Leah here from leah4sci.com/mcat and in this video I'm going to show you a time-saving
shortcut for balancing acid base reactions.
An acid base reactions is one in which an acid like nitric acid reacts with a base like
sodium hydroxide to give us water and salt, in this case NaNO3.
You can balance this the same way that I teach in the other balancing videos on my website
leah4sci.com/reactions where we write a checklist down the middle for all of the atoms present
in the reactant and products.
The problem is similar to what we saw on the last video in balancing combustion where an
atom that shows up in more than one place can be very confusing.
A combustion that is typically oxygen but here notice that we have oxygen in both the
acid and the base.
We have hydrogen in the acid and the base, same thing in the products.
What you wanna recognize is that in an acid base reaction, not every single atom actually
reacts.
Not everything changes so if you can identify what doesn't change and just keep is as a
unit the entire process becomes simpler.
Think about what you have in your acid.
An Arrhenius or bronsted lowry acid, it's simply an H+ down to some conjugate in this
case Na3 minus.
An Arrhenius base is simply an OH minus bound to a spectator ion in this case sodium.
The other thing to recognize is that our acid has a polyatomic ion nitrate.
Nitrate does not react or change, it simply something that dissolves in a solution so
that we have nitrate in the reactants, and nitrate on the products.
So instead of breaking it up what if we just pretend that nitrate is an atom and when balancing
we'll simply look for how many NO3 minus up here on the reactants and products.
Sodium is not a polyatomic ion but it's also something that doesn't change which leaves
us with the oxygen and hydrogen.
We have the acidic hydrogen which is different from the basic hydrogen to give us water.
But what if we don't look at water as a brand new molecule?
What if we look at water almost like the H+ cation bound to the OH minus anion and balance
them separately?
That means we're going to look for the number of acidic or purple hydrogens and the number
of basics or green hydroxides discounting the fact that they both have hydrogen because
we're looking at them separately.
Let's start balancing.
How many acidic protons do we have?
We have 1 in the reactants for nitric acid and 1 in the product as part of the HOH.
We have 1 nitrate in the reactants and 1 nitrate in the products.
We have 1 hydroxide in the reactants and 1 hydroxide in the HOH of the products and sodium
1 and 1.
Look at that, there's one of everything.
This reaction happens to be totally balanced so we have 1 and 1 reacts to give us 1 and
1.
I purposely chose a simple example so that we can focus on the trick without having it
too complicated.
I realized it appears to have taken more time in this simple example but watch how much
time it saves us for something more complicated if for example we're asked to balance the
acid base reaction of phosphoric acid reacting with calcium hydroxide to give us water and
the salt Ca3(PO4)2.
Soon as you identify that this is an acid base reaction, identify your acidic, basic,
and poly atomic components.
Identify your acidic hydrogen and basic hydroxide.
Identify those same components as the H-OH in your products then look for any poly atomic
in this case it's phosphate so we're gonna treat phosphate as a single unit.
This allows us to write a very simple list.
We have the acidic hydrogen, the phosphate anion, we don't even need the charges because
we're just looking for whole numbers.
The calcium cation and the basic hydroxide.
Now let's balance.
On the reacting side we have 3 acidic hydrogens, 1 phosphate anion, 1 calcium, and 2 hydroxides.
In the products we have 1 water molecule which is 1 H-OH molecule, 3 calcium and 2 phosphate
anions.
Let's start with a non water Anion and cation first.
We have a 1 to 2 ratio for phosphates in the reactants and products so let's double the
phosphates in the reactants placing a 2 in front of phosphoric acid.
That doubles my acidic protons from 3 to 6 and gives me 2 phosphates.
I have 1,2,3 calcium in the reactants and products so let's triple the reactants by
placing in 3 infront of calcium hydroxide.
Updating all the numbers that gives me 3 calcium and 3 times 2 for 6 hydroxides.
Phosphate is balanced, calcium is balanced.
The only thing left unbalanced is that I have 6 acidic protons and 6 hydroxides in the reactants
but just 1 and 1 in the products but 1 to 6 ratio just multiply by 6, if you place a
6 in front of the water that means I have 6 H-OH molecules in the products giving me
6 protons, 6 hydroxides, and look at that 6 hydrogens, 2 phosphate 3 calcium, 6 hydroxide,
everything is balanced now let’s update update.
We have 2 phosphoric acid reacting with 3 calcium hydroxide to give us 6 water plus
1 calcium phosphate.
Once you're comfortable with this, I challenge you to redo this problem using the method
of writing out every single atom then coming back letting me know in the comments if the
shortcut saved you any amount of time.
And then be sure to join me in the next video where we look at how to balance redox equations.
You can find that video along with this entire video series practice quiz and cheat sheet.
First, give this video a thumbs up, then visit my website leah4ssci.com/reactions for the
MCAT stoichiometry reactions, practice quiz and cheat sheet.
The link again leah4sci.com/reactions.