Redox Reactions

CSEC Chemistry

Define oxidation 4 ways.

1) Gain of oxygen

2) Loss of hydrogen

3) Loss of electrons

4) Increase in oxidation number

Define reduction 4 ways.

1) Loss of oxygen

2) Gain of hydrogen

3) Gain of electrons

4) Decrease in oxidation number

OIL RIG

• Oxidation is loss of electrons and hydrogen

• Reduction is gain of electrons and hydrogen

What is a redox reaction?

It is a chemical reaction in which one reactant is reduced and the other is oxidised.

Define oxidation number( oxidation state) of an element.

This is the theoretical charge that an atom of an element would have if all the compound containing it were comprised of ions.

What is oxidising agent?

1) The reactant which CAUSES OXIDATION of the OTHER reactant while ITSELF IS REDUCED.

What is the reducing agent?

1) The reactant which CAUSES REDUCTION of the OTHER reactant while ITSELF IS OXIDISED.

What is the oxidation number of any atom in a neutral substance that has only one kind of atom (e.g. O2, Fe, P4, S8) ?

Zero (0)

What is the oxidation number of simple ions (e.g. Cl-, K+)?

The charge on the ion.

What is the oxidation number of hydrogen when it is combined with a non-metal (e.g. CH4, NH3, H2O, and HCl) ?

+1

What is the oxidation number of hydrogen when it is combined with a metal (e.g. LiH, NaH, CaH2, and LiAlH4) ?

-1

What is the usual oxidation of number of oxygen in a compound?

-2 Except when in peroxides (H2O2) where it is -1.

The sum of the oxidation numbers in a polyatomic ion is equal to

the charge on the ion

What is the oxidation number of fluorine?

-1

How is rust formed?

When iron and its alloy, steel, come into contact with oxygen and moisture the iron is oxidised by the oxygen to form hydrated iron (III) oxide (Fe2O3 . xH20) (rust).

Why do certain fruits and vegetables turn brown when cut and exposed to the air?

Enzymes in the plant cells on the surface of the fruit/vegetable begin to oxidise certain chemicals in the cells to brown compounds known as melanins.

What is the action of sodium sulfite and sulfur dioxide used in certain foods for preservation?

They are reducing agents and thus prevent oxidation e.g. preventing wine from being oxidized to vinegar(sour).

What is the chemical principle behind the breathalyzer test?

Any ethanol vapours in the driver’s breath will reduce the orange dichromate((VI) ion (𝐶𝑟2 𝑂7^(2−) to the green chromium (II) ion 𝐶𝑟^(3+)

How does bleach work ?

Bleach operates by oxidising the coloured chemicals resulting in them becoming colourless. ClO- (aq) + coloured dye -> Cl- (aq) + colourless dye

What is the oxidation state of sulfur in SO2 ? (NB. You have to work it out not guess)

+4

What is the oxidation state of manganese in MnO4^- ?

+7

What is the oxidation state of chromium in K2Cr2O7 ?

+6

What is the oxidation state of nitrogen in the nitrate ion (NO3^-)?

+5

List examples of common oxidising agents.

Bromine, chlorine, concentrated sulfuric acid, nitric acid, oxygen, potassium manganate, potassium dichromate, hydrogen peroxide

List examples of common reducing agents.

Carbon, carbon monoxide, hydrogen, hydrogen sulphide, metals, potassium iodide, sulphur dioxide, ammonia.

What is the colour change of potassium manganate when it is added to a reducing agent? Give reason for the change.

Manganate(VII) (MnO4^-) ion is reduced to manganese (II) Mn2+ ion. Purple to pale pink (or colourless)

What is the colour change of potassium dichromate when it is added to a reducing agent? Give reason for the change.

Dichromate(VI) (Cr2O7^2-) ion is reduced to chromium (III) ion(Cr3+) Orange to green

What is the colour change when iron (III) sulfate is in contact with a reducing agent? Give reason for the change.

Yellow-brown to pale green The yellow-brown Fe3+ ions are reduced to pale green Fe2+ ions.

What is the colour change when iodine is in contact with an oxidising agent? Give reason for the change.

Colourless to brown I- is oxidised to I2

What is the colour change when iron (II) sulfate solution is in contact with an oxidising agent? Give reason for the change.

Pale green to yellow-brown Fe2+ is oxidised to Fe3+

How can the presence of a reducing agent be confirmed?

Test with a strong oxidising agent with visible colour change.

How can the presence of an oxidising agent be confirmed?

Test with a strong reducing agent with visible colour change.