Introduction to How to Calculate Oxidation Numbers Video Lecture - Class

in this video we're going to learn how
to figure out the oxidation numbers for
the different elements in a chemical
compound the oxidation numbers are the
numbers that I've written here above
each one of the elements now if you want
to learn more about what oxidation
numbers are or why they're important
check out my video called what are
oxidation numbers in this video we're
going to work through the process of how
you figure out what these numbers are so
here are the rules that we're going to
use to figure out oxidation numbers now
just so you know every teacher and
textbook has their own version of these
rules but they all work in pretty much
the same way so if you learn mine you'll
still get the answer right 100% of the
time even if these are a little bit
different from your teachers I'm going
to talk through a few of these rules
right now and then I'll introduce the
rest just as we work through practice
problems the first is this an element by
itself always has an oxidation number of
0 here's what I mean by that there are a
lot of chemical compounds that have just
one element that element is not combined
with any other elements that's what I'm
calling an element by itself so that's
something like CL - it doesn't matter
how many atoms of that element you have
just as long as it's only that element
and none others so CL - an element by
itself oxidation number of 0 sodium na
by itself 0 s 6 just sulfur nothing else
P for phosphorous and nothing else
zero so that's probably the easiest rule
here it's always zero for the oxidation
state if you have an element by itself
with nothing else the other rule here is
about monatomic ions these are ions that
are made of only one and so like this
for monatomic ions their oxidation
number is the same as their ion charge
so for k+ here it's oxidation number is
going to be plus 1 for n 3 - it'll have
an oxidation number of minus 3 and
mg 2 plus here is going to have an
oxidation number of plus 2 now keep this
in mind when we write oxidation numbers
we write this the sign first so plus
minus and then the number after this is
the opposite of how we write ionic
charges so just keep that in mind the
charge might be 2 Plus on magnesium but
the oxidation number is plus 2 now for
the rest of these rules we usually use
more than one together so I'll just talk
about these as we use them in example
problems here is our first example KCl
we want to figure out the oxidation
numbers of the elements in this compound
ok so let's take a look at our rules
here K potassium is in this column in
the periodic table it's in group 1a so
there's this rule that elements in group
1a are always plus 1 so that is
potassium Jacque sedation number then we
have CL over here that is one of the
halogens halogens usually minus 1
positive with oxygen well CL certainly
isn't with oxygen here so we'll give it
a minus 1 oxidation number now I want to
use this to show you a third rule that's
this rule right here the sum of
oxidation numbers for a neutral compound
equals 0 KCl is a neutral compound here
it doesn't have a charge after so that
means that the sum of these oxidation
numbers is going to equal 0 and that's
definitely true here plus 1 minus 1
equals 0
we'll use this later on when we do other
examples but just keep that in mind the
sum of oxidation numbers for a neutral
compound should always equal 0 mg o mg
is in this column in the periodic table
group 2a
and group 2a elements are always plus 2
so there's that oxygen here we have a
rule for oxygen
it is usually minus 2 it is minus 1
in peroxide h2o2 hydrogen peroxide is
the most common peroxide is probably the
only one you'll ever see but anyway this
oxygen is definitely not in hydrogen
peroxide so it's fair to say that it's
oxidation number will be minus 2 plus 2
4 mg minus 2 4 oh and they add together
to make 0 because this is a neutral
compound co carbon monoxide let's figure
this one out ok so see here there isn't
any rule for carbon so we'll have to
figure it out based on what we do know
okay so we do know oxygen oxygen is
usually minus 2 unless we're in a
peroxide definitely not a peroxide so we
can safely say that oxygens oxidation
number is minus 2 now let's use this
other piece of information that we know
and that's the sum of oxidation numbers
for a neutral compound should equal 0
so whatever carbons oxidation number is
should add together with oxygens to make
0 so we can figure out the carbons
oxidation number should be plus 2 minus
2 from oxygen gives us 0 nh3 ok and
there isn't any information about that
so just like in the last example we'll
have to figure out its oxidation number
using what we do know here we've got
hydrate there's a rule for hydrogen
hydrogen is plus 1 with nonmetals and
minus 1 with metals so nitrogen is
definitely a nonmetal which means it in
this case hydrogen is going to have a
plus 1 oxidation number ok but there are
3 hydrogen's each one of them has a plus
1 so what we have to do is we have to
multiply this plus 1 times 3 for the 3
hydrogen's and that's going to give us
plus 3 now let's keep this rule in mind
that the sum of oxidation numbers for a
neutral compound should equal 0 NH 3 is
definitely a neutral compound we don't
see any charge here so
whatever nitrogens number here is needs
to add together with plus three to make
zero so that means that nitrogen here
has to be minus three that adds there to
get zero so it's oxidation number is
minus three hydrogen's is plus one CAC -
so calcium here is in this column group
2a so we know that it's number is always
plus two we don't know what carbons
number is but we can figure it out what
we do know is that calcium's number plus
whatever carbons number is needs to
equal zero because this is a neutral
compound it doesn't have a charge that
means that the total number on carbon is
going to be negative two but there are
two carbons here so we have to divide
this number between the two of them so
we're going to take this and divide it
by two to get an oxidation number of
minus one for each one plus two for
calcium - one for carbon no.2 one -
check this out it is an ion so this is
going to be a little bit different in
this case the sum of oxidation numbers
for a polyatomic ion equals the ion
charge so let's see how we'll use that
here nitrogen we don't know what its
oxidation number is going to be but
oxygen here is usually minus two so we
have minus two here but we have two
oxygens so you're going to minus 2 times
2 is going to give us minus four
altogether now
whatever nitrogens is when we combine it
with minus four we need to get not zero
but we need to get minus one because
that's what the ion charge of no.2 1
minuses so this means that we will have
a 3 plus 3 plus 3 minus 4 gives us minus
1 which means that oxidation number here
on night
is going to be +3 hno3
okay let's take a look at the rules we
got hydrogen hydrogen here is combined
with nonmetals nitrogen and oxygen so
that means that it's oxidation number is
going to be plus one oxygen over here is
going to be minus two but we have three
oxygens so we got to take this minus 2
times 3 to get minus 6 ok so now we have
plus 1 from the hydrogen plus whatever
nitrogen is minus 6 has got to give us 0
because this is a neutral compound here
so that means that in this case
nitrogens oxidation number will be plus
5 1 plus 5 minus 6 equals 0 now I'm
going to start talking about some more
challenging examples that use some rules
here that are a little less common ZN h
2 ok zinc here we don't know what its
charge will be but hydrogen we have a
rule for that now we have been used two
hydrogen's number being plus 1 because
it's been with nonmetals but here it's
with a metal with zinc so that means
that it's oxidation number is going to
be minus 1 but we have 2 of these
hydrogens so it's going to be minus 1
times 2 give us minus 2 since this is a
neutral compound whatever zinc is plus
minus 2 has to give us 0 so that means
that zinc's oxidation number must be
plus 2 here so we got plus 2 for zinc
and minus 1 for hydrogen because a
hydrogen was with metal so it has a
minus 1 oxidation number BR o 3 1 - this
is a tricky one but it's pretty common
so BR bromine is one of the halogens the
rule for the halogens is they are
usually minus 1 but they are positive
with oxygen so I'm not going to put 1
minus in here because it's with oxygen
so let's go to oxygen instead oxygen is
usually minus 2
is not in peroxide so let me put that
here - - now we have three oxygens so
that's going to be times three to give
us - six
now whatever bromine is - six has got to
give us minus one because this whole
thing is a polyatomic ion that has a
charge of minus one so that means that
bromine is charge must be plus five plus
five minus 6 gives us minus one so this
is an exception where the halogens
instead of having an oxidation number of
minus one have a positive oxidation
number because they're paired up with
oxygen keep that in mind
clf3 okay so here we have two halogens
in the same compound we have fluorine
and chlorine how are we going to figure
this out well this is one of these
things that sometimes happens where
fluorine combines with another one of
the halogens we're going to zero in on
this rule first that fluorine is always
always always minus one so I'm going to
put a minus one right there multiply it
by three because I have three of them
and get minus three
now chlorine the other halogen says that
these other halogens are usually minus
one but not always they can be positive
with oxygen they can also be positive
with fluorine here so fluorine gives us
minus three chlorine must be plus three
so that these can add together to give
us zero since this is a neutral compound
which means that chlorines oxidation
number must be plus three and it's plus
three instead of minus 1 because it's
paired with fluorine which is always
always always minus one no matter what
so that's how to work through these
rules to figure out what the oxidation
numbers of elements are in a comp
if you want a little bit more practice
I'm going to have another video with
more practice problems if you just are a
little bit shaky on this doing more and
more practice problems is the best way
to get good really fast