Exam tip 1
A very common mistake is to confuse the species being oxidised/reduced with the agent. Remember: the oxidising agent gets reduced, and the reducing agent gets oxidised. Write it down in the exam to be sure!
9701 · 6.1
Redox processes: electron transfer and changes in oxidation number (oxidation state) — common mistakes
Common exam mistakes on 9701 Redox processes: electron transfer and changes in oxidation number (oxidation state). Learn what loses marks, then practise the topic with Examiner’s Ink.
Is there a difference between 'oxidation state' and 'oxidation number'?
For the purposes of A-Level Chemistry, the terms are used interchangeably. 'Oxidation number' refers to the specific value (e.g., +2), while 'oxidation state' can be seen as the condition of the atom having that number. You can use either term in an exam.
Why does oxygen have a +2 oxidation state in $\text{OF}_2$?
The rules for assigning oxidation numbers have a hierarchy. Fluorine is the most electronegative element, so it is always assigned an oxidation number of -1 in its compounds. For the neutral molecule to have an overall oxidation number of 0, the single oxygen atom must have an oxidation number of +2 to balance the two -1 charges from the fluorine atoms.
Can a reaction be just oxidation or just reduction?
No. Electrons are fundamental particles; they cannot be created or destroyed in a chemical reaction. If one species loses electrons (oxidation), another species must gain them (reduction). The two processes are inextricably linked and always occur together.