Chemistry

1. 1.

   Explain why molten sodium chloride conducts electricity but solid sodium chloride does not.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Conduction requires mobile charged particles. Melting frees the ions from fixed lattice positions; it does not turn them into free electrons.

   Marking points

   • Charge carriers are ions.
   • Solid ions are fixed in a lattice.
   • Molten ions move and carry charge.

   Examiner tip: Name the mobile particles; 'particles move' alone does not identify a current carrier.

2. 2.

   Predict the shape of CO2 and explain why its polar bonds do not produce a permanent molecular dipole.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Treat each double bond as one region of electron density. The symmetric linear geometry makes the two dipole vectors sum to zero.

   Marking points

   • Two bonding regions around carbon give a linear shape.
   • Each C=O bond is polar towards oxygen.
   • Equal opposite bond dipoles cancel.

   Examiner tip: Nonpolar molecules can still contain polar bonds.

3. 3.

   Compare the H-N-H and H-O-H bond angles in ammonia and water using electron-pair repulsion.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The reference arrangement is tetrahedral. Lone pairs occupy more space than bonding pairs; replacing another bond pair with a lone pair further compresses the angle between the remaining bonds.

   Marking points

   • Both central atoms have four electron-pair regions.
   • Water has two lone pairs; ammonia has one.
   • Greater lone-pair repulsion compresses water's bond angle more.

   Examiner tip: Distinguish electron-pair geometry from molecular shape.

4. 4.

   Ethanol has a higher boiling point than dimethyl ether although they share a molecular formula. Explain this difference.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The formula alone cannot describe connectivity. Ether's oxygen can accept a hydrogen bond from water, but ether molecules cannot supply the required O-H hydrogen to each other.

   Marking points

   • Ethanol contains an O-H group.
   • Ethanol molecules form hydrogen bonds with each other; pure ether lacks an O-H donor.
   • More energy is needed to overcome ethanol's intermolecular attractions.

   Examiner tip: Boiling separates molecules; it does not break their covalent bonds.

5. 5.

   A material is hard, has a very high melting point and does not conduct when solid. Evaluate whether these observations prove it is diamond.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Use each observation to constrain a structural model. Hardness and thermal stability support an extended network, but physical properties are not a unique fingerprint of chemical composition.

   Marking points

   • The observations are consistent with a giant covalent network.
   • Many strong covalent bonds require much energy to disrupt.
   • Diamond lacks mobile electrons or ions.
   • They do not prove diamond: other materials such as silica can fit; composition/structural evidence is needed.

   Examiner tip: Separate 'consistent with' from 'uniquely proves'.

6. 6.

   Explain why graphite conducts along its layers and is soft, whereas diamond is insulating and hard, despite both being carbon.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Relate two different properties to two different structural features: electron mobility explains conduction, while connectivity and interlayer attraction explain mechanical behaviour.

   Marking points

   • Graphite carbon bonds to three neighbours, leaving delocalised electrons.
   • Delocalised electrons carry charge within layers.
   • Weak attractions between layers allow sliding.
   • Diamond has a rigid three-dimensional covalent network with electrons localised in bonds.

   Examiner tip: Do not describe graphite's bonds within a layer as weak.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.