Terms
This topic becomes a whole lot easier once you have your terminology down packed!
An oxidising agent is also called the oxidant. They are substances which oxidise another species and they themselves are being reduced during reactions.
Because the oxidising agent is being reduced, it gains electrons. OIL RIG - Reduction is a gain of electrons. |
A reducing agent is also called the reductant. They are substances which reduce another species and they themselves are being oxidised during reactions.
Because the reducing agent is being oxidised, it looses electrons. OIL RIG - Oxidation is a loss of electrons. |
Oxidation numbers
An oxidation number is the hypothetical charge that an atom would have if all the bonds of different elements were 100% ionic
During oxidation - the oxidation number of a species goes up
During reduction - the oxidation number of a species goes down
An easy way to remember this is by remembering reduction. If something is being reduced, it is going down. Oxidation is simply the opposite.
During reduction - the oxidation number of a species goes down
An easy way to remember this is by remembering reduction. If something is being reduced, it is going down. Oxidation is simply the opposite.
Changes in oxidation numbers can be used to determine which element is oxidised and which is reduced.
If the oxidation number of an element in a reaction increases, it has been oxidised. It has lost electrons - therefore lost negative charges so becomes more positive. If the oxidation number of an element in a reaction decreases, it has been reduced. It has gained electrons - therefore gained negative charges and becomes more negative.
If the oxidation number of an element in a reaction increases, it has been oxidised. It has lost electrons - therefore lost negative charges so becomes more positive. If the oxidation number of an element in a reaction decreases, it has been reduced. It has gained electrons - therefore gained negative charges and becomes more negative.
Determining oxidation numbers
Oxidation numbers are assigned by a set of rules
Rules |
Examples |
1. In their elemental form, elements have the oxidation number of zero |
O2, Mg, Al, Cl2 and C all have an oxidation number of zero |
2. The oxidation number of an element in a monatomic ion (an ion with one atom) is the same as the charge on the ion |
Zn in Zn2+ has an oxidation number of +2 F in F- has an oxidation number of -1 |
3. Hydrogen in compounds has an oxidation number of +1, except in metal hydrides (a metal bonded to hydrogen), where it is -1 |
Hydrogen in water H2O and hydrogen in CH4 has an oxidation number of +1 In NaH, the oxidation number of hydrogen is -1 |
4. Oxygen in compounds has an oxidation number of -2, except in hydrogen peroxide, H2O2, where the oxidation number is -1 |
Oxygen in CaO, H2SO4, KMNO4, and ZnO has an oxidation number of -2 Oxygen in hydrogen peroxide has an oxidation number of -1 |
5. For polyatomic ions (these are ions containing more than one atom), the sum of the oxidation numbers equals the charge of the ion |
For NH4+, the sum of the oxidation numbers is +1 For MnO4-, the sum of the oxidation numbers is -1 |
6. The sum of the oxidation numbers in a molecule is zero |
The sum of the oxidation numbers in each of the following is zero, CO2, CaCl2, H2SO4, H2O |
The sum of the oxidation number of the atoms in a compound it zero
What are the oxidation numbers of the atoms in the following compounds?
What are the oxidation numbers of the atoms in the following compounds?
MgO
Oxygen has an oxidation number of -2 (rule number 4). The total oxidation number for the molecule is zero (rule number 6). This means that magnesium has an oxidation number of +2. -2 + ? = 0 |
MnO2
Oxygen has an oxidation number of -2 (rule number 4) and there are 2 of them so the oxidation number of oxygen is -4 (-2 x 2). Because the overall oxidation number of the molecule is zero, the oxidation number of Mn must be +4. ? - 4 = 0 |
HOCl
Hydrogen has an oxidation number of +1 (rule number 3), oxygen has an oxidation number of -2 (rule number 4) therefore, Cl must has an oxidation number of +1. +1 -2 + ? = 0 |
H2O2
Each hydrogen has an oxidation number of +1, there are 2 of them which totals +2. Therefore the oxidation number of both oxygen's is -2, each being -1. Remember oxygen has an oxidation number of -2 except in H2O2 where it is -1. +2 -2 = 0 |
Mg(IO3)2
Each oxygen has an oxidation number of -2, there are 6 of them, therefore the total oxidation number of oxygen is -12 (-2 x 6 = -12).
Magnesium has an oxidation number of +2, therefore the total oxidation number of I is +10. +2 +? -12 = 0 |
Cl2
All elements have an oxidation number of zero.
|
The sum of the oxidation numbers of the atoms in a polyatomic ion is equal to the charge on the ion
NB: The charges of these ions should be written as superscript, due to formatting, they are not.
MnO42-
Each oxygen has an oxidation number of -2, therefore the total oxidation number of oxygen is -8. The sum of the oxidation numbers for polyatomic ions equals the charge of the ion, therefore Mn must have an oxidation number of +6 ? -8 = -2 |
Cr2O72-
Each oxygen has an oxidation number of -2, therefore the total oxidation number of oxygen is -14 (-2 x 7). The sum of the oxidation numbers for polyatomic ions equals the charge of the ion, therefore the total oxidation number of Cr must be +12, each Cr will have an ON of +6. ? -14 = -2 |
NH4+
Each hydrogen has an oxidation number of +1, there are four of them so the total oxidation number of the H is +4. The sum of the oxidation numbers for polyatomic ions equals the charge of the ion, therefore N must have an oxidation number of -3 ? +4 = +1 |
SO42-
Each oxygen has an oxidation number of -2, the total oxidation number being -8 (-2 x 4 = -8) The sum of the oxidation numbers for polyatomic ions equals the charge of the ion, therefore S must have an oxidation number of +6 ? -8 = -2 |
NO3-
Each oxygen has an oxidation number of -2, the total oxidation number must therefore be -6 (-2 x 3 = -6). The sum of the oxidation numbers for polyatomic ions equals the charge of the ion, therefore N must have an oxidation number of +5 ? -6 = -1 |
S2O32-
Oxygen has an ON of -2, the total ON is therefore -6. The sum of the oxidation numbers for polyatomic ions equals the charge of the ion, therefore the total ON of S must be +4, each S must be +2. ? -6 = -2 +4 - 6 = -2 +4/2 = +2 |
Redox couples
This table shows species on the left hand side of the table in their oxidised form and on the right hand side, their reduced form
Is this an Oxidation or reduction reaction?
Remember OIL RIG - oxidation is loss of electrons. If something is losing electrons it is losing negative charges, hence becoming more positive. It is acting and the reductant.
In chemistry, language is important so here is how we explain this answer:
This is an oxidation reaction because the oxidation number of S changes from -2 in H2S to 0 in S which means that S is experiencing an increase in oxidation number and loss of 2 electrons, so H2S is acting as a reductant.
In chemistry, language is important so here is how we explain this answer:
This is an oxidation reaction because the oxidation number of S changes from -2 in H2S to 0 in S which means that S is experiencing an increase in oxidation number and loss of 2 electrons, so H2S is acting as a reductant.
H+/MnO4- -> Mn2+
This is a reduction reaction because the oxidation number of Mn changes from +7 in MnO4 to +2 in Mn2+ which means Mn is experencing a decrease in oxidation number and a gain of 5 electrons so MnO4 is acting as an oxidant. |