Topic 6 Tests

Question 1

How can a tertiary alcohol be distinguished from a primary (or secondary) alcohol?

Answer 1

Heat both with acidified potassium dichromate. The tertiary alcohol will remain in a yellow solution but primary and secondary alcohols will turn the solution green.

Question 2

Explain why, during the oxidation of primary and secondary alcohols, the solution changes from yellow to green.

Answer 2

The yellow chromium (VI) ions are reduced to green chromium (III) ions.

Question 3

The oxidation of alcohols using potassium dichromate does not distinguish between primary and secondary alcohols. Suggest how they could be distinguished between.

Answer 3

Oxidise the alcohols under distillation using acidified potassium dichromate and then add Fehling’s reagent. If the solutions turns red it is primary, if it remains blue it is a secondary alcohol.

Question 4

Explain why the Fehling’s reagent turns red for primary alcohols and remains blue for secondary alcohols.

Answer 4

Fehling’s is a mild oxidising agent so in the presence of an aldehyde the blue copper(II) ions are reduced to red copper (I) ions. Ketones cannot be oxidised so remain blue.

Question 5

Describe the process of testing which halogen is present in a halogenoalkane.

Answer 5

1. Add drop of the halogenoalkane to aqueous sodium hydroxide and heat in a water bath. [Hydrolysis of the halogenoalkane]
2. Acidify with dilute nitric acid.
3. Add a few drops of silver nitrate solution.

Question 6

Describe the colours of the following solid precipitates:

1. AgCl(s)
2. AgBr(s)
3. AgI(s)

Answer 6

1. White
2. Cream
3. Yellow

Question 7

What is the general test for an alkene?

Answer 7

Add an excess of alkene to bromine water then it will go from brown to colourless.