A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Alkenes: Bonding and Electron Density — mark scheme explained
The short answer
Alkenes are a class of hydrocarbons characterized by the presence of at least one double covalent bond between carbon atoms. This section delves into the bonding in alkenes, focusing on the nature of the double covalent bond and the concept of high electron density.
The question
Identify the alkene with the molecular formula C 4 H 8 and draw its structural formula.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For identification questions, marks are typically awarded for correctly naming the compound or structure. For explanation and description questions, marks are given for clear and concise answers that demonstrate a deep understanding of the topic. For writing equations, marks are awarded for correct reactants, products, and balancing.
What the command words demand
- Identify
- Recognize and name a specific compound or structure.
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Describe
- Give a clear account of the properties or characteristics of a substance or reaction.
- Write
- Formulate and present a balanced chemical equation for a given reaction.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed explanations, and check your work.
- Identify the general formula for alkenes, which is C n H 2n .1 mark
- Verify that the given molecular formula (C 4 H 8 ) matches the general formula for alkenes.1 mark
- Draw the structural formula of butene, which is an alkene with four carbon atoms and one double bond.2 marksThe structural formula of butene can be drawn as CH 3 -CH=CH-CH 3 .
Final answer: Butene (CH 3 -CH=CH-CH 3 )
Work through every step correctly and you earn all 4 marks.
Another worked example
Explain the bonding in a double bond of an alkene, including both sigma and pi bonds.
- Describe the formation of the sigma (σ) bond.2 marksThe sigma (σ) bond is formed by the head-on overlap of sp 2 -hybridized orbitals between two carbon atoms.
- Describe the formation of the pi (π) bond.2 marksThe pi (π) bond results from the side-by-side overlap of p-orbitals above and below the plane of the molecule.
- Summarize the nature of the double bond in alkenes.1 markA double bond in alkenes consists of one sigma (σ) bond and one pi (π) bond, with the π bond contributing to the high electron density around the double bond.
Final answer: The double bond in alkenes consists of one sigma (σ) bond formed by the head-on overlap of sp 2 -hybridized orbitals and one pi (π) bond formed by the side-by-side overlap of p-orbitals.
Work through every step correctly and you earn all 5 marks.
Common mistakes
Misidentifying alkenes from molecular formulas or structural diagrams.
Why it happens: Students may not recognize the presence of a double bond in the structure, leading to incorrect identification.
Fix: Always look for the presence of at least one carbon-carbon double bond when identifying alkenes.
Forgetting to mention both sigma and pi bonds when describing double bonding in alkenes.
Why it happens: Students may focus only on the sigma (σ) bond or the pi (π) bond, leading to an incomplete description of the double bond.
Fix: Always include both the sigma (σ) bond and the pi (π) bond when describing the double bond in alkenes.
Misunderstanding the concept of high electron density in alkenes.
Why it happens: Students may not fully grasp why the π bond contributes to high electron density and how this affects reactivity.
Fix: Practice explaining that the electrons in the π bond are more accessible due to their side-by-side overlap, leading to a region of high electron density around the double bond.
Incorrectly writing balanced chemical equations for electrophilic addition reactions.
Why it happens: Students may make errors in balancing the equation or identifying the correct products.
Fix: Always ensure that the reactants and products are correctly identified and that the equation is balanced with respect to both atoms and charges.
Failing to explain the planar geometry around a double bond in alkenes clearly.
Why it happens: Students may struggle to articulate the relationship between sp 2 -hybridization and the planar structure of alkenes.
Fix: Practice explaining that the carbon atoms involved in the double bond are sp 2 -hybridized, leading to a trigonal planar geometry with one sigma (σ) bond and one pi (π) bond.
Confusing alkenes with other types of hydrocarbons, such as alkanes or alkynes.
Why it happens: Students may not clearly distinguish between different classes of hydrocarbons based on their structural features.
Fix: Review the key characteristics of alkenes (unsaturated, double bond) and compare them with those of alkanes (saturated, single bonds) and alkynes (triple bonds).
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Identify And Draw Alkene | Identify the C4H8 alkene and draw its structural formula correctly. | 4 |
| Explain Alkene Bonding | Describe how sigma and pi bonds form the double bond in alkenes. | 5 |
| Explain Reactivity Difference | Explain why alkenes react more readily than alkanes using the pi bond's electron density. | 5 |
| Balance Organic Equation | Write a balanced equation for propene reacting with hydrogen bromide, showing correct reactants and products. | 4 |
| Explain Molecular Geometry | Describe the shape around an alkene double bond and explain why it is planar. | 5 |
| Total across these question types | 23 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.