Video Transcript
We generally think of reactions
occurring in one direction where the reactants combine to form the products. But we’ll see in this video how
some reactions can go in the opposite direction where products can react to form
reactants. In this case, the reaction is
reversible. In this video, we will learn how to
define reversibility in chemical reactions, identify examples of reversible and
irreversible processes, and discuss how changing the conditions of a reaction can
cause it to go in the reverse direction.
At the start of the 1800s,
scientists thought reactions only went in one direction, which seems reasonable. After all, for a lot of processes,
you can’t get things to react in the opposite direction. Imagine trying to uncombust a
combustion reaction, like when methane burns in oxygen to form carbon dioxide and
water. We’d have to force carbon dioxide
and water to react to form methane and oxygen again. Impossible! But there was a French chemist
named Claude Louis Berthollet that was studying a reaction where sodium carbonate
reacts with calcium chloride to form calcium carbonate and sodium chloride.
He took a trip to a salt lake where
he made an interesting observation. There was sodium carbonate forming
at the edge of the lake. He eventually realized that the
large amount of salt in the salt lake or sodium chloride was reacting with the
calcium carbonate that’s found in the shells of marine organisms. In other words, the reaction was
going in the reverse direction of what he was studying in the lab. As it turns out, there’s many
reactions that don’t just go in the forward direction where the reactants combine to
form the products. There’s many reactions that can go
in the reverse direction as well, where the products can combine to form the
reactants.
When the reaction can only go in
the forwards direction, like the combustion of methane, it’s called an irreversible
reaction. But when the reaction goes both in
the forward direction and the reverse direction, we call it a reversible
reaction. We can indicate a reversible
reaction in our chemical equation by using two single-headed arrows. So what makes it so reactions can
sometimes go in the forward direction and other times go in the reverse direction
like Berthollet saw with his reaction? Well, it turns out that changing
the conditions that a reaction is reacting under can sometimes cause a reaction to
reverse. To get a feel for this, let’s take
a look at some classic examples of reversible reactions.
Anhydrous copper(II) sulfate is a
white powder, but when it’s exposed to water, it forms a hydrated salt and turns
blue. This makes it extremely useful as a
test to identify if the substance is water. When you heat this blue hydrated
copper(II) sulfate, it drives the water out of the salt, causing the reaction to go
in the reverse direction, which gives us the white powder, the anhydrous copper(II)
sulfate, once again.
NH4Cl or ammonium chloride is
another white powder. When it’s heated, it decomposes,
forming two colorless gases, ammonia and hydrogen chloride. If you perform this reaction in a
stopper test tube, you’ll notice that as the reaction proceeds, a white powder will
form at the cool end of the test tube. This white powder that’s forming is
in fact ammonium chloride, the same substance we started off with. When the ammonia and hydrogen
chloride gases cool down enough, they react with each other, causing the reaction to
go in the reverse direction and form ammonium chloride again.
Now, this isn’t to suggest that all
reactions go in the forward or reverse direction because we add or remove heat. There are plenty of other
reversible reactions that occur due to changes in other kinds of conditions, like pH
indicators. pH indicators are extremely useful in acid–base reaction experiments
like titrations. We can add a few drops of indicator
to our analyte in our titration experiment so that we can roughly monitor the pH of
the solution. When we first add the pH indicator
to our solution, it’s one color. But as we perform the titration
experiment, in this case, adding a base to an acid, the pH will change. At a certain point when we’ve added
enough of our base, the pH indicator will suddenly change color.
As it turns out, these pH
indicators are large organic molecules that have two forms. In one form, the indicator has a
hydrogen, and it’s one color. In the other form, the indicator
has lost that hydrogen, and it’s another color. The color of the indicator changes
depending on the concentration of hydrogen ions in the solution. When there’s more H+ ions in the
solution, meaning that the solution is acidic, the indicator is one color. But as we add the base during this
experiment, it will react with the H+ ions in the solution, causing the
concentration of H+ ions to decrease, which will make the solution more basic, and
the indicator will change color.
Since the pH is related to the
concentration of hydrogen ions in the solution, we can use the color of the pH
indicator to give us a rough estimate of the pH of the solution. The exact pH that this color change
happens depends on the properties of the indicator. We have many different molecules
that can act as indicators, meaning we have indicators that we can use for a variety
of pH values.
Now, these examples shouldn’t
suggest that for all reversible reactions, we have the forward reaction occurring in
some conditions and the reversed reaction occurring in other conditions. Sometimes both the forward and
reverse reactions are happening at the same time. This is common in reactions that
involve weak acids and weak bases. For example, when we introduce the
weak acid hydrofluoric acid to water, it will dissociate and form H+ ions and F−
ions. But then the F− ions are capable of
reacting with the H+ ions to form hydrofluoric acid. Then these newly formed molecules
of hydrofluoric acid can dissociate to form H+ ions and F− ions. Well, some of the existing F− ions
will again react with the hydrogen ions to form hydrofluoric acid.
In other words, the forward and the
reverse reaction are occurring at the same time. When this happens, the forward and
reverse reactions will continue to occur until the system reaches an equilibrium
where the concentration of hydrofluoric acid, hydrogen ions, and F− ions are no
longer changing. Now this equilibrium isn’t a static
equilibrium where the rate of the forward reaction is zero and the rate of the
backwards reaction is zero. Rather, both reactions are still
occurring. Their rates are just equal to each
other, so there’s no net change in the concentrations of our chemical species. This kind of equilibrium is called
a dynamic equilibrium.
Now there’s one last topic to
discuss before we practice what we’ve learned, and that’s the role of energy in
reversible reactions. During the course of a chemical
reaction, some or all of the bonds between the atoms in our reactants break apart so
that the atoms can become rearranged to form the products. The process of breaking bonds
requires energy. A process that must absorb energy
is endothermic. Conversely, the process of making
bonds is exothermic. It releases energy. When a reaction releases more
energy than it absorbs, that reaction is exothermic, which corresponds to a negative
change in enthalpy. But when a reaction absorbs more
energy than it releases, the reaction is endothermic, which corresponds to a
positive change in enthalpy.
Now what does this mean for
reversible reactions? Well, let’s say that our forward
reaction is endothermic, so the reaction would have a positive change in
enthalpy. What would the reverse reaction
be? When we look at our diagram, we can
see that when we go from products to reactants, the reaction is endothermic because
we have a positive change in enthalpy. But when we go the other way,
forming the reactants from the products, the enthalpy change is negative, meaning
that the reaction would be exothermic. This will always be true for
reversible reactions. In one direction, the reaction will
be endothermic, and in the other direction, the reaction will be exothermic.
However, when we look at the
overall change in enthalpy for the reaction, that will be the same, only the sign
will be different. In other words, if in the forward
direction the enthalpy change is 100 joules, the enthalpy change will be equal to
negative 100 joules in the reverse direction. So now we’ve discussed all we need
to know about reversible and irreversible reactions, so let’s try some practice
problems.
Which of the following statements
about reversible reactions is true? (A) A reversible reaction is often
a combustion reaction. (B) A reversible reaction is
indicated by a single double-headed arrow in a chemical equation. (C) A reversible reaction is a
chemical reaction that can proceed in both directions. (D) A reversible reaction always
involves hydrated and anhydrous salts. (E) A reversible reaction is
endothermic in both directions.
A reversible reaction is a reaction
that not only goes in the forward direction where the reactants combine to form the
products, but also the reverse direction where the products combine to form the
reactants. We can indicate this in our
chemical equation by using two single-headed arrows. With this information, answer
choice (C) is clearly the correct answer. A reversible reaction is a chemical
reaction that can proceed in both directions, the forward direction and the reverse
direction. But let’s take a quick look through
our other answer choices to see why they’re false.
Answer choice (A) says that a
reversible reaction is often a combustion reaction. An example of a combustion reaction
is the combustion of methane, where methane burns in oxygen to produce carbon
dioxide and water. If this reaction were reversible,
it would have to go in the reverse direction, where carbon dioxide reacts with water
to form methane and oxygen. It’s hard to imagine this process
happening. Since combustion reactions don’t
typically go in the reverse direction, they’re not often reversible. Answer choice (B) says that
reversible reactions are indicated by a single double-headed arrow. But we’ve seen how reversible
reactions are indicated by two single-headed arrows in a chemical equation.
Answer choice (D) says that a
reversible reaction always involves hydrated and anhydrous salts. There are multiple examples of
reversible reactions that involve hydrated and anhydrous salts. For example, anhydrous copper(II)
sulfate can react with water to form hydrated copper(II) sulfate. This reaction can be reversed when
the hydrated copper(II) sulfate is heated. But there are plenty of examples of
reversible reactions that don’t involve these hydrated salts, for instance, the
reaction of any weak acid or base with water such as hydrofluoric acid. So reversible reactions sometimes
involve hydrated and anhydrous salts, but not always.
Our final answer choice says that a
reversible reaction is endothermic in both directions. Recall the endothermic means that
the enthalpy change for the reaction is positive. This means that, overall, the
reaction takes in energy. Looking at the sketch of an energy
diagram, we can see that the forward reaction where A plus B reacts to form C plus D
is endothermic since the change in enthalpy is positive. But what about the reverse
direction where C plus D reacts to form A plus B? In this case, we would get a
negative change in enthalpy, meaning that the reverse reaction is exothermic not
endothermic. So a reversible reaction will be
endothermic in one direction, but exothermic in the other direction.
So of the statements we looked at,
the only one that was true about reversible reactions is that a reversible reaction
can proceed in both directions.
What is the backwards reaction for
the following reversible reaction? CoCl2 plus 6H2O in equilibrium with
CoCl2 6H2O.
A reversible reaction is a reaction
that can go in both directions. That means that not only can this
reaction go in the forward direction, where the anhydrous cobalt(II) chloride reacts
with water to form hydrated cobalt(II) chloride, but the reaction can also go
backwards, which is what this question is asking us for. In the forward direction, like
we’ve just seen, the reactants form the products. But in the backwards reaction, the
products form the reactants. So the backwards reaction for this
reaction would be our products, CoCl2 6H2O, reacting to form our reactants, the
anhydrous cobalt(II) chloride and water.
Now let’s sum up everything we
learned with the key points for this lesson. When a reaction goes in the forward
direction, the reactants form the products. And when a reaction goes in the
reverse direction, the products form the reactants. If a reaction can go in both the
forward and reverse direction, it’s a reversible reaction, which we indicate with
two single-headed arrows. But if a reaction can only go in
the forward direction, it’s an irreversible reaction, which we can indicate in our
chemical equation by using a single double-headed arrow. Changing the conditions that a
reaction is occurring in can cause the reaction to reverse. Reversible reactions are always
endothermic in one direction and exothermic the other direction.
