This video explains and demonstrates the half-reaction method for balancing redox reactions in neutral, acidic, and basic solutions, emphasizing the importance of balancing both atoms and charge.
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in this video we're going to focus on
the half reaction method used to
balance redox reactions
we're going to consider an example
using a neutral solution an acidic
solution and a basic solution
so let's start with a neutral solution
let's say
if aluminum metal
reacts with
nickel plus two
and it produces the aluminum plus three cation
cation
and nickel metal
go ahead and balance
this reaction using the half reaction method
method
now when balancing a redox reaction
you need to make sure that not only the
particles are balanced the atoms and the
ions but also
that you have charge balance the charges
must be balanced on both sides
in this example we have one aluminum
atom and ion on both sides
and we have a nickel particle
on the left and one on the right
so the atoms are balanced but however
the charges are not the left side
contains a net charge of positive two
the right side contains a total charge
of positive three
so this reaction is not balanced yet
so what we're going to do is we're going
to separate it into half reactions
let's start with the first one aluminum
now in order to balance it we'll need to
balance the charge
the net charge on the left is zero the
net charge on the right is three
so the difference between these two
numbers is three we're going to add
three electrons to the side with the
higher charge
and so the first half reaction
is balanced whenever the electrons are
on the right side
this half reaction represents an
oxidation reaction
now let's move on to the second half reaction
reaction
nickel plus two
turns into nickel
now in order to balance the charge we
gotta add two electrons to the left side
so whenever the electrons are on the
left side you have a reduction half reaction
reaction
now before we can add the two half
reactions we need to make sure the
number of electrons are the same on both sides
sides
the least common multiple of two and
three is six
so we need to get six electrons on both
reactions let's multiply the first one
so we're gonna have is two a l
which becomes two a l plus three
plus six electrons
and then below that three
nickel two plus cations plus six electrons
electrons
which becomes uh three nickel atoms
now we're going to add these two half reactions
reactions
notice that the electrons cancel
when that happens
the charges will be balanced
so on the left side we have these two
we have two aluminum atoms
reacting with three
nickel ions
and on the right side
two aluminum cations are being produced
and three nickel atoms
so notice that the number of nickel
particles is the same on both sides
and the number of aluminum particles is
the same
and the total charge is the same on the
left side it's 3 times 2
which is positive six and on the right
side it's two times three which is
positive six
so in balance in a redox reaction you
need to make sure that the number of
atoms are the same on both sides and
also the total charge has to be the same
on both sides
zinc
reacts with bromate
to produce
the zinc cation
balance this reaction under acidic conditions
conditions
under acidic conditions you can add h plus
plus
and water to balance a half reaction
so let's start with zinc
in order to balance this half reaction
all we need to do
now let's move on to the second half reaction
reaction
where bromate
now we already have one bromine atom on
both sides
so the next thing we need to do is
balance the oxygen atoms
and to do that we got to add water
so we need to add three water molecules
to the left side i mean the right side
in order to balance it so we have three
oxygen atoms on both sides
but now
we have six hydrogen atoms on the right side
side
so we need to add six
h plus ions to the left
now that all the atoms are balanced
we need to balance the charges
the net charge on the left side
is negative one
then that charge on the right side
is six
the difference between five and negative
one is six if you take five
and subtract it by negative one this is
equal to six
so therefore we need to add six
electrons now should we add it to the
left side or the right side
you should always add electrons to the
side with the higher total charge
so in this example we're going to add
six electrons
to the left side
and so now the second half reaction is balanced
balanced
now we need to make the number of
electrons equal on both sides so
therefore all we need to do
is multiply the first half reaction by three
three
and so it's going to be three
zinc atoms
producing three zinc ions
and six electrons
so now
we can add the two half reactions
to do so we need to cancel the six
electrons on both sides
and everything else we can bring down
so this is on the left side of the arrow
so we can bring it down
on the left side and then we have these
and on the right side
one bromide ion
so now let's make sure
that everything is balanced in our final answer
answer
so we have one bromine atom on both sides
sides
we have three
zinc particles on both sides
six hydrogen atoms three times two is six
six
and now let's check the total charge the
total charge
on the left side is six plus negative one
one
so it's positive five
on the right side it's three times two
which is six
plus negative one and so that two is
positive 5.
so now the total charge is the same and
the number of particles is the same on
both sides
this
now let's try another example
and let's balance it using the half
reaction method
under basic conditions
you can add hydroxide and water to
balance a half reaction
but it might be difficult to do so
an easier technique is to start by
balancing it under acidic conditions
and then add hydroxide to both sides of
the reaction
so i'll illustrate that with this example
example
so let's start with the first half reaction
reaction aluminum
aluminum produces
produces
now in order to balance the number of
atoms all we need to do is add four
and now the particles are balanced
now we need to balance the charge
the total charge
on the right side is negative one on the
left side it's four times negative one
or negative four
so these two numbers differ by three
now should we add the three electrons to
the left side or to the right side
which number is higher
on a number line negative four or
negative one
negative one is a higher number so we
need to add three electrons to negative
to the right side
so now the charges are balanced the net
charge on the right side is negative one
plus negative three which is negative four
four
so for this one
i'm going to balance it first as if it's
under acidic conditions so i'm going to
add four water molecules
on the right side so that the number of
oxygen atoms
will be the same on both sides so we
have four oxygen atoms on both sides
now i have eight hydrogen atoms on the
right side so i'm going to add eight h
plus ions
under basic conditions
the hydrogen ion is virtually
non-existent so we need to get rid of it
so i'm going to add eight hydroxide ions
when you mix h plus and oh minus you're
gonna get water
so therefore
these two will combine and form eight
on the right side
i'm still gonna have the four water molecules
molecules
but i'm also gonna have eight hydroxide ions
ions
now if you have the same substance on
both sides
you could simplify the equation
let's subtract both sides by four water molecules
and so now we have the balance reaction
under basic conditions eight minus four
is four
so it's four water molecules plus a per
chlorine ion produces one chloride ion
and eight hydroxide ions
so notice that the number of atoms
is balanced in this reaction
we have eight hydrogen atoms
on both sides
and we have eight oxygen atoms on the
right side
we have four from water and four from
the chlorine ion the perchlor ion
now all we need to do
is balance the charges
so the net charge on the left side is
negative one
the total charge on the right side is
negative one plus negative eight or
negative nine so these two numbers
differ by eight and negative one is
higher than negative nine so i'm going
now we need to make the number of
electrons equal
so 3 times 8 is 24.
that's the least common multiple between
3 and 8. so i need to
multiply this reaction by 8 to get 24
electrons and this one by 3 to get 24
electrons as well
so then this is going to be 32
hydroxide ions
plus 8
aluminum atoms
and that produces
plus
now let's move on to this one let's
multiply everything by three
so we're going to have 24 electrons
electrons plus
plus
12 water molecules
plus three perchlorate
ions and that's going to produce
three chloride ions
and 24
so now that we have the same number
of electrons on both sides we can add
the two half reactions
so let's cancel
the number of electrons
and so on the left we have 32 hydroxide ions
ions
plus eight aluminum atoms
12 water molecules
and three perchlorate ions
on the right side
three chloride ions and 24 hydroxide ions
ions
so notice that
we can reduce this reaction because we
have hydroxide on both sides
so what we need to do is subtract both
sides by 24
so the final answer
is going to be 32 minus 24 which is eight
so we have eight hydroxy ions eight
aluminum atoms
12 water molecules
three per chlorate ions
and that yields three chloride ions
and that's about it
now let's make sure that
the reaction is balanced
so we have eight aluminum atoms on both sides
sides
we have three chlorine atoms on both sides
sides
now on the right side we have eight
times four
or 32 hydrogen atoms
on the left side here we have 8 hydrogen atoms
atoms
and 12 times 2 is 24 24 plus 8 is 32
so we got 32 hydrogen atoms on both sides
sides
now for oxygen we have 8 times 4 or 32
oxygen atoms on the right side
here we have 8
this is 12 and 3 times 4 is 12.
so 12 plus 12
plus 8
that's 24
plus 8 which is 32.
now the last thing that we need to do
is check
the charges
so this is 3 times negative 1
plus negative eight so the total charge
on the left side is negative eleven
and here this is eight times negative
one plus negative three
so the total charge on the right side is
negative eleven
so once the number of atoms
and the total charge is the same
on the right side once you have
once the mass is balanced and the
charges are balanced
then the redox reaction is balanced as well
well
so this is the final answer
so now you know how to balance a redox reaction
reaction
using the half reaction method under
neutral solutions acidic solutions and
basic solutions
so that's it for this video that's all i
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