Oxidation and Reduction

Question 1

COMMON OXIDISING AGENTS

Answer 1

• Acidified KMnO4
  (or KMnO4 / H+)
  ✔ Acidified K2Cr2O7
  (or K2Cr2O7 / H+)
  ✔ Iron (III) salts – Fe3+
  ✔ Dilute or concentrated nitric acid
  ✔ Concentrated sulphuric acid (usually
  hot)
  ✔ Sodium chlorate (I), NaClO (BLEACH)
  ✔ Oxygen
  ✔ Chlorine
  ✔ Manganese (IV) oxide, MnO2

Question 2

COMMON REDUCING AGENTS

Answer 2

✔ Potassium iodide
✔ Hydrogen sulphide
✔ Iron (II) salts – Fe2+
✔ Concentrated hydrochloric acid
✔ Hydrogen
✔ Carbon
✔ Carbon monoxide
✔ Reactive metals

Question 3

SUBSTANCES THAT CAN BEHAVE AS BOTH OXIDISING AND REDUCING AGENTS

Answer 3

SULPHUR DIOXIDE – SO2
ACIDIFIED HYDROGEN PEROXIDE (H2O2/H+)

Question 4

ACIDIFIED HYDROGEN PEROXIDE (H2O2/H+)

Answer 4

Acidified Hydrogen Peroxide is usually an oxidising agent. If it reacts with a stronger oxidising agent than
itself, it acts as a reducing agent.

✔With aqueous potassium iodide it acts as an oxidising agent and oxidises the iodide ions to iodine.
✔ With both KMnO4 / H+ and K2Cr2O7 / H+, both stronger oxidising agents, it acts as a reducing agent.

The acidified hydrogen peroxide reduces the purple MnO4– ion to the colourless Mn 2+ ion; and the orange Cr2O72– ion to the green Cr 3+ ion.

Question 5

SULPHUR DIOXIDE – SO2

Answer 5

Sulphur dioxide is usually a reducing agent. However, if it reacts with a stronger reducing agent than itself, it
will act as an oxidising agent.

✔ With both KMnO4 / H+ and K2Cr2O7 / H+ it acts as a reducing agent

✔ With hydrogen sulphide, a stronger reducing agent, it acts as an oxidising agent and oxidises the
hydrogen sulphide to yellow sulphur.

Question 6

OXIDATION AND REDUCTION REACTIONS IN EVERYDAY LIFE

Answer 6

✔ Rusting
✔ The action of bleach
✔ The browning of fruits and vegetables
✔ The use of sulphur dioxide and sulphites in food preservation

Question 7

RUSTING- HOW?

Answer 7

When oxygen and water are present, iron is oxidised to form hydrated iron (III) oxide which is
commonly called rust.

Question 8

The action of bleach

Answer 8

Bleach is added to clothes to remove stains.
There are 2 types of bleaches:

✔ Chlorine bleaches which contains chlorate (I) ions
✔ Oxygen bleaches containing hydrogen peroxide

When added to stained cloth, the bleach oxidises the stain or dye to a colourless form which is then
washed out.

Question 9

THE BROWNING OF FRUITS AND VEGETABLES

Answer 9

When some vegetables and fruits are cut enzymes in the cells on the cut surfaces are exposed to oxygen in the air. These enzymes oxidize phenols (colourless compounds) to polymers (melanins) which are coloured and cause the surface to appear brown.

Question 10

THE USE OF SULPHUR DIOXIDE AND SULPHITES IN FOOD PRESERVATION

Answer 10

they are reducing agents used as food perservatives and they prevent the growth of microbes (spoliage) by preventing oxidation and inhibiting enzyme action.

they prevent fats from getting rancid, prevent fruits from getting brown, prevent bacterial action, etc.