Potassium dichromate oxidation

A pool of acidified potassium dichromate(VI) is placed in a Petri dish. A few drops of cyclohexanol are added to the pool. There are clear signs of reaction as the cyclohexanone is formed.

Lesson organisation

This is a quick microscale experiment to illustrate that alcohols like cyclohexanol react with acidified dichromate(VI). No attempt is made to identify the product. The experiment takes about 15 minutes.

Chemicals

Cyclohexanol (HARMFUL)

For the potassium dichromate(VI) solution (Note 1:

Potassium dichromate(VI) (OXIDISING, VERY TOXIC, DANGEROUS FOR THE ENVIRONMENT), 2 g

Concentrated sulfuric acid (CORROSIVE)

Refer to Health & Safety and Technical notes section below for additional information.

Apparatus

Eye protection: goggles

Each group of students will need:

Petri dish, 5.5 cm size, plastic

Dropping pipettes, 2

White paper

Health & Safety and Technical notes

Wear goggles throughout.

Cyclohexanol, C6H11OH(s) or (l) (HARMFUL) - see CLEAPSS Hazcard. Cyclohexanol has a melting point of 20 – 22°C (about room temperature). If this experiment is done on a warm day the cyclohexanol is liquid and therefore easier to sample. In cold laboratories, warm the cyclohexanol first!

Potassium dichromate(VI), K2Cr2O7(s), (OXIDISING, VERY TOXIC, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.

Concentrated sulfuric acid, H2SO4(l), (CORROSIVE) - see CLEAPSS Hazcard.

1 To make up the the acidified dichromate(VI) solution: dissolve 2 g of potassium dichromate(VI) in 80 cm3 of deionised or distilled water and slowly add 10 cm3 of concentrated sulfuric acid to the solution, with cooling. Label the solution TOXIC and CORROSIVE.

Procedure

a Place a Petri dish on a piece of white paper.

b Collect 1 cm3 of acidified potassium dichromate(VI) solution in a dropping pipette.

c Carefully, use the pipette to place ten drops of potassium dichromate(VI) in the centre of the dish.

d Use another pipette to place one drop of cyclohexanol into the pool of dichromate(VI) solution.

e Observe the pool over the next few minutes. Look for signs of reaction and of new substances being formed.

Teaching notes

A great deal of surface activity is observed.

This is due to the differences in physical properties such as surface tension and viscosity of the product (cyclohexanone) compared to the reactant (cyclohexanol). A green colour, due to Cr3+(aq), will also be seen.