Edexcel A-Level Chemistry AS Paper 1, November 2021: Question 5

7 marks · Medium difficulty · Short Open Response

Explain the difference in melting temperature between diamond and iodine, and describe the key bonding feature in graphite that allows it to conduct electricity.

Practise this question

Question

Question text

5 This question is about crystalline solids.

(a) Iodine and diamond are crystalline solids at room temperature.

Explain why diamond has a much higher melting temperature than iodine.

(5)

(b) Graphite is also a crystalline solid at room temperature.

Unlike diamond, graphite conducts electricity.

Describe the key feature of the bonding of the carbon atoms in graphite that

results in it being an electrical conductor.

(2)

… *P67083A01424*

(Total for Question 5 = 7 marks)

Mark scheme

Show the mark scheme

Question Answer Additional Guidance Mark

Number

5(a) An explanation that makes reference to the following (5)

points:

• iodine is (simple) molecular (1) Allow iodine is made up of (I2) molecules

• diamond is a giant (covalent / lattice) structure Do not award diamond molecules

(1)

(with 4 covalent bonds per carbon atom)

• iodine molecules are held together by weak Allow weak intermolecular forces

London forces / dispersion forces / van der (1)

Waal’s forces / instantaneous induced dipole-

dipole attractions

• carbon atoms in diamond are held together by (1) Do not award strong intermolecular forces

(strong) covalent bonds

• strong covalent bonds require more energy to Award converse argument for less energy need to

(1)

break than intermolecular forces break intermolecular forces

Single sentences may contain more than one marking

point. For example ‘iodine molecules are held

together by weak intermolecular forces’ scores (2)

Number

5(b) An answer that makes reference to the following points: Mark independently (2)

Marks could be scored in a diagram

• one electron free to move / delocalised (within

the layer to carry the current) (1) Ignore just ‘free electrons’

• each carbon is (covalently) bonded to three other Allow uses three (outer shell) electrons in bonding

carbons

the carbon atoms are arranged in layers which

allow the flow of electricity through them (1)

(Total for Question 5 = 7 marks)

How to answer it

Structure and Bonding in Crystalline Solids

What this question tests

This question assesses your understanding of bonding and structure types at AS Level. You must be able to contrast giant covalent lattices (diamond, graphite) with simple molecular lattices (iodine), link physical properties (melting point, electrical conductivity) to internal bonding types, and use precise chemical terminology regarding intermolecular forces versus covalent bonds.

Part (a): Melting Temperatures of Diamond vs. Iodine

Explain why diamond has a much higher melting temperature than iodine. (5 marks)

✅ Model Answer Points

Iodine is a simple molecular structure (made of I₂ molecules).
Diamond is a giant covalent / lattice structure with 4 covalent bonds per carbon atom.
Iodine molecules are held together by weak London forces (dispersion forces / induced dipole-dipole attractions).
Carbon atoms in diamond are held together by strong covalent bonds.
A large amount of energy is required to break the strong covalent bonds in diamond, whereas very little energy is needed to overcome the weak intermolecular forces in iodine.

💡 Key Knowledge

Structure Identification: Always state the structure type first before discussing bonding forces.
Intermolecular vs Intramolecular: Ensure you never confuse the forces *between* molecules with the bonds *within* molecules or giant lattices.

🧠 Exam Technique

This is a 5-mark comparative question. To secure full marks, structure your answer clearly: state the structure of both substances, name the specific forces/bonds present in both, and explicitly compare the energy required to disrupt them.

❌ Common Errors

Calling diamond "molecular" or stating it consists of "diamond molecules".
Describing intermolecular forces as "strong covalent bonds between iodine molecules".
Vague phrasing like "diamond has strong bonds" without specifying they are covalent bonds.

Mark Allocation: 1 mark for iodine molecular structure | 1 mark for diamond giant covalent structure | 1 mark for weak intermolecular forces in iodine | 1 mark for strong covalent bonds in diamond | 1 mark for energy comparison (more energy to break covalent bonds than intermolecular forces).

Part (b): Electrical Conductivity of Graphite

Describe the key feature of the bonding of the carbon atoms in graphite that results in it being an electrical conductor. (2 marks)

✅ Model Answer Points

Each carbon atom is covalently bonded to three other carbon atoms (leaving spare electrons).
There is one electron per carbon atom free to move / delocalised (within the layer) to carry the electrical current.

Alternative valid point: The carbon atoms are arranged in layers which allow the flow of electricity through them.

💡 Key Knowledge

Graphite's physical properties stem from its sp² hybridization. Each carbon uses 3 of its 4 outer shell electrons for covalent bonding within a hexagonal layer, leaving the 4th electron delocalised in a p -orbital perpendicular to the layers.

🧠 Exam Technique

Avoid generic statements like "graphite has free electrons". Examiners strictly require you to link it back to the bonding structure—specify that each carbon is bonded to three atoms and that there is a delocalised/free electron within the layer.

❌ Common Errors

Writing "free ions" instead of free/delocalised electrons.
Stating each carbon is bonded to four atoms (confusing graphite with diamond).
Just writing "has free electrons" without mentioning bonding context or delocalisation.

Mark Allocation: 1 mark for delocalised / free to move electron (to carry current) | 1 mark for carbon bonded to three other atoms (or reference to layers facilitating electrical flow).

Topics

Physical Chemistry · Topic 2: Bonding and Structure

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.