Oxidation number: rules and the structure method

Find the oxidation number of every atom in H₂O, H₂O₂, CO₂, NH₄⁺, HNO₃, H₂SO₄, SO₄²⁻, MnO₄⁻ and Cr₂O₇²⁻ by the algebraic rules and by the structure method, where the bond electrons are given to the more electronegative atom. Free for every student.

Also searched as: oxidation number class 11 · oxidation number class 11 · oxidation number by structure method · rules for oxidation number · oxidation state of sulphur in h2so4 · oxidation number of cr in k2cr2o7

About this experiment

Aim

To find the oxidation number of an atom in a molecule or ion, first by the algebraic rules and then by the structure (Lewis structure) method, and to see that the two methods agree.

Theory

The oxidation number of an atom is the charge it would have if all the bonding electrons were given to the more electronegative atom of each bond. It is a bookkeeping number: it shows how many electrons an atom has lost (positive) or gained (negative) compared with the free atom. In a bond between two atoms of the same element the two electrons are shared equally, so each atom gets one.

Rules (algebraic method): the oxidation number of an atom in an element is 0; of a monatomic ion it is the charge; of H is +1 (−1 in metal hydrides); of O is −2 (−1 in peroxides, +2 in OF₂); of F is always −1. The sum of the oxidation numbers equals the charge of the species: 0 for a molecule and the charge for an ion.

Structure method: draw the Lewis structure, then give every bonding pair to the more electronegative atom. Oxidation number = (valence electrons of the free atom) − (lone-pair electrons + bonding electrons given to it). In H₂SO₄ all the S–O bond electrons go to oxygen, so sulphur has lost all its six valence electrons: +6.

Formulas used

• Σ (oxidation numbers) = charge of the species
• oxidation number = valence electrons − (lone-pair electrons + bonding electrons assigned to the atom)
• H = +1, O = −2, F = −1, alkali metals +1, alkaline earth metals +2
• increase in oxidation number = oxidation; decrease = reduction

Procedure: what to do in the virtual lab

1. Choose H₂SO₄ and show the structure only. Count the bonds around S.
2. Show the arrows: each arrow points to the atom that takes the bonding pair. Which atoms are the electron takers in H₂SO₄?
3. Show the oxidation numbers and check them with the algebraic equation in the readouts.
4. Compare H₂O with H₂O₂. Why is oxygen −2 in one and −1 in the other?
5. Do Cr₂O₇²⁻ and MnO₄⁻ and check that the sum of the oxidation numbers equals the charge.

Observations to make

• An arrow always points from the bond towards the more electronegative atom; for two atoms of the same element there are two dots instead.
• The oxidation numbers always add up to the charge of the molecule or ion.
• In H₂O₂ the O–O bond is shared equally, so oxygen is −1 and not −2.

What you can change in this experiment

• Molecule or ion: Water, H₂O / Hydrogen peroxide, H₂O₂ / Carbon dioxide, CO₂ / Ammonium ion, NH₄⁺
• What to show: The structure only / Arrows: bond electrons go to the more electronegative atom / The oxidation number of every atom
• Turn the molecule slowly (on / off)

Viva questions on Oxidation number

Questions an examiner may ask in the Class 11 chemistry practical. Run the experiment in the lab to find the answers yourself, then check them in the Learn tab.

1. What is the oxidation number of Mn in KMnO₄ and Cr in K₂Cr₂O₇?
2. Why is the oxidation number of oxygen −1 in H₂O₂?
3. How is oxidation number different from formal charge?
4. What is the sum of the oxidation numbers in an ion?

Frequently asked questions

Syllabus: Class XI Chemistry, Chapter 7 — Redox Reactions, §7.1.2 (oxidation number)

What you do in the lab

Change the controls and watch the chemistry behave in real time in 3D. Live readouts and graphs show the numbers behind it, and the Learn tab has the theory, procedure, observations and viva questions. Works on a phone or a laptop, in English and Hindi.

Topics: oxidation number · oxidation state · structure method · rules for oxidation number · electronegativity · redox reactions · oxidation number of sulphur · oxidation number of chromium

More from this unit

• Galvanic cell: Mg | Mg²⁺ || Cu²⁺ | Cu
• Galvanic cell: Zn | Zn²⁺ || Ag⁺ | Ag
• Copper with concentrated nitric acid
• Dichromate oxidises iron(II)
• Fractional oxidation number: S₄O₆²⁻, C₃O₂, Fe₃O₄ and Pb₃O₄
• Iodometric titration of copper(II)
• Iron nail in copper sulphate
• Redox titration: KMnO₄ vs Mohr's salt

Also try these chemistry experiments

• Electron, proton and neutron: how they were discovered (beams in electric and magnetic fields) (Class 11)
• Volume strength of hydrogen peroxide: molarity, normality and % w/v (Class 11)
• Electrochemistry: cell potential, Nernst equation, ΔG and equilibrium constant (Class 12)
• Daniell cell (galvanic cell): electrons, ions, salt bridge and cell potential over time (Class 12)