak London forces between molecules, requiring little energy to overcome.

💡 Key Knowledge

Physical state and boiling point are dictated by structure type. Giant covalent networks require the continuous disruption of robust covalent bonds across the entire lattice, whereas simple molecular substances only require separating discrete molecules by breaking weak intermolecular forces.

🧠 Exam Technique & Guidance

Structure your answer by explicitly naming the structure type for both substances first, then contrasting the type of bonding/forces broken when boiling occurs. This secures all 4 available marks systematically.

❌ Common Errors

  • Describing SiO₂ as consisting of "molecules" or having "intermolecular bonds".
  • Confusing covalent bonds inside SiCl₄ molecules with the intermolecular London forces broken during boiling.

Topics

Physical Chemistry · Organic Chemistry · Topic 2: Bonding and Structure · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.