Oxidizing agents cause another substance to be oxidized. The oxidizing agent accepts electrons, meaning the oxidizing agent itself is reduced as a result.
Reducing agents cause another substance to be reduced. The reducing agent donates electrons, meaning that the reducing agent itself is oxidized.
How do you determine the oxidizing and reducing agents in an equation?
All you need to do is write the oxidation numbers of each element or ion in each compound and see which reactant has an element with an oxidation number that increased or decreased. If the oxidation number increased, then that compound was oxidized, and is therefore the reducing agent; and if the oxidation number decreased, then that compound was reduced, and is therefore the oxidizing agent.
Let's try this in a question then.
Identify the oxidizing agent and reducing agent in the following reaction:
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
We first determine the oxidation number of each element:
Reactants
Zn is in its free state, so its oxidation number is 0.
Cu is bonded to the sulphate ion, which has a charge of 2- and oxidation number of -2. Since the sum of oxidation numbers in a neutral compound must add up to zero, we know that:
Oxidation number of Cu + (-2) = 0
Oxidation number of Cu = 0 - (-2)
Oxidation number of Cu = +2
(always remember to include the + or - sign in the oxidation number)
The sulphate ion has a charge of 2- and oxidation number of -2. Oxygen has an oxidation number of -2 here. So, we can calculate the oxidation number of the sulfur in the ion as well:
S + 4(-2) = -2
S = -2 - (-8)
S = +6
Products
Zn here is bonded to the sulphate ion, which has an oxidation number of -2. Since the sum of oxidation numbers in a neutral compound must add up to zero, we know that Zn must have an oxidation number of +2.
SO₄ is unchanged.
Cu is in its free state, so it has an oxidation number of 0.
0 +2 +6 -2 +2 +6 -2 0
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
So, the oxidation number of zinc increases while the oxidation number of Copper decreases. Therefore, Zinc is oxidized and Copper Sulphate is reduced, meaning that Zinc is the reducing agent and Copper is the oxidizing agent.
In summary, oxidizing agents oxidize other substances and are reduced themselves. They gain electrons and their oxidation number decreases
Reducing agents reduce other substances and are oxidized themselves. They lose electrons and their oxidation number increases.
Common Oxidizing Agents
Nitric Acid (concentrated)- forms a brown gas. The products can be NO2(g), water, nitrate ions.
Sulphuric acid (hot and concentrated)- produces a gas with a characteristic smell. Products can be SO2(g), sulphate ions
Potassium manganate (VII)- colour changes from purple to colouress. Products include Mn2+ ion.
Potassium dichromate (VI)- colour changes from orange to green. Products include the Cr3+ ion.
Iron (III) salts- colour changes from yellow to pale green. Products include the Fe2+ ion.
Hydrogen Peroxide-effervescence, colourless oxygen gas is evolved. Products include O2 gas.
I2(aq)- colour changes from brown to colourless. Products include the I- ion.
Common Reducing Agents
Hydrogen sulphide- a yellow colloidal suspension is formed. The products include sulfur and water.
Sulphur dioxide- there is no significant observable change. The products include H2SO4 or the sulphate ion.
Sulphite- no observable change. The products include H2SO4 or the sulphate ion.
HCl (concentrated)- A yellow green gas is formed. The products include chlorine.
Fe2+ - turns yellow or brown. Products include the Iron (III) ion.
Hydrogen peroxide- effervescence, colourless hydrogen gas evolved. Products include hydrogen gas.