Oxidation and Reduction in Terms of Oxygen/Hydrogen Transfer

Understanding Oxidation and Reduction

  1. Oxidation and reduction can be understood from the aspect of:
    1. Losing or gaining oxygen 
    2. Losing or gaining hydrogen
    3. Transferring of electron
    4. Changing of oxidation number

Oxidation and Reduction in Terms of Oxygen Transfer

  1. Oxidation is the process of gaining oxygen.
  2. Reduction is the process of losing oxygen.
  3. For example, in the extraction of iron from its ore:
    1. Iron(III) oxide loses oxygen. Iron(III) oxide is reduced to iron. This is a reduction process.
    2. Carbon monoxide gains oxygen. Carbon monoxide is oxidised to become carbon dioxide. This is an oxidising process.

Oxidation and Reduction in Terms of Hydrogen Transfer
  1. Oxidation is the process of losing hydrogen.
  2. Reduction is the process of gaining hydrogen.
  3. For example, ethanol can be oxidised to ethanal:
    1. Ammonia loses hydrogen. Ammonia is oxidised to become nitrogen. This is an oxidation process.
    2. Bromine gains hydrogen. Bromine is reduced to become hydrogen bromide. This is a reduction process.

Example:

Combustion of Magnesium in Air

  • Magnesium is oxidised to become magnesium oxide.
Displacement of copper(II) oxide by Carbon

  • Copper(II) oxide is reduced to become copper metal
  • Carbon is oxidised to become carbon dioxide.

Displacement of lead(II) oxide by Zinc

  • Zinc is oxidised to become zinc oxide.
  • Lead(II) oxide is reduced to become lead metal

Reaction between Magnesium and Steam

  • Water is reduced to become hydrogen gas.
  • Magnesium is oxidised to become magnesium oxide.
Hydrogen Sulphide Reacts with Chlorine

  • Hydrogen sulphide is oxidised to become sulphur.
  • Chlorine is reduced to become hydrogen peroxide.

Copper(II) oxide Reacts with Ammonia

  • Copper(II) oxide is reduced to become copper(II) metal.
  • Ammonia is oxidised to become nitrogen gas.

Oxidation and Reduction in Terms of Electron Transfernts

  1. Oxidation occurs when a reactant loses electron.
  2. Reduction occurs when a reactant gain of electron.

Example:

CuO + Mg → Cu + MgO
In this reaction,
  1. copper (II) ion in copper(II) oxide gains 2 electrons to form copper metal.
    Half equation:
    Cu2+ + 2e → Cu

    This is a reduction process.

  2. Magnesium metal loses 2 electrons to form magnesium ions in magnesium oxide.
    Half equation:
    Mg → Mg2+ + 2e

    This is a oxidation process.

Summary

  1. Magnesium is oxidised because it loses2 electrons to form magnesium ion, Mg2+.
  2. Copper(II) oxide is reduced because copper(II) ion gains 2 electrons to form copper metal, Cu.

Oxidation and Reduction in Term of Changes of Oxidation State

Oxidation States (Oxidation Numbers)

Oxidation state shows the total number of electrons which have been removed from an element (a positive oxidation state) or added to an element (a negative oxidation state) to get to its present state.

Oxidation State of Some Elements

1. The oxidation state of an element is zero.
Example

Element
Oxidation State
Mg
0
H2
0
Br2
0

2. For a simple ion with single atom, the oxidation state is equal to the charge.
Example

IonOxidation State
Cu2+ +2
Br -1
O2- -2
Al3+ +3

3. Some elements almost always have the same oxidation states in their compounds:

Example 1:
The oxidation state of oxygen is always -2 except peroxide, which is -1.

CompoundOxidation state of oxygen
H2O-2
H2SO4-2
ZnO-2
KClO3-2
H2O2-1


Example 2:
The oxidation state of hydrogen is always +1 except hydride, which is -1.

CompoundOxidation state of hydrogen
NH3+1
HCl +1
NaOH +1
MgH2 -1
NaH -1

4. The sum of the oxidation states of all the atoms or molecule in a neutral compound is zero.

Example
:

IonSum of Oxidation State
H2O 0
CO2 0
NH3 0

5. The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.

Example
:

IonSum of Oxidation State
NO3 -1
CO32- -2
PO43- -3
NH4+ +1
Working Out the Unknown Oxidation State of an Element in a Compound
  1. The sum of the oxidation state of each element in a compound are equal to the charge of the compound.
  2. This rule can be used to find the unknown oxidation number of an element is a compound.

Example 1

Find the oxidation state of all the elements in a Chlorate(V), ClO3 ion.

Answer:
Oxidation number of O = -2

Oxidation number of Cl = x

x + 3(-2) = -1
x = -1 + 6 = +5

Example 2
Find the oxidation state of all the elements in a Potassium manganate(VII), KMnO4 ion.

Answer:
Oxidation number of K = +1`

Oxidation number of O = -2

Oxidation number of Mn = x

(+1) + x + 4(-2) = 0
x = -1 + 8
x = +7

Example 3
Find the oxidation state of all the elements in an Ammonium ion, NH4+ ion.

Answer:
Oxidation number of H = +1

Oxidation number of N = x

x + 4(+1) = +1
x = +1 – 4
x = -3
Using the Oxidation States in Naming Compounds
  1. You will have come across names like iron(II) sulphate and iron(III) chloride. The (II) and (III) are the oxidation states of the iron in the two compounds: +2 and +3 respectively. That tells you that they contain Fe2+ and Fe3+ ions.
    Example

    Formula

    Name of the compound
    FeCl2Iron(II) chloride
    FeCl3Iron (III) chloride
    MnO2Manganese(IV) oxide
    Mn(NO3)2Manganese (II) nitrate
    PbCl2Lead(II) chloride
  2. Transition metals always show difference oxidation state as shown in the table below.

    Metal

    Oxidation state

    Fe+2, +3
    Cu+1, +2
    Mn+2, +4, +6, +7
  3. Non-metal elements (except fluorine) usually have more than one oxidation state.

    Oxidation and Reduction in Term of Changes of Oxidation State

    Oxidation involves an increase in oxidation state
    Reduction involves a decrease in oxidation state
    1. Another way to determine oxidation and reduction is to see the change of the oxidation state after a reaction.
    2. An atom is said to be oxidised when its oxidation state increases.
    3. An atom is said to be reduced when its oxidation state decreases.

    Example:

    1. The magnesium’s oxidation state has increased by 2, from 0 to +2. Therefore, it has been oxidised.
    2. The hydrogen’s oxidation state has decreased by 1, from +1 to 0. Therefore it has been reduced.
    3. The chlorine is in the same oxidation state on both sides of the equation – it hasn’t been oxidised nor reduced.

    Example:

    There is no change of oxidation state for all elements. This isn’t a redox reaction.

    Example:

    1. In this example, we can see that the oxidation state of chlorine has increased and also decreased.
    2. Chlorine is oxidised and reduced, at the same time.
    3. This is a good example of a disproportionation reaction. A disproportionation reaction is one in which a single substance is both oxidised and reduced.