Video Transcript
In this lesson, we will learn how
to construct and understand equations for chemical reactions. We will also learn to write
chemical reactions using both words and chemical symbols.
Learning to understand chemical
reaction equations can be a bit like learning a new language. Our equations involve words and
symbols that might not always be familiar. So, let’s review some of the key
features that you might come across when looking at reaction equations.
You may see reaction equations
written in the form a bit like this, where you have one or more substances on the
left. In this case, two substances A and
B, some kind of arrow, and then some more substances or just one substance on the
right-hand side. Let’s look more closely at some of
these features. All of the substances to the left
of our arrow are referred to as reactants. These are the substances which go
on to react in our equation.
On the right-hand side of our
arrow, we have the products. Notice that we may not have the
same number of reactants as we do products and vice versa. Now, let’s take a closer look at
our reaction arrow. The arrow drawn here is perhaps the
most common sort of reaction arrow that you might come across. It denotes that the reactants
become the products. You may also come across the
equilibrium arrow which looks like this. This denotes that the reactants are
becoming the products, but also the reverse reaction may be taking place as well,
where the products turn back into the reactants.
You may also see it drawn where the
two arrows are not the same size. We won’t go into understanding
these right now, but just know that they are similar equilibrium arrows. We also have this plus symbol in
our reaction equation which is used to separate different chemical species. You may see this on the reactant
side and on the product side. Sometimes you may also see text or
symbols drawn above the reaction arrow. This gives us extra information
about the reaction. For example, we might find a symbol
for heat here, or even a specific reaction condition such as a specific temperature
or perhaps the presence of a catalyst.
These are things which are needed
for the reaction to occur but are not themselves reactants or products. Something else you might come
across, our state symbols. These are written in brackets and
as a subscript to our products or reactants. The state symbols you’re likely to
come across are s for solid, l for liquid, g for gas, and aq for aqueous, meaning
dissolved in water. These state symbols tell you which
physical state each of our species is in at the time.
Water is a great example for
understanding some of these state symbols. H2O with the gas state symbol is,
of course, steam. H2O with the liquid symbol is
liquid water. And H2O with the solid state symbol
is, of course, ice. You can see from these examples why
it might be important to include a state symbol in some cases. HCl is another good example, where
HCl aqueous is hydrochloric acid, but HCl gas is hydrogen chloride gas, a very
different reactant or product. So, this is why you might see state
symbols included in your reaction equation.
Now that we’ve looked at some of
the general features of our equations, let’s study some examples. Let’s begin by looking at two
simple reaction equations. On the left of our arrow, we have
the reactants. And on the right, we have our
products. Let’s have a look now at how we
might read these equations out loud.
You can start by just reading
sodium plus chlorine, but how do we say this reaction arrow in words? It turns out there are lots of
different options, but we’ll just go through a few. The most common way to say this
reaction arrow is “react to form,” as in sodium plus chlorine react to form sodium
chloride. You can also have similar phrasing
such as “react to make” or just the word “form,” like sodium plus chlorine form
sodium chloride. But what about in our second
example?
In this example, we only have one
reactant to the left of our arrow. So, we can’t really use the word
react when talking about our arrow. React implies that you have more
than one substance. So when reading this kind of
equation, you could use the word “decomposes” or “yields.” So, you could say calcium carbonate
decomposes to form calcium oxide plus carbon dioxide. If you ever get stuck trying to
understand the reaction equation, particularly one with symbols, it can help to try
to read it out loud, or at least read it through in your head. This can be helpful when writing
your own reaction equations, for example. Let’s look at how to do that
now.
You might need to write out a
reaction equation from an experiment that you’re doing. Let’s try writing our own reaction
equation from this experimental detail. Potassium hydroxide reacts with
sulfuric acid. Potassium sulfate and water are
formed in the reaction. So, where do we start? Let’s begin by identifying what our
reactants are.
Remember, the reactants are on the
left of our reaction arrow. We can see the word react in our
first sentence. And we’re told that potassium
hydroxide reacts with sulfuric acid. So, here we have two reactants. We can write these out with a plus
symbol in between to start our reaction equation. Now that we’ve found all of our
reactants, we can add in our reaction arrow. We aren’t told that this reaction
is in equilibrium with anything, so we can draw a normal reaction arrow.
Next, we need to find our
products. In the second sentence, we’re told
what is formed in our reaction. So, these must be our products. Let’s add potassium sulfate and
water to the product side of our reaction. Now that we’ve added our products,
our reaction equation appears complete. To check that we’ve done it right,
we can try reading it aloud and comparing it with the information from the
experiment that we’ve been given. Potassium hydroxide plus sulfuric
acid react to form potassium sulfate and water. This appears to match our
experimental detail so it must be correct.
You might be asked to write your
chemical equation using chemical symbols instead of words. Here, all we need to do is to
convert our word equation into chemical symbols. Let’s do this one item at a
time. Let’s start with our first
reactant, potassium hydroxide. We know from the periodic table
that the symbol for potassium is K. Now we just have to remember the
symbol for hydroxide. Luckily, the clue is in the
name. The first part “hydr” means we have
hydrogen and “oxide” means we have an oxygen.
You may remember that potassium
hydroxide is KOH. If you hadn’t remembered, you could
work it out by looking at the ion that potassium forms K+ and remembering that
hydroxide is OH-. So together, they form KOH. The plus in our equations stays the
same. So next, we need to convert
sulfuric acid into the chemical symbol form. Sulfuric acid is, of course,
H2SO4. Again, if we want to think about
the ion forms, hydrogen, of course, tends to form H+. So, SO4 must have a charge of two
minus to balance out our two hydrogens.
Our reaction arrow stays the
same. So now, we’re on to the products,
potassium sulfate and water. We already know that potassium is K
and sulfate is SO4. We can use the charges on the ions
to work out what the formula is for potassium sulfate. Because our sulfate has a charge of
two minus, we’re going to need two potassium cations to balance the charge. So potassium sulfate is K2SO4. Finally, we have water, which is
H2O. So here we’ve written a reaction
equation from experimental detail and then converted that word equation into a
chemical symbol equation. Let’s try a few more for
practice.
An experiment shows that hydrogen
peroxide decomposes to form water and oxygen gas. Which of the following word
equations describes the chemical reaction that has occurred in this experiment? (A) Hydrogen peroxide decomposes to
form water plus oxygen. (B) Hydrogen peroxide plus oxygen
react to form water. (C) Hydrogen peroxide decomposes to
form peroxide plus hydrogen. (D) Hydrogen peroxide decomposes to
form hydrogen and oxygen peroxide. Or (E) hydrogen peroxide plus water
react to form oxygen.
This question is asking us to form
a word equation from the experimental detail given. Let’s start by finding our
reactants. Reactants are always on the
left-hand side of our reaction arrow. In this case, we only have one
reactant, since only one substance is listed and we have the phrase decomposes to
form. Decomposes means that one substance
breaks down into one or more other substances. So here, we have the beginning of
our reaction equation. Hydrogen peroxide decomposes to
form something.
Now, let’s look for the
products. The question tells us that the
products are water and oxygen gas. So these should be written on the
right of our reaction arrow. So here, we’ve written the reaction
equation from the details given to us in the experiment. Now we just have to match it with
one of our potential answers. We can see that the correct
matching answer is (A). Remember that the order of the
reactants or the products makes no difference.
When aqueous silver nitrate is
mixed with aqueous sodium chloride, a white solid precipitate of silver chloride is
produced. The symbol equation for this
reaction is shown below. AgNO3 blank plus NaCl blank react
to form AgCl blank plus NaNO3 blank. Which state symbols should be added
to each compound to complete the equation?
This question is asking us to add
state symbols to a reaction equation. So, let’s remind ourselves what we
mean by state symbols. State symbols tell us which
physical state a substance is in at the time. We use the letter s for solid. The letter l for liquid, g for gas,
and aq for aqueous, meaning dissolved in water. We are given our chemical reaction
equation in the chemical symbol form, but our experimental detail uses the full word
names for various compounds. To match these up, let’s convert
our chemical symbol equation into a word equation to make life easier.
Our periodic table tells us that
silver is Ag. Our compound contains nitrogen and
oxygen, so that must make it a nitrate. So, here, we have silver
nitrate. Na is the symbol for sodium and Cl,
the symbol for chlorine. So, this gives us sodium chloride,
the sort of thing you’d sprinkle on your fries. Next, we have silver and chlorine
again, so this must be silver chloride. And finally, we have sodium again
and nitrate, sodium nitrate. Next, we need to find each of these
compounds in the experimental detail to work out which state it is in.
Silver nitrate is first. And the question tells us that it
is aqueous. Aqueous means dissolved in water,
and the state symbol for aqueous is aq. Next, we have sodium chloride. This is also listed as aqueous. Next, we’re looking for the first
product, silver chloride. The question tells us that this
forms as a white solid precipitate. So, this is a solid, so we use the
letter s. But what about the sodium
nitrate? It’s not specifically listed. We know that sodium nitrate didn’t
precipitate as a solid. So it must have remained in
solution as aqueous. So, we can label this also as
aqueous. So, here we have our reaction
equation, complete with state symbols.
Let’s summarize the key points. Reactants are written on the left
of our arrow and products on the right. Our reaction arrow usually means
“react to form” or sometimes “decompose to form.” The arrow can also show if our
reaction is in a state of equilibrium. State symbols can be added to our
equation to show the state that substances are in. Sometimes, reaction equations are
displayed with words and sometimes with chemical symbols.
