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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the general formula for alkenes, which is C n H 2n .
- S2
Step 2: Verify that the given molecular formula (C 4 H 8 ) matches the general formula for alkenes.
- S3
Step 3: Draw the structural formula of butene, which is an alkene with four carbon atoms and one double bond.
- S4
The structural formula of butene can be drawn as CH 3 -CH=CH-CH 3 .
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the general formula for alkenes, which is C n H 2n .
- S2
Step 2: Verify that the given molecular formula (C 4 H 8 ) matches the general formula for alkenes.
- S3
Step 3: Draw the structural formula of butene, which is an alkene with four carbon atoms and one double bond.
- S4
The structural formula of butene can be drawn as CH 3 -CH=CH-CH 3 .
Final answer: Butene (CH 3 -CH=CH-CH 3 )
Common mistakes
- Misidentifying alkenes from molecular formulas or structural diagrams. — 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. — 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. — 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. — 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. — 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. — 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
- Full worked solution (all marking points)4 marks