Mastering the Half Reaction Method to Balance Redox Reactions

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 and nickel metal go ahead and balance

this reaction using the half reaction 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

becomes al plus 3. 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 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 now before we can add the two half

reactions we need to make sure the number of electrons are the same on both 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 by two and the second one by three 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 which becomes uh three nickel atoms

now we're going to add these two half reactions notice that the electrons cancel

when that happens the charges will be balanced so i'm going to put this up here

so on the left side we have these two we have two aluminum atoms reacting with three

nickel ions and on the right side we have those two species

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 now let's move on to our next example zinc

reacts with bromate to produce the zinc cation

and bromide balance this reaction under acidic conditions

under acidic conditions you can add h 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

is add two electrons now let's move on to the second half reaction

where bromate becomes bromide 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 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 plus negative one which is positive five 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 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

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

two species and on the right side we have three zinc ions

one bromide ion and three water molecules so now let's make sure

that everything is balanced in our final answer so we have one bromine atom on both

sides we have three zinc particles on both sides

six hydrogen atoms three times two is six and now let's check the total charge the

total charge on the left side is six plus negative 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 so therefore

this is the balanced chemical reaction now let's try another example

and let's balance it using the half reaction method under basic conditions

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 so let's start with the first half reaction

aluminum produces this polyatomic ion

now in order to balance the number of atoms all we need to do is add four hydroxide ions to the left side

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 now let's move on to our next example perchlorate turns into chloride

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

to both sides when you mix h plus and oh minus you're gonna get water

so therefore these two will combine and form eight water molecules

on the right side i'm still gonna have the four water molecules

but i'm also gonna have eight hydroxide 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

to add eight electrons to the left side 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 eight aluminum hydroxide ions

plus 24 electrons now let's move on to this one let's

multiply everything by three so we're going to have 24 electrons

plus 12 water molecules plus three perchlorate

ions and that's going to produce three chloride ions and 24

hydroxide ions 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 plus eight aluminum atoms

12 water molecules and three perchlorate ions on the right side

we have this stuff three chloride ions and 24 hydroxide 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 hydroxide ions

so these two will disappear 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

eight aluminum hydroxide 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

we have three chlorine atoms on both 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 and 12 times 2 is 24 24 plus 8 is 32 so we got 32 hydrogen atoms on both

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.

so we got 32 oxygen atoms on both sides 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

so this is the final answer so now you know how to balance a redox 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 got and thanks for watching you