oxidation of alcohols ms answers

thanol can be oxidised by acidified potassium dichromate(VI) to ethanoic acid in a two-step process.

ethanol—-ethanal— ethanoic acid

(a) In order to ensure that the oxidation to ethanoic acid is complete, the reaction is carried out under reflux. Describe what happens when a reaction mixture is refluxed and why it is necessary, in this case, for complete oxidation to ethanoic acid.

A mixture of liquids is heated to boiling point for a prolonged time 1

Vapour is formed which escapes from the liquid mixture, is changed back into liquid and returned to the liquid mixture

Any ethanal and ethanol that initially evaporates can then be oxidised

Write a half-equation for the overall oxidation of ethanol into ethanoic acid.

CH3 CH2 OH + H2 O CH3 COOH + 4H+ + 4e

The boiling points of the organic compounds in a reaction mixture are shown in the following table. Compound ethanol ethanal ethanoic acid Boiling point / °C (oh) 78 (co) 21 (cooh) 118

Use these data to describe how you would obtain a sample of ethanal from a mixture of these three compounds. Include in your answer a description of the apparatus you would use and how you would minimise the loss of ethanal. Your description of the apparatus can be either a description in words or a labelled sketch.

Mixture heated in a suitable flask / container A labelled sketch illustrating these points scores the marks

1 With still head containing a thermometer

Water cooled condenser connected to the still head and suitable cooled collecting vessel

Collect sample at the boiling point of ethanal

Cooled collection vessel necessary to reduce evaporation of ethanal

A student obtained a sample of a liquid using the apparatus in part (c). Describe how the student could use chemical tests to confirm that the liquid contained ethanal and did not contain ethanoic acid.

-tollens reagent where silver mirror is produced if ethanal present, no silver mirror if no ethanal present

Reagent to confirm the absence of ethanoic acid

-Add sodium hydrogencarbonate or sodium carbonate Result; no effervescence observed; hence no acid present