Leah here from leah4sci.com and in this MCAT video we're going to look at oxidation and
reduction reactions.
What are oxidation and reduction reactions or rather what sets them up apart from the
other reactions you see and the answer is simple, the molecules that react are going
to have a change in oxidation number.
When something gets oxidized or reduced its oxidation number will go up or down, in the
last video we looked at how to find the oxidation number as well as how to recognize common
oxidizing and reducing reagents.
So let’s it a step further and see how this works with redox reactions.
First a quick review of my favorite mnemonic LEO the lion says GER and this tells us a
loss of electrons is oxidation, a gain of electrons is reduction or loss of electrons
can also mean loss of hydrogen or gain of oxygen atom and gain of electron can also
be gain of hydrogen or loss of oxygen atom.
For more details on this see the oxidation number video link below or visit my website
leah4sci.com/reactions
Take a look at this partial reaction where Sn2+ goes to Sn4+, this is an incomplete reaction
because we know that charge cannot be created or destroyed, only transferred and if we add
up the charges on both sides, Sn2+ gives us plus 2, Sn4+ gives us plus 4 and this doesn't
match up, what we're missing here are negative electrons, the one that's higher will be brought
down by adding negative, by adding electrons, what's the difference between 4 and 2?
2 electrons.
Positive 4 is greater than positive 2 by 2 that means we have to add +2 electrons to
the product sign so that we now have a net charge on the right of +4 - 2 = +2 + 2 = +2
so we know we have the correct number of electrons.
Is this an oxidation or reduction reaction?
Well let's see.
What happens to Tin + 2?
When Tin losses 2 negatives, it gets more positive by 2 which is why the charge goes
up to by 2 and we have 2 electrons that are now lost floating away into solution to be
picked by something else because Tin lost electrons this is an oxidation reaction.
Now take a look at this reaction, if 2Fe3+ reacts to give me 2Fe2+ what happens?
Iron started with the positive 3 and went down to positive 2, how do we figure out if
it's oxidation or reduction?
Let's see what happened to the electrons.
The net charge in the reactant side is 2 times positive 3 which is +6, the net charge on
the product is 2 times positive 2 which is +4, the side with a higher charge has to be
brought down with electrons and in this case it has to be brought down with 2 electrons,
that means in this reaction, 2 electrons react with 2Fe3+ to give us 2Fe2+.
Because the Iron 3+ is gaining electrons, the Iron is getting reduced that means this
is a reduction reaction.
One way you can recognize this is electrons on the product side is oxidation, electrons
in the reactant side as reduction but what if you don't see it this way?
These are both half reactions, meaning we have the oxidation half, the reduction half
but in reality they happen together because redox reactions happen in pairs.
The balance redox reaction is Sn2+ aqueous reacts with 2Fe3+ aqueous to give us Sn4+
aqueous + 2Fe2+ aqueous.
We know that electrons are transferred but without seeing this, how would you know which
side gain and lost electrons?
The proper way to do this is to separate it into the half reactions to write out the equation
for Sn2+ to 4+, write out the equation for Fe3+ to 2+ and figure it out as we did before
but guess what, you don't have this kind of time on the MCAT so if you understand what's
going on, here's how I like to do it instead.
When Tin got oxidized it went up in charge, the charge went from +2 up to +4, what if
we spell up with an O?
OP, in charge for oxidation, when Iron got reduced it went from +3 down to +2 or we can
say that +3 was reduced to +2, Iron 3 is reduced to Iron 2 and if you can recognize this you
don't have to write it all out, all you do is compare which charges higher or lower,
+2 goes up to +4 oxidation, Iron +3 is reduced to Iron +2 that's the reduced or reduction.
What if it's not obvious?
What if you were given the reaction of H2 gas reacting with O2 gas to give you water,
this is a redox reaction but what is getting oxidized and what is getting reduced?
In order to figure this out you have to assign the oxidation number to the individual atoms
and then figure out which one went up and which was reduced in charge.
Remember from the last video that a standard state element or a neutral molecule made up
of one type of atom is going to have an oxidation number of 0, that means each hydrogen has
an oxidation number of 0 and each oxygen also 0.
When looking at water we have to break it apart and we can remember that oxygen typically
has an oxidation number of -2 or that when hydrogen is bound to nonmetal it has an oxidation
number of +1 and 2 hydrogens at +1 balances out a -2.
Now we can compare the atoms directly.
Hydrogens oxidation number went from 0 to +1, it went OP from 0 to +1, OP = Oxidation,
oxygen oxidation number was reduced from 0 down to -2, it was reduced telling us that
oxygen underwent reduction and as we discussed in the last video, if hydrogen is getting
oxidized it is also the reducing agent, if oxygen is getting reduced it is also the oxidizing
agent.
Redox will come up in a lot of the common types of reactions so let’s look at them
one at a time.
I like to think of disproportionation reactions as being disproportionate or uneven because
one atom can be oxidized and another of the same atom can be reduced for example, why
does a peroxide solution typically come in a dark bottle so that it's not exposed to
light and so that it doesn't decompose by this reaction, H2O2 liquid will break down
to give us H2O liquid plus O2 gas, what in this reaction underwent oxidation and reduction?
We know that the peroxy anion has an oxidation number of -1 per oxygen, this is the O2 negative
2 exception but the remaining oxygens, if in a molecule will have -2, if in a standard
state element will have an oxidation number of 0, hydrogen bound to a nonmetal is +1,
hydrogen bound to nonmetal is +1, hydrogen did not undergo redox, oxygen did and specifically
oxygen underwent oxidation and reduction.
The oxygen that winds up in water went down or was reduced to -1 to -2, that's the reduction
reaction.
The oxygen that became O2 gas went up in charge from -1 to 0 giving us up for oxidation.
Take a look at this reaction, we know it's a single replacement because on one side we
have chlorine with nothing, on the other side it's attached to something, same thing with
the bromine, bromine with nothing, bromine with something.
How is this a redox reaction?
Notice that we have ionic compounds in the reactants of products where potassium is simply
a spectator on both sides, what's actually reacting is chlorine and bromine, chlorine
in its standard state is 0, bromine in its standard state is 0, if bromine is balanced
by a positive spectator, it's got a charge of -1 or simple recognize Halogens as -1 same
with the chlorine on the product side, so how is this a redox reaction?
Chlorine went from 0 down or reduced to -1 making it the reduction half reaction and
bromine went up or OP from -1 to 0 making it the oxidation half reaction.
A combination or synthesis reaction is when we bring 2 things together haven't we already
seen that in the formation of water from its standard state elements?
In this case, hydrogen and oxygen combined would give us water and we've seen that hydrogen
gets oxidized while oxygen gets reduced.
A decomposition reaction is a reaction where something breaks apart, what if we reverse
this synthesis reaction showing that 2 water molecules can break apart to give me H2 gas
O2 gas since this is the reverse of synthesis we would expect that hydrogen getting oxidized
on a forward reaction would get reduced in the reverse and that oxygen getting reduced
in the forward should get oxidized in the reverse, lets prove it.
Lets first assign oxidation states, Hydrogen + 1 Oxygen - 2, hydrogen and oxygen are 0
in their elemental states so what do we have?
Hydrogen goes down or gets reduced from +1 to 0, oxygen goes up increasing from -2 to
0 which proves that in the reversed reaction hydrogen gets reduced and oxygen gets oxidized.
Last but not least we have combustion which is a burning reaction so lets look at a simple
combustion of reacting methane with 2 oxygen gas to give us carbon dioxide and 2 water.
What is getting oxidized and what is getting reduced?
Let's start with oxidation numbers.
Remember that hydrogen bound to a metal is negative but nonmetal is positive, this gives
me +1 times 4 for the hydrogen and therefore, -4 for the carbon, oxygen in its standard
state is 0 but in the products, oxygen in a compound is -2 and since we have 2 of them,
2 times -2 is negative 4, carbon has to be positive 4 and what do we have hydrogen +1,
oxygen -2.
Noticed that hydrogen hasn't changed, it started and ended as +1 but carbon and oxygen have
different oxidation numbers.
Carbon started off as negative 4 and went up by 8 so that it's final oxidation number
is +4, carbon went OP giving us the oxidation reaction, oxygen started with 0 but went down
or was reduced to -2 giving us the reduction half of the reaction.
For even more on reactions be sure to join me on the next video where we look at chemical
reactions in much more detail.
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