OCR A Level Chemistry B (Salters)
OCR A Level Chemistry B Colour by Design (CD) Exam Questions
Free OCR A Level Chemistry B exam questions for Colour by Design (CD), with a mark scheme for every pack. Covers colour and conjugation, benzene, electrophilic substitution, azo dyes, aldehydes and ketones, fats and oils, chromatography and synthesis.
Colour by Design (CD) exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About OCR A Level Chemistry B Colour by Design (CD)
Colour by Design is a Year 2 storyline of OCR A Level Chemistry B (Salters) that explores how dyes and pigments are made and why they are coloured. It covers the delocalised structure of benzene and electrophilic substitution, the formation of diazonium salts and azo dyes, and how chromophores and extended conjugation produce colour. It also includes the reactions of aldehydes and ketones, the chemistry of fats and oils, gas chromatography and planning organic synthesis routes. Examiners like mechanisms of electrophilic substitution, the conditions for diazotisation and coupling, tests for carbonyl compounds and synthesis routes that combine reactions from across the course.
What you need to know
- Colour and conjugation Coloured organic molecules contain a chromophore, often an extended delocalised system. Absorbing visible light promotes electrons to higher energy levels; more conjugation lowers the energy gap and shifts absorption to longer wavelengths.
- Benzene and electrophilic substitution Benzene's delocalised ring is more stable than the Kekule structure. It undergoes electrophilic substitution: nitration with concentrated nitric and sulfuric acids, halogenation with a halogen carrier, and Friedel-Crafts reactions with AlCl3.
- Azo dyes Phenylamine reacts with nitrous acid (NaNO2 and HCl) below 10 C to form a diazonium salt, which couples with a phenol in alkaline solution or an aromatic amine to give an azo dye containing N=N.
- Aldehydes and ketones Aldehydes are oxidised by Fehling's solution (red precipitate) and Tollens' reagent (silver mirror); ketones are not. Both undergo nucleophilic addition with HCN and are reduced by NaBH4.
- Fats and oils Triglycerides are esters of propane-1,2,3-triol and fatty acids. Unsaturated oils contain C=C bonds and can be hydrogenated; hydrolysis gives glycerol and fatty acids or their salts.
- Analysis and synthesis Gas chromatography separates components by retention time. Plan multi-step routes using known reactions, giving reagents and conditions for each step.
Common mistakes in Colour by Design (CD)
- Letting diazotisation warm above 10 C. The diazonium salt decomposes, releasing nitrogen.
- Saying colour arises from light emitted by the dye. It results from absorption of some visible wavelengths, with the remaining light reflected or transmitted.
- Drawing a full delocalised ring in the intermediate of electrophilic substitution.
- Expecting ketones to react with Tollens' reagent or Fehling's solution.
- Forgetting the alkaline conditions needed for coupling with phenols.
Try three Colour by Design (CD) exam-style questions
1. Give the reagents and conditions to make benzenediazonium chloride from phenylamine, and write the structure of the azo dye formed with phenol. (4 marks)
Show the mark scheme answer
NaNO2 and HCl (nitrous acid); below 10 C (about 5 C); couple with phenol in alkaline (NaOH) solution; product C6H5N=NC6H4OH (4-hydroxyazobenzene).
2. 0.78 g of benzene (Mr 78.0) is nitrated and 0.95 g of nitrobenzene (Mr 123.0) is obtained. Calculate the percentage yield. (3 marks)
Show the mark scheme answer
Moles benzene = 0.78 / 78.0 = 0.0100 mol; theoretical mass = 0.0100 x 123.0 = 1.23 g; yield = 0.95 / 1.23 x 100 = 77.2%.
3. Explain why increasing the extent of conjugation in a dye molecule changes its colour. (2 marks)
Show the mark scheme answer
Greater delocalisation decreases the energy gap between electron energy levels; so light of lower frequency (longer wavelength) is absorbed and a different complementary colour is seen.
Colour by Design (CD) FAQs
What does OCR A Level Chemistry B Colour by Design (CD) cover?
It covers the origin of colour in organic molecules, benzene and electrophilic substitution, diazonium salts and azo dyes, aldehydes and ketones, fats and oils, gas chromatography and synthesis.
Which papers is Colour by Design examined in?
In the Salters A Level, content from any storyline can be assessed in all three papers: Fundamentals of chemistry, Scientific literacy in chemistry and Practical skills in chemistry.
How do I revise synthesis for Colour by Design?
Make a reaction map that links aromatic compounds, carbonyls, alcohols, acids, esters, amines and azo dyes, then practise writing two and three step routes with reagents and conditions.
