Chemistry · Redox Equilibrium · NEET
Use the simple additive method. Take H as +1 and O as -2. Let carbon be x. Add all the numbers and set the total equal to zero for a neutral molecule (or equal to the charge for an ion). Example: CH4 gives x + 4(+1) = 0, so x = -4. Example: CO2 gives x + 2(-2) = 0, so x = +4. This is the fastest method NEET expects and it works for CH4, CH3Cl, HCHO, HCOOH, CO2 and more.
Carbon is more electronegative than hydrogen, so in every C-H bond the shared electrons are counted as belonging to carbon. Carbon has 4 C-H bonds, so it is assigned 4 extra electrons, giving it a charge of -4. By the additive rule: x + 4(+1) = 0, so x = -4. This is why NEET 2020 marked C in CH4 as -4.
Set carbon = x. There are 4 H atoms (+1 each) and 1 O atom (-2). So x + 4(+1) + 1(-2) = 0, which gives x + 4 - 2 = 0, so x = -2. This -2 value sits between methane (-4) and formaldehyde (0), showing carbon getting oxidised step by step as more oxygen is added.
When a molecule has more than one carbon and they are bonded differently, the simple additive rule gives an AVERAGE value for all the carbons together. NCERT notes that a fractional oxidation state is really the average of carbons that are actually in different states. For NEET you divide the total carbon oxidation number by the number of carbon atoms. Example: in C3O2, the three carbons share a total of +4, giving an average of +4/3. Use the average only when the question does not ask about one specific carbon.
A C-C bond contributes ZERO to oxidation number because the two carbons are the same element and share the electrons equally. So you can ignore C-C bonds and just count bonds to H (each gives -1 to that carbon) and bonds to O, Cl, N (each gives +1 to that carbon). This structural method is useful when NEET asks for the oxidation state of ONE particular carbon in a chain, not the average.
What is the change in oxidation number of carbon in the following reaction? CH4(g) + 4Cl2(g) -> CCl4(l) + 4HCl(g)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. When carbon bonds to less electronegative atoms like hydrogen, it takes the shared electrons and becomes negative. In CH4 it is -4, the most negative value carbon can have.
The highest is +4, seen in CO2, CCl4 and carbonates. Carbon has 4 valence electrons, so at most it can lose or share all 4 with more electronegative atoms, giving +4.
No. Valency of carbon is fixed at 4 (it forms 4 bonds). Oxidation number changes with the partner atom and ranges from -4 to +4. Do not confuse the two in NEET redox questions.
Use H = +1, O = -2, halogen = -1, and set the molecule sum to zero or the ion charge. Solve for carbon. For a single carbon in a chain, count -1 per C-H bond and +1 per C-O/C-Cl bond, ignoring C-C bonds.
