What would you like tell me which is oxidising agent in the following ask?

Mg (s) + Ni(NO3)2 (aq) ------------> Ni (s) + Mg(NO3)2 (aq)

0 ..........+2...2(-1) ......................... 0 ......... +2 ...2(-1)

|.............|.......................................|..............| Oxidation (reducing agent)

................|.......................................| Reduction (oxidizing agent)

You have to understand basic naming convention and how to balance a compound to understand how to determine whether a reaction is oxidation or reduction and to be able to pick-out the oxidizing agent. As you can see, Mg went from a charge of 0 on the reactant side of the equation to a charge of +2 on the product side of the equation, so Mg is oxidation reaction (Oxidation is a loss of electrons), and therefore, is the reducing agent because it causes the reduction of the other substance.

Ni went from +2 on the reactant side of the equation to 0 on the product side, so Ni is the reduction reaction (Reduction is a gain of electrons), and therefore, is the oxidizing agent because it causes the oxidation of the other substance.

NOTE: An easy way to remember: Once you find out which element is the oxidation reaction and which one is the reduction reaction, the the element that is the oxidation reaction is the reducing agent and the element that is the reduction reaction is the oxidizing agent.

MnO2 (s) + 4HCl (aq) ------------------> MnCl2 (aq) + 2H2O (l) + Cl2

+4....................-1...................................+2..................................0

HCI --------------> Cl2 Went from -1 to 0, and therefore, is the oxidation reaction and the reducing agent.

MnO2 -----------> MnCl2 went from +4 to +2, and therefore, is the reduction reaction and the oxidizing agent.

Hope this helps,

Bill