Video Transcript
In this video, we will learn what a
dynamic equilibrium is and investigate how one is established. So what is a dynamic
equilibrium? When a reaction occurs, it may go
in one direction only to produce products, for example, A reacting to form B. These
are irreversible reactions. Or a reaction can move in two
directions, both forward and backwards. These are called reversible
reactions. Not all reactions have the ability
to be reversible.
In a reversible reaction, reactants
are forming products and at the same time products are forming reactants. Both A and B will be present
together in the same vessel. And we say they are in equilibrium
with each other. Reversible reactions can be
described as being in either static equilibrium or dynamic equilibrium.
In a static equilibrium, there is
no movement or exchange between the reactants and products. In other words, the rate or speed
of the forward reaction and the backward reaction are both zero, whereas in a
dynamic equilibrium, the rate or speed of the forward reaction and the backward
reaction are equal and constant. In other words, the reactants are
being converted to products and products are being converted to reactants at the
same speed. And the speed is not changing.
However, it is very important to
note that the concentrations of the reactants and products may be equal or they may
not be equal. What is most important to remember
is that the rate of the forward and reverse reactions are equal. And this has nothing to do with the
concentration of the reactants and products. The concentrations of the reactants
and products and the rate of forward and backward reactions can be changed if the
conditions under which the reaction occurs are changed.
We will discuss changing conditions
a bit later on. For now, we must consider that a
dynamic equilibrium occurs under constant conditions. Let’s investigate in what kind of
system a dynamic equilibrium occurs.
Dynamic equilibrium can be reached
in a closed system rather than an open system. What’s the difference? The processes or reactions which
occur in an open system occur in an open vessel, and in a closed system in a sealed
or closed vessel. In an open system, both energy and
matter can enter or leave the system. In a closed system, energy can
enter or leave, but matter cannot. A reaction which occurs in a test
tube, which releases gases to the atmosphere, is an example of an open system. Another example of an open system
is water boiling in a pot on the stove with the lid off. Corresponding examples of closed
systems would be the same reaction occurring in a test tube but with a stopper in
place or a boiling pot of water with the lid on.
Now, in an open system, visible
changes might be obvious to an observer. For example, in an open pot, water
is free to leave the pot to the surroundings. And over time, the level of water
will appear to decrease. However, in a closed system, it may
look like nothing is happening, even though the forward and backward processes are
occurring at the same time. A true equilibrium can be set up in
a closed system because matter cannot leave. And if there is constant movement
in the forward and backward directions for the process or reaction, we say this
equilibrium is dynamic.
Now that we know what a dynamic
equilibrium is and that it occurs in a closed system, let’s look closely at a
reaction that reaches dynamic equilibrium. Let’s walk through what happens
when a reaction in a closed system begins to the point where it reaches dynamic
equilibrium. We’ll use the example of nitrogen
gas reacting with hydrogen gas to form ammonia. The equation is N2 gas plus three
H2 gas reacting reversibly to give two NH3 gas. In industry, the synthesis of
ammonia gas by this reaction is called the Haber process.
We are going to use a sketch graph
to understand the formation of this dynamic equilibrium. We are going to plot the rate of
the reaction over time. Because we are just going to use
sketch graphs, we are not going to use actual values or units. But we are just going to get an
understanding of the shape of the curves.
Now, imagine some nitrogen gas and
hydrogen gas are placed together in a closed system, in other words, in a sealed
vessel. At time equals zero, there is no
product as the reaction has not yet begun. Under the right conditions, the
reaction begins, with the forward reaction producing ammonia very quickly. In other words, the forward
reaction begins very fast or with a high rate. Because there is no ammonia
initially, the rate of the backward reaction is zero. As soon as ammonia forms, some of
it will break down to reform the reactants. In other words, a reversible
reaction occurs.
Although the forward reaction is
still occurring quite fast, the rate of the forward reaction decreases slowly over
time as more and more product forms. Oppositely, the rate of the reverse
or backward reaction is initially very slow. But over time, this reaction occurs
faster as there is more ammonia to decompose back into the reactants. After a certain amount of time,
let’s call it time equals 𝑥, the rates of the forward and reverse reactions are the
same. Let’s call that rate or speed
𝑝.
After time 𝑥, the forward and
backward reactions are in equilibrium. We now know that at a dynamic
equilibrium, the forward and reverse reactions are both occurring and are occurring
at the same speed or rate. And we know that the concentration
of the reactants and products are not necessarily the same. Now, let’s investigate what happens
to the concentrations of reactants and product over time by drawing another sketch
graph.
If we plot concentration versus
time, at time equals zero, there’s a high concentration of the reactants nitrogen
and hydrogen in the vessel. And no product has formed yet. In other words, the concentration
of the product ammonia is zero. As the forward reaction begins to
make product, so the concentration of the reactants decreases and the concentration
of the product ammonia increases. After some time, let’s call it time
equals 𝑥, the concentrations of the reactants and product remain constant. This is because the reaction is at
dynamic equilibrium.
Notice, however, that the
concentration of the reactants at equilibrium, concentration 𝑄, and the
concentration of the product ammonia at equilibrium, concentration 𝑆, are not
necessarily the same value. In this example, the concentration
of the reactants at equilibrium is higher than that of the product. And this is because of the
particular set of conditions that this reaction occurred under. The specific conditions of
pressure, temperature, or concentration will directly affect the concentration of
the reactants and product at equilibrium.
Let’s clear some space to
investigate this a bit further. The first of these three graphs we
saw a few moments ago, where the concentration of the reactants at dynamic
equilibrium was higher than that of the product ammonia. But there are two other possible
scenarios where the reactants and product have the same concentration at dynamic
equilibrium or where the product ammonia has a higher concentration than that of the
reactants nitrogen and hydrogen.
If we took the concentration value
of the reactants and product in each case and put them as a ratio of concentration
of product divided by concentration of reactants, we’d get the following three
results, where the ratio is less than one, equal to one, or greater than one. These ratios give us an indication
of the equilibrium position. The equilibrium position can be
described as the ratio of the concentration of product to reactants. When the ratio gives a value less
than one, we know that the graph looks like this. When the equilibrium position gives
a value of one, we know that the concentrations of the reactants and product are
equal, a dynamic equilibrium. And a value greater than one
indicates that the product concentration at dynamic equilibrium is higher than that
of the reactants.
Whether we get a graph like the
first, second, or third one depends on the equilibrium position, which depends on
the conditions under which the reaction occurs. In other words, the pressure,
temperature, and concentration directly affect the concentration of the reactants
and products at equilibrium. These conditions can cause the
equilibrium to shift towards the left, the reactant side of the equation, or towards
the right, the product side of the equation. If any of these conditions are
changed, the equilibrium position will also be changed. And the concentrations of the
reactants and product will change. The details of how these three
conditions influence the equilibrium position is a discussion for another video.
Other reaction conditions, such as
the presence of a catalyst, do not have an effect on the equilibrium position. In other words, they do not
influence the concentration of reactants and product at equilibrium. However, a catalyst does affect the
rate at which the equilibrium is reached. The presence of a catalyst will
cause the equilibrium to be reached faster.
Now, it’s time to practice an
equilibrium problem.
If a reversible reaction is allowed
to reach equilibrium in a closed system, which of the following will be true? (A) The concentration of the
products will gradually increase. (B) Increasing the pressure inside
the closed system will not affect the equilibrium. (C) Increasing the temperature will
not affect the equilibrium. (D) The concentration of the
reactants and products will be the same. Or (E) the rate of the forward
reaction will be the same as the rate of the backward reaction.
A reversible reaction is one in
which reactants form products and at the same time products are forming
reactants. When this occurs in a closed
system, which is one in which energy can enter or leave but matter cannot, we say an
equilibrium is reached. We can plot the progress of the
reaction in terms of rate of reaction versus time from time equals zero. At time equals zero, reactants form
products very fast. And at time equals zero, before any
products are formed, the backward reaction hasn’t yet begun.
When some products are formed, some
of it will react to reform the reactants. And the rate of the forward
reaction begins to decrease. As more product forms, so the rate
of the backward reaction increases. From a certain point in the
reversible reaction, let’s call it time equals 𝑥, an equilibrium will occur. And the rate of the forward and
backward reactions will be the same. We can see that from time equals
𝑥, both the black and green curves have the same 𝑦-value, in other words, the same
rate of reaction. Option (E) is correct. The rate of the forward reaction
will be the same as the rate of the backward reaction.
Let’s confirm this by having a look
at the other answer options. We can do this by doing another
sketch graph, this time of concentration versus time. At time equals zero, before the
reaction has begun, when there is only a reactant in the vessel, the concentration
of the reactants is very high and the concentration of the products is zero. As the reaction begins to proceed,
so the concentration of the reactants decreases as they are converted into
products. As more reactants are converted to
products, so the concentration of the products increases.
However, from time equals 𝑥, the
concentration of the reactants remains constant and the concentration of the
products remains constant. An equilibrium is formed where the
rate of conversion of reactants to products is the same as the rate of conversion of
products to reactants. If we look carefully, however,
we’ll see that the concentration of the reactants and products are not necessarily
the same at equilibrium. So we can rule out option (D).
We can also rule out option (A)
because the question asks about equilibrium. And at equilibrium, the
concentration of the products will not increase but will remain constant. Now, concentration–time graphs can
look different from the one drawn here. The reactant concentration can be
higher than that of the products at equilibrium, as drawn in this graph. Or the reactants and products can
have the same concentration at equilibrium. Or the products can have a higher
concentration than the reactants.
Taking a ratio of the concentration
of the products to reactants will give us a value either less than one, equal to
one, or greater than one, depending on the graph. These values, which represent the
equilibrium position, depend on the temperature, pressure, or concentration
conditions under which the reaction occurs. In other words, temperature,
pressure, and concentration do influence equilibrium. So we can rule out the remaining
two answer options as they are incorrect.
Finally, if a reversible reaction
is allowed to reach equilibrium in a closed system, the rate of the forward reaction
will be the same as the rate of the backward reaction. And we call this dynamic
equilibrium.
Now, let’s summarize everything we
have learnt. We learnt that a dynamic
equilibrium is an equilibrium in which the forward and backward or reverse reaction
rates are equal. We learnt that the concentrations
of the reactants and products at dynamic equilibrium are not necessarily equal. And we learnt that a dynamic
equilibrium occurs in a closed system. We defined a closed system as being
one from which energy can enter or leave but matter cannot.
We also walked through how a
dynamic equilibrium occurs from the beginning of a reaction and visualized this
using a rate-versus-time graph. And lastly, we looked at three
different concentration–time graphs showing three different equilibrium
positions. And we learnt that equilibrium
position is influenced by concentration, pressure, and temperature.
