Fractional oxidation number: S₄O₆²⁻, C₃O₂, Fe₃O₄ and Pb₃O₄
See why some atoms seem to have a fractional oxidation number (+2.5, +4/3, +8/3) and find the real whole-number states of each atom with the structure method. Free for every student.
Also searched as: fractional oxidation number · fractional oxidation number · average oxidation number · oxidation number of fe in fe3o4 · oxidation number of sulphur in s4o6 · mixed oxides
About this experiment
| Subject | Chemistry |
|---|---|
| Class | 11 |
| Unit | 7. Redox Reactions |
| NCERT section | 7.1 |
| Level | NCERT level |
| Time | about 15 minutes |
| Access | Free, no card needed |
Aim
To understand why the algebraic method sometimes gives a fractional oxidation number, and to find the real whole-number oxidation numbers of the atoms from the structure.
Theory
In S₄O₆²⁻, C₃O₂, Fe₃O₄ or Pb₃O₄ the algebraic method gives a fraction: +2.5, +4/3, +8/3 and +8/3. A fractional value is only an average. A real atom can only have a whole number of electrons more or less than the free atom, so the atoms in the species must have different whole-number oxidation numbers that average to the fraction.
The structure method finds them. In the tetrathionate ion O₃S–S–S–SO₃ the two middle sulphur atoms are bonded to each other and to the end sulphur atoms, so they are 0, and the two end sulphur atoms, bonded to three oxygen atoms each, are +5. The average is (5 + 0 + 0 + 5)/4 = +2.5.
In carbon suboxide O=C=C=C=O the two end carbon atoms are +2 and the middle one is 0: the average is +4/3. In the mixed oxides Fe₃O₄ (FeO·Fe₂O₃) and Pb₃O₄ (2PbO·PbO₂) the metal is present as two kinds of ions: Fe²⁺ and 2 Fe³⁺, Pb²⁺ ×2 and Pb⁴⁺. In all four, the sum of the oxidation numbers still equals the charge.
Formulas used
- average oxidation number = (sum of the oxidation numbers of the atoms of the element) / (number of those atoms)
- S₄O₆²⁻: (+5 + 0 + 0 + 5) / 4 = +2.5
- C₃O₂: (+2 + 0 + 2) / 3 = +4/3
- Fe₃O₄: (+2 + 3 + 3) / 3 = +8/3
Procedure: what to do in the virtual lab
- Choose S₄O₆²⁻ and read the algebraic equation in the readouts: you get +2.5.
- Show the arrows and the oxidation numbers and note that the four sulphur atoms are not alike.
- Do C₃O₂, then Fe₃O₄ and Pb₃O₄: find the atoms with different oxidation numbers.
- Check that the average of the real values gives the fraction.
Observations to make
- No atom has a fractional oxidation number in the structure: only the average is a fraction.
- In the two oxides the metal ions are of two kinds, so the oxide is a mixed oxide.
- The sum of the oxidation numbers is always the charge of the species.
What you can change in this experiment
- Species: Tetrathionate ion, S₄O₆²⁻ / Carbon suboxide, C₃O₂ (O=C=C=C=O) / Magnetite, Fe₃O₄ (FeO·Fe₂O₃) / Red lead, Pb₃O₄ (2PbO·PbO₂)
- What to show: The structure only / Arrows: bond electrons go to the more electronegative atom / The oxidation number of every atom
- Turn the picture slowly (on / off)
Viva questions on Fractional 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.
- Why do we get a fractional oxidation number for sulphur in Na₂S₄O₆?
- What is the real oxidation number of the atoms in Fe₃O₄?
- What is a mixed oxide?
- What is the average oxidation number of C in C₃O₂?
Frequently asked questions
What is the aim of the Fractional oxidation number experiment?
To understand why the algebraic method sometimes gives a fractional oxidation number, and to find the real whole-number oxidation numbers of the atoms from the structure.
Which formulas are used in the Fractional oxidation number experiment?
average oxidation number = (sum of the oxidation numbers of the atoms of the element) / (number of those atoms); S₄O₆²⁻: (+5 + 0 + 0 + 5) / 4 = +2.5; C₃O₂: (+2 + 0 + 2) / 3 = +4/3; Fe₃O₄: (+2 + 3 + 3) / 3 = +8/3
What viva questions are asked on Fractional oxidation number?
Why do we get a fractional oxidation number for sulphur in Na₂S₄O₆? What is the real oxidation number of the atoms in Fe₃O₄? What is a mixed oxide? What is the average oxidation number of C in C₃O₂?
How do I do the fractional oxidation number experiment for Class 11 online?
Open the Fractional oxidation number virtual lab on PractiSci, follow the procedure above, change the values and read the live graph. It matches the Class 11 chemistry practical, with the aim, theory, observations and viva questions on this page.
Can I do the Fractional oxidation number experiment online?
Yes. Open it as a 3D virtual lab in your browser or on your phone, change the values and read the results from live graphs. This lab is free.
Syllabus: Class XI Chemistry, Chapter 7 — Redox Reactions, §7.1.2 (oxidation number; fractional oxidation numbers appear in the exercises)
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: fractional oxidation number · average oxidation number · mixed oxides · tetrathionate · magnetite Fe3O4 · structure method · redox reactions
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)
- Iodometric titration of copper(II)
- Iron nail in copper sulphate
- Redox titration: KMnO₄ vs Mohr's salt
- Redox titration: KMnO₄ vs oxalic acid
Also try these chemistry experiments
- Ionic bond: how sodium and chlorine make NaCl (Class 11)
- Shapes of molecules (VSEPR): CO₂, BF₃, CH₄, NH₃, H₂O, PCl₅, SF₆, XeF₄ (Class 11)
- Schrödinger wave equation: wave function, probability and quantised energy (Class 11)
- Stoichiometry: limiting reagent, mass of product and percentage yield (Class 11)