Jonathan Fong

**U.C.Berkeley**

M.Ed.,San Francisco State Univ.

Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.

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So in our example here, we're given Cu2+ plus Br- yields CU+ plus BrO3-. So then what we'll do, is we'll break it into half reactions. So the first method is to use Half-reaction method.

So we'll take Cu2+, and we'll match it with the Copper, Cu+. Then we'll match up the Br- with the BrO3-. Then you just follow your steps.

So all those are equal. So all we need to do is balance out the charge for the first half-reaction with Copper. So we have the electron on the reactant side.

For the second half reaction, you need to have 3 waters to balance out the oxygens. You need to have 6H+ to balance out the Hydrogens. Then you double check the charges. Then so we need to add 6 electrons to the product side of this second half-reaction to make it, so that they are equal charges.

So let's check it up. So that means that the first half-reaction we need to multiply everything by 6. So to multiply everything by 6. Take 6, another 6, and another 6. So now we just add them up.

So we can cancel out the electrons, then we get our answer which is 6Cu2+ plus Br- plus 3H20 yields 6Cu+ plus BrO3- plus 6H+. During the course of the half-reaction, we figured out that 6 moles of electrons were transferred. So that is the long way. That's if you have an unbalanced Redox equation that you're given.

Now, what if you were already given the balanced equation, and then you wanted to find out how many electrons were transferred? Here is the trick. So what we'll do is, take a look at the number of atoms of the things that you have oxidized or reduced.

So the on Coppers, you have 6 atoms of Copper. That's identical on both sides. It has to be. You have 1 atom of Bromine, and that's identical on both sides. So 6 and 1, between 6, and 1, the lowest common multiple is 6. So that means that 6 moles of electrons were transferred. So you just take the lowest common multiple between the number of atoms of each of those particular elements that got oxidized or reduced, then you got your answer.

So you can try with any other equation that you want. So if you take a look here, let's try this one. So let's do another example. We have 2Al + 3Cl2 yields 2AlCl3.

So over here we have 2 Aluminum atoms, we have 6 Chlorine atoms, the lowest common multiple, between 2 and 6 is 6. So 6 moles of electrons were transferred in this way that we balanced the equation.

If you check it out, you break it into half-reactions, and then you actually balance it. You'll find that 6 moles of electrons were indeed transferred during the course of this reaction.

So this trick is actually very easy. Just find the number of moles of atoms; those are things that get oxidized and reduced. Then you got the lowest common multiple. You end up with the number of moles of electrons transferred. So that shortcut will hopefully save you a lot of time, and make your life a little bit easier. Have a good one.