CH 115 Fall 2014Worksheet 23

1.  List a few rules for determining the oxidation numbers for elements in a compound.

Any thing in its elemental state has an oxidation state of 0.

Oxygen has an oxidation state of -2 except in H2O2.

Hydrogen has an oxidation state of +1.

All other elements generally have an oxidation state equal to their monatomic ion charge.

Oxidation states in a compound must add up to the charge on that compound.

2.  Determine the oxidation numbers of each element in the following compounds.

a). MgBr2 – Mg = +2, Br = -1

b). Fe2O3 – O = -2, Fe = +3

c). AlN – N = -3, Al = +3

d). SO3 – O = -2, S = +6

e). PO43- - O = -2, P = +5

f). Cr2O72- - O = -2, Cr = +6

g). HClO2 – O = -2, H = +1, Cl = +3

h). CuSO4 – O = -2, Cu = +2, S = +6

3.  What does it mean for something to be oxidized? Reduced? What is an oxidation-reduction reaction? What is an oxidizing agent? A reducing agent?

OIL RIG or LEO GER – oxidation is loss of electrons and reduction is gain of electrons. Can also think of oxidation as an increase in oxidation number or reduction as a decrease in oxidation number.

An oxidation-reduction reaction is one that has movement of electrons from an oxidized substance to a reduced substance. You cannot have oxidation without reduction.

An oxidizing agent is the compound that was reduced.

A reducing agent is the compound that was oxidized.

Agents are ALWAYS reactants.

4.  For each of the following, determine which species was oxidized and which was reduced. Also indicate the oxidizing and reducing agents and write the half reactions for each.

The first thing you should do on these problems is assign oxidation states/numbers to each element in the reaction. This will help you see the oxidation and reduction.

a). Cr+ + Sn4+ Cr3+ + Sn2+

oxidation – Cr+ ® Cr3+ + 2e-

reduction – Sn4+ + 2e- ® Sn2+

oxidizing agent – Sn4+

reducing agent – Cr+

b). 3 Hg2+ + 2 Fe (s) 3 Hg2 + 2 Fe3+

oxidation – 2 Fe ® 2 Fe3+ + 6e-

reduction – 3 Hg2+ + 6e- ® 3 Hg2

oxidizing agent – Hg2+

reducing agent - Fe

c). 2 As (s) + 3 Cl2 (g) 2 AsCl3

oxidation – 2 As ® 2 AsCl3 + 6e-

reduction – 3 Cl2 + 6e- ® 2 AsCl3

oxidizing agent – Cl2

reducing agent - As

d). 3 I2(aq) + 2 S2O32-(aq) 6 I-(aq) + S4O6 (aq)

oxidation – 2 S2O32- ® S4O6 + 4e- (sulfur is the element oxidized)

reduction – 3 I2 + 6 e- ® 6 I-

oxidizing agent – I2

reducing agent – S2O32-

e). 2 MnO4-(aq) + 5 H2O2(aq) + 6 H+(aq)2 Mn2+(aq) + 5 O2(g) + 8 H2O(l)

oxidation – 5 H2O2 ® 5 O2 + 10 e- (oxygen is oxidized)

reduction – 2 MnO4- + 10 e- ® 2 Mn2+ (manganese is reduced)

oxidizing agent – MnO4-

reducing agent – H2O2