Chemistry Class

Question

Identify the oxidation number for this reaction: C12H26 + O2 → H2O + CO2

Is this an example of a redox reaction? How do you know?

Answer 1

C₁₂H₂₆ +O₂ → H₂O + CO₂

Reactants:

The oxidation number of the H in is +1 in C₁₂H₂₆ this creates a +26 charge that must be canceled by the 12 Carbon atoms. 26/12 is not a round number, this is because the 10 C in the middle of this chain will have an oxidation number of -2 (-2 x 10 = -20) while the 2 C at the ends of the chain will have an oxidation number of -3 (-3 x 2 = -6... -6 + -20 = -26).

In O₂ the oxygen has an oxidation number of 0, as all free elements have.

Products:

The oxidation number of the H is +1 in H₂O this creates a +2 charge that must to be canceled by the one oxygen atom, so the oxygen atom must have an oxidation number of -2.

The oxidation number of the O in CO₂ is - 2, this means that there is a -4 charge that must be canceled out by the C atom, thus C in CO₂ must be +4.

Yes, this is a redox reaction, we know this because the oxidation number of Oxygen changes from 0 to -2, it has been reduced. The Carbon changes from -2 or -3 to +4, it has been oxidized.

Answer 2

You can tell it's a redox reaction because the O2 goes from zero to -2. Carbon changes its oxidation number from -3 and -2 to +4.

Carbons attached to hydrogens get a -1 for each hydrogen, but no charge for bonds to other carbons.

There is an excellent discussion in Calculating the oxidation state of a carbon – Master Organic Chemistry