Oxidation states and the rules of oxidation are a must know to do these questions:
1) [O] number of an element is always zero
2) [O] number of a simple ion is equal to the charge on that ion
3) [O] number of H is always (+I) for non-metals and (-I) for metals (when combined)
4) Total [O] number of any compound is zero
5) Total [O] number of any complex ion is equal to the charge of that ion
So i.e. the one that's being reduced is the one that gains electrons (positively charged to a lower positive charge such as III-I or to neutral etc.).
so say for (a)
Reactants: Mg(s) has an oxidation state of 0; H has an oxidation state of I and Cl has an oxidation state of -I (due to valencies).
Products: MgCl2, Mg now has an oxidation state of II since it has lost 2 electrons and Cl still has -I. H_2 has an oxidation state of 0 (hint hint reduced but not the metal).
Do that for all of them haha good practice ^^'
