Assign oxidation numbers and identify oxidised and reduced species.
Method and worked examples
Oxidation is electron loss and an increase in oxidation number; reduction is electron gain and a decrease in oxidation number. Both occur together in a redox reaction. An element in its uncombined form has oxidation number zero. A monatomic ion has oxidation number equal to its charge. In usual compounds hydrogen is+1 and oxygen −2, with stated exceptions such as metal hydrides or peroxides.
The sum equals zero for a neutral compound and the ion charge for a polyatomic ion. Solve for an unknown using that total. The oxidising agent accepts electrons and is reduced. The reducing agent donates electrons and is oxidised. Oxidation number is bookkeeping; it does not imply that a covalently bonded atom carries that full ionic charge.
Example 1
Find Mn oxidation number in MnO4−, with oxygen −2.
x+4(−2)=−1 gives x=+7.
Example 2
Identify agents in Zn+Cu2+→Zn2++ Cu.
Zn loses electrons and is the reducing agent; Cu2+ gains electrons and is the oxidising agent.
Watch out:The oxidising agent accepts electrons and is reduced.
Oxidation numbers and redox identification | Year 11 Chemistry | Ascent Arcade