Leah here from leah4sci.com/mcat and in this video we're going to look at Oxidation numbers.
Oxidation number tells us the electrons gained or loss if every atom owns its own electrons.
Huh?
I like to think of oxidation number as the imaginary charge of every atom.
This is slightly similar to formal charge which we covered in Organic Chemistry.
And this will help us later when we're trying to figure out if an atom got oxidized or reduced.
Say you have NaCl, it's a salt, it appears neutral but we know this is an ionic compound
that breaks up to give us the sodium positive ion and the chlorine negative ion.
What about NaOH?
This breaks up to give us Na+ and OH- . The negative is assumed over the entire hydroxide.
To find out where the negative comes from we can use the formal charge shortcut, should
minus has that I teach on the tutorial linked below.
To see that Hydrogen should have 1 has 1 attached, no formal charge, but Oxygen should have 6,
has 7 attached for a formal charge of minus 1, that's the source of our negative charge.
But what if it's less obvious?
What if you're asked to find the oxidation number of every atom in methane or water?
How do we figure it out?
There's no standard set of rules as long as you have the concepts and know how to attack
the atoms you'll be okay.
But here are the rules I like to follow; an atom in its standard state or a stand alone
atom will have an oxidation number of zero.
Monoatomic ions will have an oxidation number that is equal to their charge.
The sum of the oxidation number in neutral molecules will add up to zero.
The sum of oxidation numbers in polyatomic ions will equal to the charge.
Within these rules we have additional rules for individual atoms, so let's go through
some examples and learn the rules as we go.
For the first rule, the standard state elements have an oxidation number of zero and that's
because they have a charge of zero.
For example sodium metal which doesn't have a charge has an imaginary charge of zero or
not very imaginary because we can clearly see it has no charge.
If I have H2 gas, we have two hydrogen atoms in their standard state, each hydrogen has
the oxidation number of zero which gives us the total charge of this molecule zero.
Say you're given a carbon compound, you don't know how many carbons there are, but no charges
were given that means each carbon has an oxidation number of zero.
Rule number 2 is very similar and that oxidation numbers don't have to be imaginary if we can
see the charge.
If I give you the sodium cation Na+ the charge on sodium is plus 1 that means the oxidation
number is also equal to plus 1.
The magnesium ion Mg2+ has an oxidation number of plus 2.
The chloride anion Cl- has a charge of negative 1 therefore an oxidation number of minus 1.
Let's tackle 3 and 4 together because even though the some of the charges is zero for
neutral or a number for anion, then we approach this is going to be the same and the first
thing we wanna know is know where to look from the periodic table when trying to understand
the oxidation numbers or the imaginary charges on the atoms.
Some simple rules for the periodic table, group 1 metals.
If part of a larger compound, are going to have an oxidation number of plus 1.
If we see potassium as an ion we know it's K+ but if potassium is part of something larger
it still has the oxidation number of plus 1.
Group 2 metals same thing with an oxidation number of plus 2.
So magnesium, calcium, Strontium, and so on.
Jumping over the 7a, the Halogens will typically have an oxidation number of minus 1 with some
exceptions.
6A will typically have an oxidation number of minus 2, 5A minus 3, and so on.
An easy way to remember this is to start on the left and count 1, 2, then start on the
right and see how many spaces you have to go back.
Zero back 1 for minus 1, back 2 for minus 2, back 3 for minus 3 keeping in mind that
it's not always the case because if I have 2 of these together, for example Nitrogen
bound to Oxygen, they can't both be negative so let's see how we determine who gets the
charge.
But first, for transition metals, don't memorize any number because they change so much, instead
look for the clues in the rest of the molecule and solve for the oxidation number of the
transition metals.
Let's break it down starting with Hydrogen.
Notice I said group 1a metals but not hydrogen.
Because hydrogen can exist with an Oxidation of plus 1 or minus 1 depending on what it's
bound to.
For example, if I ask you to solve for the oxidation number of hydrogen in HNO3 or NaH
we're gonna get two completely different numbers.
When Hydrogen is bound to a metal, it gets a negative 1 oxidation number.
If Hydrogen is bound to a non metal it has a positive 1 oxidation number.
That means in NaH, Hydrogen follows sodium which is a metal, that means Hydrogen should
have an oxidation number of minus 1 and if you break it down you'll see that we have
NA+ H- , remember rules 3 and 4 the sum of oxidation numbers have to be equal to the
sum of the charges or the net charge of the molecule, so Na+ and H- gives us zero.
With nitric acid we have hydrogen bound to a non metal, it's bound to polyatomic ion
so that hydrogen is plus 1 nitrate you should recognize as NO3 minus or minus 1 or once
again the sum of the charges equals zero.
Another way that you can recognize this is in many cases where hydrogen is at the end,
it's negative but in the beginning it's positive.
But what about the NO3 minus?
what is the oxidation numbers of these individual atoms?
Oxygen is typically going to have an oxidation number of negative 2 and when oxygen is bound
to something else for example NO3 minus, the other atom will have to adapt to the oxygen.
The way to figure this out is to recognize that the charge on the nitrate ion is negative
1 so the oxidation numbers have to equal to negative 1.
This means we have 1 nitrogen of unknown oxidation number plus three oxygens of oxidation number
negative 2 that almost equal to negative 1, that's Nitrogen, 3 Oxygens, for a charge of
negative 1.
Let's simplify, we have nitrogen plus 3 times negative 2 which is minus 6 equals negative
1.
Let's isolate the nitrogen, add 6 to both sides which cancels it out, negative 6 positive
6 equals zero.
Negative 1 plus 6 is equal to positive 5, Nitrogen equals plus 5 which means the oxidation
number of oxygen in Nitrate is positive 5.
What about the oxidation of every atom in carbon dioxide?
We know that Oxygen should be minus 2 but what about carbon?
Carbon next to oxygen have to adopt 2 oxygen so let's solve, setting the sum of oxidation
number equal to the charge which in this case is zero.
If no charge is given, if the molecule is neutral, then it has to add up to zero.We
have one carbon with unknown oxidation number plus 2 oxygens which an oxidation number of
minus 2 equals zero.
Let's simplify, carbon minus 2 times 2 which is negative 4 equals zero.Isolate carbon by
adding 4 to both sides minus 4 plus 4 equals zero.
Zero plus 4 is equal to 4.
Carbon equals positive 4 which gives us the oxidation number of carbon in carbon dioxide,
that's plus 4.
Peroxy or peroxides are the one exception to the oxidation number for oxygen.
I like to think of peroxy as 1 per oxy or per oxygen, that means oxygen has a negative
one oxidation number instead of negative 2.
The most common peroxide you've seen are H2O2 or the peroxide anion which is O2 2 minus.
Let's start with the anion.
If we look at O2 2 minus, that means we have 2 oxygens and I put the x there so it won't
mix it up with zero equals negative 2.
So 2 of the unknown equals minus 2 divided by 2, divide by 2, the unknown oxygen is equal
to negative 1.
Each oxygen has an oxidation state of minus 1 and you'll get the same thing when you solve
for it in hydrogen peroxide.
We mentioned that halogens or typically negative 1, for example in HCl, hydrogen bound to a
non metal will give us an H+ next to a Cl minus.
But if a halogen is bound to something like an Oxygen, the oxygen wins.
You should recognize HClO4 as a strong acid to memorize where we know that this is really
an acidic hydrogen and the ClO4 minus polyatomic ion.
To find the oxidation number of chlorine, we'll set it up the same way.
Cl O4 is equal to negative 1 where we have chlorine plus 4 times negative 2 for the 4
oxygens equals minus 1, that's chlorine plus 4 times negative 2 is negative 8 equals negative
1.
Isolate chlorine by adding 8 to both sides and that gives us an oxidation number of chlorine
is equal to negative 1 plus 8 which is positive 7.
Definitely not what you expect when you look at chlorine in group 7a on the periodic table.
Be sure to join me in the next video where we look at redox pairs and common oxidizing
and reducing reagents.
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MCAT stoichiometry video series along with the practice quiz and cheat sheet.