Methods to calculate oxidation numbers of carbon, oxygen and hydrogen atoms
Find oxidation numbers of atoms of CH3COOH using lewis structure
Mark direction of electrons attracting of bonds due to electronegativity difference
Electronegativity values of carbon, hydrogen and oxygen - Pauling Scale
Find oxidation numbers of atoms of CH3COOH using algebraic equation
Methods to calculate oxidation numbers of carbon, oxygen and hydrogen atoms
There are two methods to find oxidation numbers of atoms.
Using lewis structure
Using algebraic equation: This method is not suitable for finding oxidation numbers of atoms in complex molecules.
Therefore, we do not use that method in this tutorial.
Find oxidation numbers of atoms of CH3COOH its using lewis structure
This method is the perfect way to find the oxidation numbers. In this method, lewis structure of relevant molecule should be drawn. So, draw the
lewis structure of acetic acid. Completed lewis structure of acetic acid is shown below.
Mark direction of electrons attracting of bonds due to electronegativity difference
Atoms which have higher electronegativity values, can attract electrons of bonds towards those atoms.
Electronegativity values of carbon, hydrogen and oxygen - Pauling Scale
Carbon:2.55
Oxygen: 3.44
Hydrogen: 2.2
Now, we can look how electrons are attracted between carbon, oxygen and hydrogen atoms.
Among carbon, oxygen and hydrogen,
oxygen is the most electronegetive element and hydrogen has the lowest electronegativity value.
Electrons of bonds are attracted toward some atoms as following image.
Due to attracting of electrons of bonds towards higher electronegative atoms, individual oxidation numbers due to bonds are marked in the
following figure.
Oxidation number of carbon atom (which made C=O bond) = +2 + 0 + (+1) = +3
Oxidation number of carbon atom (which made three C-H bonds) = -1 + (-1) + (-1) + 0 = -3
Oxidation number of oxygen atom (which made C=O bond) = -2
Oxidation number of oxygen atom (which made O-H bond) = -1 + (-1) = -2
Oxidation state of each hydrogen atom = +1
Find oxidation numbers of carbon atom of CH3COOH using algebraic equation
Following parameters are important to use this methodology to find oxidation number of unknown atom. This method is not accurate as the
previous method.
In this methodology, you can find oxidation number of only one atom. Therefore, oxidation numbers of all other atoms should be known or
assumed to continue.
In this case, we are going to find the oxidation number of both carbon atoms. So, oxidation numbers of oxygen atoms and hydrogen atoms
should be known or assumed.
Because, oxygen is the most electronegative element in CHCOOH molecule, we can take the oxidation number of oxygen atom as -2.
Also, because Hydrogen atom is the most electropositive element, it's oxidation number should be +1. Now, we can substitute those values in the
algebraic equation.
Summation of oxidation states of all atoms in a molecular or anion = overall charge of the molecule or ion
Σ(number of atoms of respective element × oxidation number of respective element) = overall charge of the molecule or ion
Summation of oxidation states of carbon atoms + Summation of oxidation states of oxygen atoms +
Summation of oxidation states of Hydrogen atoms = 0
Because oxidation number of carbon atoms is taken as unknown, take it as x to solve the algebraic equation.
(2 × x) + (-2 × 2) + (+1 × 4) = 0
x = 0
As compared with oxidation number of carbon atom which we found in the lewis structure method, this method has given us a different answer.
As you are informed earlier, algebraic methodology does not give you the perfect answer, it gives you an average value in this case to suit to
both carbon atoms.
