Professor Dave here, let’s learn how to write chemical equations.
We know a little bit about chemical reactions, as well as the written equations we can write
to depict them.
But there are a few different ways we can write these equations.
We should be able to write them using chemical formulas, which abbreviate all the elements
present in each compound, and sometimes list the phases of each compound, but we should
also be able to write these equations using words, because this helps us understand how
we might verbally communicate the details of a chemical reaction to someone else.
Let’s therefore make sure that we can do two things.
We must be able to look at a chemical reaction that is written with symbols, and translate
it into written words.
And we must also be able to do this in reverse, by taking a sentence that describes a chemical
reaction, and making sure we are able to display this transformation using the symbols we know
in chemistry.
Take this reaction for example.
We have NA solid plus H2O on the reactants side and then NaOH in the aqueous phase and
H2 in the gas phase.
Let’s turn this equation into a sentence comprised of normal words.
Starting from the left, NA is sodium, and it’s a solid, so we might simply say solid
sodium metal.
Now what does the sodium react with?
We all know H2O is water, and the L means liquid, so let’s add “reacts with liquid
water”.
Now the forward arrow separates the reactants from the products, so we need words that indicate
that a chemical reaction has occurred.
So let’s say “to produce”.
And now we list the products.
NaOH is sodium hydroxide, and it’s in the aqueous phase, so we write aqueous sodium
hydroxide, and H2 is hydrogen, which is a gas, so we write hydrogen gas.
And now we have completed a sentence that describes in words what the equation describes
in symbols.
Let’s go the other way around.
Say we have the following sentence.
Solid iron reacts with fluorine gas to produce solid iron three fluoride.
Let’s identify some key words here.
“Reacts with” tells us that iron and fluorine are the reactants, and “produce” tells
us that iron three fluoride is the product.
So those should be separated by an arrow.
Now let’s translate the words into chemical symbols and phase symbols.
The symbol for iron is Fe, and it’s a solid, so let’s write Fe with the subscript S in
parentheses.
Then we put a plus symbol, because the iron is reacting with something else.
Fluorine has the symbol F, and fluorine gas is F2, which we know from our ability to draw
Lewis dot structures.
So let’s write F2 with the subscript G in parentheses.
Then the forward arrow, separating the reactants from the products, and lastly we have iron
three fluoride.
So we know the product has iron and fluorine in it, but in what ratio?
If we recall, this roman numeral three is telling us the oxidation state of the iron
atom, which will be plus three.
Fluoride ions are minus one, so we will need one iron and three fluorides to make a neutral
molecule, so we write FeF3 with the subscript S. Lastly, we might want to balance the equation.
We can see that we have two fluorines on the left and three on the right, so let’s put
a three here and a two there so that we have six on each side.
Now we also have two irons on the right, so let’s put a two here so that we have two
irons on the left as well.
And that’s all there is to it, so we should now be able to write chemical equations either
with symbols or with words. Let's check comprehension.