Leah here from leah4sci.com/mcat and in this MCAT video we're going to look at conventions
for writing chemical equations.
Chemistry is all about the reactions but when you're given a reaction such as this precipitation
equation, what exactly are we looking at?
Every number, every letter, every symbol has a very specific purpose and is very important.
Let's look at one more, the dissociation for acetic acid, same set of symbols going on
but a little bit different, so let’s break it down.
When you look at a reaction equation, you'll see some sort of arrow in the middle, that's
going to separate the start of the reaction and the end of the reaction.
Whatever you see on the left of the reaction arrow, no matter what the arrow looks like,
those are called the reactants.
The reactants react either with themselves or with each other to give you what we get
on the right side of the equation which are your products.
Products are the result of your reaction, you'll notice that we have a small letter
or letters after each reactant and this refers to the phase or the physical state of what’s
reacting.
You should know your three phases of matter starting with solid written as a lowercase
S, this is something with a definite shape and size that you can see, that you can touch
like a book or a table.
Liquid is a lower case L, liquids take the shape and size of their container but they
have a definite volume for example, water can take the shape of a tall glass or a bowl
and gas lower case G does not have a definite shape or volume and that means it can expand
to fit it's container for example you can have oxygen in a tank but if the tank leaks
out then the oxygen will just disperse into the environment.
You'll also see one more physical state and that is aqueous written as a lower case AQ
in parentheses.
Aqueous refers to something that is dissolved in aqua, in water, for example, if you have
a salt like NaCl, it's solid.
You can touch it but if you dissolve it in water it appears to disappear and that's because
the ions break apart so that sodium and chlorine no longer interact with each other but instead
they interact with the aqueous or water solution.
In our precipitation reaction, you'll notice that calcium chloride and potassium carbonate
are both aqueous to form potassium chloride which is Aqueous and calcium carbonate which
is solid.
In our acid dissociation reaction we have aqueous acetic acid dumped into a liquid solution
of water and when that happens the acid dissociates to give us the aqueous hydronium ion and an
acetate which is an aqueous conjugate base.
If you look at the product in the first reaction, there is a two in front of the KCl but none
of the others have such a number, this number is called the stoichiometric coefficient or
simply the coefficient.
The coefficient tells us how many of that species are going to react but it doesn't
tell us the number meaning if there is one it's not that one molecule and that's it reacts,
instead it tells us the ratio but if most of the reactants and products don't have numbers,
how many reacts?
That's a trick question, if you don't see a number in front of the reactant or product
it's not that there’s zero, it's that it's understood to be one but since so many reactions
involve just one of something, we keep that one invisible to keep things simple but in
reality for the first reaction, we have on calcium chloride reacting with one potassium
carbonate to yield two potassium chloride and one calcium carbonate but that doesn't
mean we have one single calcium chloride reacting, remember we said it's a ratio when you carry
out these experiments on lab, you'll have them on a test tube, beaker or flask, say
we fill up this erlenmeyer flask with water and then we add in some acidic acid, it's
not one single acidic acid that we dump in to solution, every drop has millions and millions
of molecules because they're so tiny so the coefficient number simply tell us for every
one of this, two of that, for every one of calcium chloride we will require two potassium
carbonate and this is where bouncing equations comes into play which we'll look at in the
next video in the second reaction for every one acidic acid that reacts, one water molecule
will be consumed and turned into hydronium for every acetate that we produce.
The coefficient is not to be confused with the smaller numbers that we see within the
reactants and products.
These numbers simply tell us how many of the atom or ion are reacting.
When we look at calcium chloride we have one calcium bound to two chloride ions to make
a neutral salt.
Potassium carbonate we have two potassium ions for every one carbonate to make a neutral
salt.
The numbers in acetic acid which is formed by covalent bonds simply tells us how many
of each are present.
You can sometimes see this written as C2H4O, you'll even see it as C2H3OH which is separating
the acidic hydrogen from the remaining hydrogens but at the end of the day it's just about
how many there are.
Three hydrogens bound to the first carbon atom, two oxygens bound to the second carbon
atom and the acidic hydrogen sitting at the end.
What we don't have here is a polyatomic ion.
Carbonate is a polyatomic ion and in this case we're only using one.
But in some reactions the molecule will require more that one polyatomic ion adding another
version of this number for example, if I have the polyatomic ion calcium hydroxide, notice
this looks a little bit different.
What we have here are parentheses of two atoms to show us that these atoms are grouped together
and you'll see this one you need more than one polyatomic ion.
The ion in this case is hydroxide which is OH- and since calcium has a charge of plus
two, we need two of the negative hydroxide to balance it, but that two outside of the
parentheses tells us that it's two of everything within.
Two hydroxide means two of the oxygen and two of the hydrogen and the parentheses tells
us only apply the two to everything within.
Even more confusing can be something like calcium nitrate because now we have numbers
inside and outside the parentheses, this is because nitrate is NO3-, there are three oxygens
bound to the nitrogen to give us that negative charge.
But that minus one is not enough to balance the plus 2 calcium so we need two of them
and the two outside tells us that there's two of everything inside meaning we have two
nitrogen and two times three oxygen which is actually six.
Again, something we'll look at in detail in the balancing videos next.
Another thing you'll notice are pluses and minuses.
When you have a plus in between two reacts or two products it tells us that there are
two different things.
In the first reaction we're told that calcium chloride reacts with potassium carbonate.
That plus tells us calcium chloride and potassium carbonate react.
The plus on the product side tells us we have two products.
Potassium chloride forms in addition to calcium carbonate.
This is not to be confused with the positive that I have here in the negative but I have
here.
If the positive and negative is sitting directly above a molecule or above an atom then that's
telling us that there is a charge.
If it's sitting separate it's telling us that we have won and the other.
Hydronium is a positive polyatomic ion, the charge is plus one and so that plus tells
us the charge, not seeing a number tells us there is an invisible one and that gives us
H3O+.
For acetate we don't have a number after the negative, that means there is an invisible
one telling us that acetate has a charge of minus one.
what about the other reactants and products?
If there is no number given assume that it's neutral because in this case it's an invisible
zero rather than an invisible one.
Why is it an invisible zero where up to know lacking a number implied one for example the
coefficient or the charge.
This is because with charge it has to be plus or minus and if we have an invisible positive
charge we still need the plus.
If we have an invisible negative charge.
We still have the minus but if there's nothing there it's not negative it's not positive
it's neutral so the invisible charge is zero telling us that it's neutral.
Finally we have the arrows in between the reactants and products, the arrows tell us
what is happening in the reaction and the arrow reads are yield so A plus B, arrow,
C plus D means A plus B yields or results in or forms C plus D. You'll see many different
types of arrows in chemistry and organic chemistry but here are the most common types you'll
see in reactions like this; If you see a one way arrow going from left to right this means
that the reaction goes to completion.
The MCAT requires you to memorize a hand full of strong acids and bases
and what sets them apart from weak the acids and bases is that the reaction goes to completion.
The strong acid HCl, if dissolved in water we'll break apart to give us H plus and Cl
minus near a hundred percent.
This means that all the hydrochloric acid dissociates and if there's none left the reaction
goes to completion, on the other hand weak acids as we see here, do not go to completion
instead the reaction will find some sort of balance where the reactants will go to products,
products will go back to reactants and this will happen back and forth back and forth
at some specific ratio based on it dissociation factor.
To show that it's going back and forth we'll have arrows pointing in two directions this
is called an equilibrium reaction where there is an equilibrium or a balance between reactants
going to products and products going to reactants and typically instead of a full arrowhead
you'll see that as just top half or bottom half of the arrow and sometimes especially
in typed texts where they don't have this symbol, they'll draw two arrows like this.
Same we have an equilibrium reaction where A and B are in equilibrium or C and D are
in equilibrium we don't always have an exact A to B, B to A. Sometimes we can say that
a reaction is product favored where even though they're in equilibrium it tends to weigh heavily
in the direction of the products.
You'll see this as the equilibrium arrow but a bigger arrow towards the favored direction
and a smaller arrow towards the less favored direction to show that you're favoring the
products.
Or lets say C plus D is an equilibrium but the reactants are favored, we'll have a bigger
arrow pointing to the reactants and a smaller arrow pointing to the products.
If the equilibrium arrow brought double headed arrow to mine, this is not typically see in
chemical equations but you've seen it in organic chemistry during resonance.
Resonance is less of a reaction and more about showing that a molecule is adopting two different
states.
For example, if we have something like this with a carbocation we know that the pi bond
can move over to the left now we show the double headed arrow, the pi bond is now on
the left and the carbocation is on the right.
the double headed resonance arrow implies that is constantly going back and forth and
for resonance you also have brackets around the two hybrids to show that it's really an
inbetween of the two rather than a reaction that is taking place as we have seen above.
You'll see many different types of chemical reactions on the MCAT but that’s another
topic for another video.
First, if you like this video make sure to give it a thumbs up, then click the subscribe
button and bell icon so that you don’t miss any of the new videos.
Then visit my website leah4sci.com/reactions for the complete stoichiometry video series
alongwith the practice quiz and cheat sheet.
The link again is leah4sci.com/reactions.