today we'd like to talk about
balancing equations, balancing equations
is a fundamental task that chemist
and students have to do the balanced equation is an equation that has the same
numbers of each atom on both sides of the equation
and there are a few rules that we need to
to obey when we're balancing equations the first rule
is that you want to count the number of atoms
on each side of the equation before you begin balancing
the reason is that sometimes an equation is already balanced and needs no further
work
the second thing that you need to know about a balanced equation is
that equations may be balanced
using coefficients the coefficients that are used
have to be whole numbers they can not be fractions
a balanced equation cannot be balanced
by changing a subscript when you change as subscript
you change the identity of the compound and that is not allowed
when you are balancing equations
the best way to begin is to start with the more complicated
molecule and work your way back to the less complicated molecule
or to a single atom once you've made your manipulations to the equation
what you want to do is to recheck, recount
to make sure that the equation is in fact balanced
the equation that I have on the board is
the reaction of titanium plus chlorine
to give titanium tetrachloride
to balance this equation let's begin by counting the atoms
I typically
write the atom in the center
on the left hand side
we have one titanium the subscript 2 in chlorine tells us that there's two
chlorines
in the titanium tetrachloride molecule
there's one titanium there are four chlorines
this equation
in starting from titanium tetrachloride since we have
our titaniums balanced we shouldn't have to worry about titanium
but our chlorines are not balanced so
if you look at the relationship between two and four
you know that in order to get four chlorines on this side you're going to
need the multiply
by two, we have to multiply
the entire molecule by two so if we put
a coefficient 2 in front of chlorine
that means that now we have four chlorines here
we have one titanium and remember that on this side we have one
and four so we balanced our equation
I mentioned to you earlier
that you can balance an equation using coefficients
you cannot balance an equation using subscripts let me share
with you
why that's the case if we have
hydrogen plus
oxygen
giving us water let's balance this equation
we have two hydrogens on the left hand side
we have two oxygens
on the left hand side on the right hand side we have two hydrogens
on the right hand side we have
one oxygen, many times I have students
who want to do this they want to change the subscript
beside of the oxygen in order to balance the equation
now let me share with you that the molecule
H2O is water the molecule
H2O2 is hydrogen peroxide
if you have
dark hair like mine and you use
hydrogen peroxide instead of water you will most likely
not happen dark hair like mine anymore
peroxide is a bleach water is not so
again you're changing the identity here so we can't balance it that way so we
can remove this
we ask yourself how should we do this properly
well since you have your hydrogens balanced
let's focus on the oxygens two
verses one what do you multiply two by
one by to get two well the answer is two
notice in a molecule you have to multiply
everything in the molecule by the same coefficient
we're going to put a two here so
we do have oxygen now at two but we've also doubled our hydrogens
so that means that now on this side we have four hydrogens
we don't worry about that cause we started with
this complicated molecule and we look
at our numbers and we see that yes we have four hydrogens
but we have a two here so that means
that if we multiply the two by two
we can balance our equation so by putting a two here
our hydrogens are now balanced and by counting
recounting we see that we have four hydrogens on each side
we have two oxygens on each side
we need to make sure that we use coefficients to balance the equations
that we never use subscripts to balance equations
there's other helpful hints that may
benefit you when balancing equations
suppose we have aluminum
plus oxygen
giving us aluminum oxide
we balance this equation
we find that we have one aluminum
and two oxygens on the left hand side
we have two aluminums and three oxygens on the right hand side
now notice here that it's no longer
a possibility to simply multiply one value
by two to double it here we have a difference between two and three
which can create a little bit of a problem always
remind my students that back when we were in fourth fifth and sixth grades
we learned how to find what was called a common denominator
for the addition of fractions how did you do that
you multiplied the denominators together
to come up with a value if we multiply two
and three together that will give us six meaning that
our initial goal should be to have six
oxygens on each side of the equation to do that
we're going to put a two here and a three here
we change the three in the right hand side
I'm sorry on the left hand side
we have six oxygens we still have
one aluminum on this side of the equation we have six oxygens
but we now have four aluminums
so the question becomes how do we complete the balancing
we do that by changing the coefficient in front of the aluminum
we do that
we balance the equation
another thing that sometimes complicates
situations has to do with reactions
in which we have combustion taking place
this reaction
is C3H8 which is propane
it's the gas that you use in gas grills reacting with oxygen
to give carbon dioxide and water when we
balance this equation and any other equation we have to remember
sometimes you will have an area where you will have
two sources for one atom you need to make sure and count
all the atoms on both sides
some people overlook this on
let's count our atoms, we have 3 carbons
8 hydrogens and 2 oxygens
we have 1 carbon
2 hydrogens and
2 plus 1 or 3 oxygens
when
we look at this equation propane C3H8
is our most complicated molecule so
if we have three carbons here in order to have three carbons on the right hand
side
we're going to put a three here and that's going to change
our carbon count to 3 if we have
8 hydrogens on the left hand side
its 8 versus 2
if we multiply 2 by 4
that will give us 8 hydrogens
we have carbon and hydrogen balanced now let's count
the oxygens we have 4
water molecules so we have
four oxygens here we have
three carbon dioxide molecules
so we have 6 oxygens here for a total of
ten so there's ten oxygens over here
if you have
10 oxygens here and 2 oxygens here
you simply have to multiply 2 by 5
to get ten
when you recheck the equation you can see that it's balanced
balancing equations
is not difficult and if you follow the rules in the guidelines that I've given
you
you should be able to do it what you do need to do
is to practice lots of different problems
so that you can acquire the skills that you need