TESTING FOR OXIDISING AND REDUCING AGENTS

Question 1

The USUAL reagents to test for an oxidising agent are

Answer 1

 Aqueous KI
 A solution containing Fe 2 + ions

If an oxidising agent is present, a colour change in the reagent is seen.

Question 2

AQUEOUS POTASSIUM IODIDE

Answer 2

The aqueous KI is normally colourless but if a few drops of it are added and the reagent
turns brown (sometimes seen also as a golden yellow colour), then an oxidising agent is present. This happens because the I– ions lose electrons to become I2 which is released
and dissolves in the solution.

The dissolved iodine gives the solution this brown to golden yellow appearance.
Tiny flecks (pieces) of solid iodine can also be seen floating around in the
solution.

Question 3

AQUEOUS POTASSIUM IODIDE equation

Answer 3

2I- (aq) - 2e- —–> I2 (aq)

Question 4

SOLUTION CONTAINING Fe 2+ IONS

Answer 4

The solution containing Fe 2+ ions is normally pale green in colour but if a few drops of it
are added and the reagent turns brown in colour (sometimes seen as dark yellow/brown
or looks like ‘rusty water’), then an oxidising agent is present. This happens because the
Fe 2+ ions lose an electron to become Fe 3+ ions which appears red-brown in solution.

Question 5

SOLUTION CONTAINING Fe 2+ IONS equation

Answer 5

Fe2+(aq) -e- ——-> Fe3+ (aq)

Question 6

The USUAL reagents to test for a reducing agent are

Answer 6

 Acidified KMnO4
 Acidified K2Cr2O7

If a reducing agent is present, a colour change in the reagent is seen.

Question 7

ACIDIFIED POTASSIUM MANGANATE (VII)

Answer 7

The acidified KMnO4 is normally purple in colour but if a few drops of it are added and
the reagent turns colourless (only 1 or 2 drops must be added or a false negative result will be obtained), then a reducing agent is present. This happens because the Mn 7+ ions (which are purple in solution) gain electrons to become Mn 2+ (which are colourless in solution).

Question 8

ACIDIFIED POTASSIUM DICHROMATE

Answer 8

The solution containing acidified K2Cr2O7 is normally orange in colour but if a few drops
of it are added and the reagent turns dark green in colour, then a reducing agent is
present. This happens because the Cr 6+ ions (which are orange in colour) gain electrons
to become Cr 3+ ions (which are dark green in colour).

Question 9

ACIDIFIED POTASSIUM MANGANATE (VII) equation

Answer 9

Mn7+(aq) + 5e- —–> Mn2+ (aq)

Question 10

ACIDIFIED POTASSIUM DICHROMATE equation

Answer 10

Cr6+(aq) + 3e- ——> Cr3+ (aq)

Question 11

to ensure potassium dichromate is acidified

Answer 11

mix it with an acid eg. dilute sulphuric acid and dip blue litmus paper in the solution and it should turn red