Chem 12
Oxidation Numbers
Name:
In ionic equations with monatomic elements, such as Cu and Zn, and with ions containing a single element, such as Cu2+ and Zn2+,
we can use ionic charges to describe the transfer of electrons. However, many redox reactions involve reactants or products with
covalent bonds, including elements that exist as covalent molecules, such as oxygen, O 2; covalent compounds, such as water, H2O;
or polyatomic ions that are not spectator ions, such as permanganate, MnO4−. For reactions involving covalent reactants and
products, you cannot use ionic charges to describe the transfer of electrons. Oxidation numbers are actual or hypothetical
charges, assigned using a set of rules. They are used to describe redox reactions with covalent reactants or products. They are also
used to identify redox reactions, and to identify oxidizing and reducing agents. Oxidation numbers can be found using Lewis
diagrams or using these rules!
Oxidation = increase in oxidation number
Reduction = decrease in oxidation number
Group 1A: cations(usually) = +1
Group 2A :cations (usually) = +2
◘ Note: (i) Oxidation numbers may be fractions or even zero. (ii) Oxidation numbers are always per atom. They will be recorded
above the atom in the formula. (iii) The oxidation number of an element may vary from molecule to molecule.
Practise:
1. In each of the following chemicals, determine the oxidation states of each element:
a) sodium nitrate
b) ammonia
c) zinc oxide
d) water
e)calcium hydride
f)carbon dioxide
g) nitrogen
h) sodium sulfate
i) aluminum hydroxide
j)magnesium phosphate
2. Give the oxidation numbers of all the elements in the following molecules and ions:
a. SO, SO2, SO3 , SO32-, SO4-2
b. ClO2, ClO1-, ClO21-, ClO31-, ClO41c. N2O, NO, NO2, N2O4, N2O5, NO2-, NO33. Determine the oxidation number of the sulfur atom:
a. H2S b. S
c. H2SO4
d. S-2 e. HS- f. SO2 g. SO3
Answers:
In each of the following chemicals, determine the oxidation states of each element:
1)
sodium nitrate: Na, +1; N, +5; O, -2
2)
ammonia: N, -3; H, +1
3)
zinc oxide: Zn, +2: O, -2
4)
water: H, +1; O, -2
5)
calcium hydride: H, -1; Ca, +2
6)
carbon dioxide: C, +4; O, -2
7)
nitrogen: N, 0
8)
sodium sulfate: Na, +1; S, +6; O, -2
9)
aluminum hydroxide: Al, +3; H, +1; O, -2
10)
magnesium phosphate: Mg, +2; P, +5; O, -2