Leah here from leah4sci.com/mcat and in this video we're going to start our discussion
on balancing redox equation. Say you're asked to balance the equation, Ag+ plus Zn solid
goes to Ag solid plus Zn2+ aqueous. Up to this point we've only been balancing the atoms
but if you look at this from The atomic perspective we have one ag and one Zinc going to Ag and
one zinc it looks balanced. But when you're dealing with redox and a change of oxidation
number it's not just the atoms that we have to balance we also have to balance, we also
have to balance the charge. If we take the net charge on the reactant side we have a
g + Zn zero for a total plus one. Net charging the products ad is zero and Zn2+ gives us
+ 2 something's off and this is why there's a more complicated approach in balancing redox.
Now i see more complicated but it doesn't have to be scary. If you have a checklist
and a logical system to follow so long as you can check off every item in the list you
know that you did it correctly and you're good to go. I don't think you're going to
see anything this crazy on the mcat, however, it is first worth knowing the steps just in
case you have to execute on one of the pieces. So how do we balance redox equations? First
thing you do is find the oxidation number of everything your reactant and your products.
We covered oxidation numbers in detail in a previous video which you can find on my
website leah4sci.com/reactions. But as a quick reminder if the element is by itself it even
has charged that the oxidation number if it doesn't have a charge the oxidation number
is 0 that tells us the oxidation number of + 1 Zn solid 0 Ag solid 0, zn 2 + plus 2.
The next thing you want to do is to identify what is oxidized what gets reduced. First
I like to pair up my species then we use the trick that I taught earlier in this series
where in the case of Ag, the charge is reduced from + 1 to 0 it goes down it suffers a reduction,
that means it's the reduction half reaction and zinc going to Zinc plus 2 goes up or should
we say op spelled with an o tells us it is the oxidation half reaction. Once you know
what's oxidized and reduced we want to break it apart so that we can balance the half reactions
before putting it back together. A word of advice this topics get so messy the neater
you can keep your paper the better. I often see students doing the reactions they have
their arrow they continue doing another reaction they have their arrow, they continue instead
i like to put my arrows down the middle so that I can always look at the right side and
identify my product always look at the left side identify reactants with no confusion.
Let's start with the oxidation of zinc. On the reactant we have Zinc solid, let's emit
the phases to keep it simple. In the products we have the Zinc2+. For reduction we have
ag + going to ag solid. Next we balance the half reactions where the order is very very
important. Start with your non oxygen and hydrogen, balance your oxygen by adding water,
then balance your hydrogen by adding H+. Now remember h plus is acidic so I'll show you
how to change this in basic conditions but i always start with adding H+ regardless.
For this reaction we have just Zinc and Ag, no oxygen no hydrogen and these are already
balance so we simply look at it verify, one zinc on the right one zinc on the left , 1
ag on the right 1 ag on the left. Once the items are all balanced we balance the charge
by adding electrons. And this is where you wanna be careful. One approach is to remember
that adding electrons is reduction and removing electrons is oxidation. But i find that difficult
in this context, instead i like to think of it in a mathematical perspective. If the only
thing we can add are electrons and electrons are negative, anytime we add an electron the
charge is going to go down so what i want to do is compare the charges on the right
and left and whichever one is higher that is where i'm going to add electrons to bring
it down because electrons will make the bigger number smaller. If i add electrons to the
smaller number it will make it even more smaller and that won't help us. Let's take a look
at the Zinc. do not charge for reactants or zero, net charge for the products are + 2.
If i add electrons to zero i make that negative and there's no way to balance that with +2.
But if i add electrons to a positive number the electron being negative will bring it
down. What we want to do is make the bigger number equals to the smaller number by asking
how many do i need to subtract to get the smaller number? + 2 minus x is equal to zero.
2 -2 = 0 and that means we add plus 2 electrons to the products. Same thing for reduction
some of the reactants plus one some of the products zero. How many electrons will bring
plus 1 down to zero? + 1 -1 = 0 that means I have to add + 1 electrons to the reactants.
Since we doubt is the transfer of electrons the same number of electrons gained by 1/2
of reaction has to be lost by the other half of the reaction. If Zinc is losing two electrons
it's giving to electrons away, then silver can simply gain one electron, it has to gain
two electrons because silver is what speaking up to electrons given up by a zinc so we have
to make sure that the electrons and the oxidation half equal the reduction half by finding the
least common multiple. In this case we have two electrons sedation we should have to in
the reduction and to do that we simply multiply the entire reduction half reaction by 2. I
don't recommend leaving it like this instead distribute that 2 as quickly as possible so
that you don't make a mistake down the line. How do we do that? Just want to play everything
by two. Two times one electron. 2 electrons, 2 x 1 ag plus, 2 ag + 2 x Ag solid is 2Ag
solid. Now that i have the same number of electrons i can cancel them out because of
their gain by one and loss by the other, they don't show up in my net or final reaction.
Two electrons in the reactants on the reduction side cancel two electrons in the products
on oxidation side and i know my reaction is balanced but we're not done yet. Next we want
to sum it up, cancel any duplicates and check everything. You want to make sure within come
all this way only to find a mistake at the end. So how do we sum it up? Just like when
you're doing math, draw line at the bottom and bring your numbers down. Again , bring
down and the middle so that everything on the reactants and products are clearly separated
and this is where that little arrow helps to avoid confusion. As you're bringing it
down i recommend crossing it out so you know for sure you didn't miss anything. On the
reactant side we have one zinc 2 Ag+ and nothing else . on the products side we have 1 Zinc
2+, and 2ag. Put your phases is back in, a metal by itself is solid, a metal with a charge
is aqueous, cancel any duplicate which we don't have in this case , but literally you'll
see water a lot , and finally check everything . Does this look like balanced? One sink one
zinc, 2Ag, 2Ag. Sum of the charges, 0 + 2 times plus 1 is +2. 1 + 2 + 0 is + 2 , we're
good to go! This is a balanced reaction. Now let's take a look at something more complicated
when we actually do have oxygen and hydrogen to balance and that's exactly what we'll look
at on the next video. First , give this video a thumbs up , make sure to subscribe to my
channel so that you don't miss out on any new MCAT videos , then visit my website leah4sci.com/reactions
for the mcat stoichiometry and reactions practice quiz and cheat sheet. The link again, leah4sci.com/reactions.