Video Transcript
In this video, we will learn how to
perform different experiments whose results could be analyzed to calculate the rate
of reaction. When we carry out a reaction, what
we’re doing is turning reactants into products. When we begin our reaction, we have
an excessive reactants and no products. Remember, of course, that we could
have any number of different reactants. As the reaction progresses, we
gradually convert our reactants into our products. How fast this process occurs is our
rate of reaction.
In chemistry, it’s often useful to
be able to measure this reaction rate. And there are lots of ways we can
do this. We could measure the rate at which
we lose one or more of the reactants. Alternatively, we could measure the
rate that we form one or more of our products. The method with which we choose to
do this depends on the state or properties of the substance we are watching. For example, if we’re measuring the
production of a gas, we might want to use a gas syringe. Regardless of which method we use,
we’ll often end up with an answer in similar units. Generally, the units we use for the
reaction rate is some kind of mass or amount over time. Examples include grams per second,
centimeters cubed per second, moles per second, or, if we’re carrying out our
reaction on an industrial scale, even tons per day.
So now let’s have a look at some
examples of ways that we could measure our rate of reaction. One factor which we know affects
the rate of reaction is the surface area when we’re dealing with solids and
sometimes liquids. So let’s have a look at an
experimental setup that could help us to measure this effect. Let’s use an example of the
reaction of a carbonate with an acid, which produces a salt, water, and carbon
dioxide. In order to set up this experiment,
we need to remember that the law of conservation of mass means that the mass of all
of the reactants added together should equal exactly the mass of all of the products
added together.
We should note, however, that in
this example reaction, one of our products is a gas. Of course, it’s pretty difficult to
try and weigh a gas. We could try and capture it and
measure its volume, but in this example, there might be an easier method. If we place our two reactants into
a conical flask and don’t stop at the top, what will happen is as we produce our
products, the carbon dioxide gas will escape. If we could measure how quickly
this carbon dioxide is produced and escapes, we could measure the rate of this
reaction. To do this, we could simply place
our reaction vessel onto a balance. As our reaction progresses and we
lose CO2, what we’re doing is losing mass. The mass, of course, isn’t
destroyed. It’s simply lost to the
atmosphere.
We can watch the rate or speed that
this mass decreases, taking measurements every 10 seconds, say. And from this, we can work out the
rate of reaction. Once the mass of our reaction
vessel stops decreasing, we know that the reaction has gone to completion. What we’ll end up with is a graph a
bit like this, showing the decrease in mass over time. But remember that we wanted to use
this to investigate the effect of surface area on rate of reaction. So what we’ll need to do is set up
another version of this experiment. But this time, we grind the calcium
carbonate into really small pieces. What we’ll discover is that by
grinding the calcium carbonate, we increase the surface area. And therefore, the rate of reaction
increases.
Of course, in order to make sure
that it’s fair, we need to have several of our variables controlled. Some of the important control
variables in this experiment include the mass of this calcium carbonate that we
start with, the volume and concentration of the hydrochloric acid that we use, the
temperature, et cetera. In this case, the independent
variable or the variable that we’re trying to test is the surface area. And of course, our dependent
variable is what we’re measuring, which is the mass of our reaction vessel. Now let’s look at a different type
of experimental setup for measuring rates of reaction.
Let’s now have a look at the effect
of acid concentration on the reaction of magnesium metal with hydrochloric acid. When we add the state symbols to
our reaction equation, we can see that one of our reactants is a solid and one of
our products is a gas. We could measure the production of
the hydrogen gas in a similar way to what we saw previously or perhaps with a gas
syringe. However, in this example, there
might be an easier way. In this reaction, our magnesium
metal is a solid. And as the reaction progresses,
that solid will disappear. So we could simply measure the time
it takes for all of our magnesium ribbon to disappear in different concentrations of
acid. What we will need is a variety of
reaction vessels with hydrochloric acid at different concentrations and some
magnesium ribbon.
But again, let’s think about our
control variables. We must make sure that we’re using
the same volume of acid in each of our experiments and again the same exact amount
of magnesium ribbon. We’ll also want to try to keep the
temperature the same across all of our experiments. Once we have a different
concentration of acid in each vessel, we can drop a piece of magnesium into one and
time how long it takes to disappear. We can then repeat this for all of
our concentrations.
In this example, the time taken for
the magnesium to disappear is our dependent variable. And our independent variable is the
concentration of acid. If you did this experiment, what
you should see is that as the concentration of the acid increases, the magnesium
ribbon disappears faster. So the rate of reaction has
increased.
Now let’s see how we could explore
the effect of temperature on rate of reaction. Let’s have a look at the example of
the disappearing cross experiment. In this reaction, sodium
thiosulfate and hydrochloric acid react to form sodium chloride, water, sulfur
dioxide, and sulfur. The trick to working out how to
measure the rate of this reaction comes by looking at the states of each of our
reactants and products.
Both of our reactants are dissolved
in water, so this makes them aqueous. Sodium chloride, of course, is also
aqueous. And we have water in its liquid
form. Sulfur dioxide is a gas. And sulfur is a solid. This, of course, means that it does
not dissolve in water. Because our sulfur does not
dissolve in water, it precipitates out of solution, which means that we’ll be able
to see it. As the reaction progresses and we
produce this sulfur, we will see the solution turn a cloudy yellow white. So how can we use this to measure
the rate of reaction?
What we’ll need is a conical flask,
sometimes called an Erlenmeyer flask, a thermometer, and a piece of paper with a
large black cross drawn on it. As we add the acid, we start our
timer. As we add the acid, we’ll also need
to swirl our flask to make sure that everything mixes. Conical flasks are ideal for
swirling because you’re much less likely to splash the contents out than if you’re
swirling, say, a beaker. You will then need to view your
reaction from the very top of the conical flask. We’ll talk about why this is
important in a minute.
As the reaction progresses and we
produce our sulfur as a product, the solution will turn cloudy. As the solution turns cloudy, it
will be harder and harder to see the black cross. Once the cross has disappeared
completely, stop the timer. So our dependent variable or the
thing that we’re measuring is the time taken for our cross to disappear from
view. So what we’re really measuring here
is the rate that our solution turns cloudy. So you may hear cloudiness in a
solution referred to as turbidity. Turbidity is just the scientific
way of saying the cloudiness of a solution.
We’ll then need to repeat this
experiment at different temperatures. And this is where we need the
thermometer. So our independent variable in this
case is the temperature. And of course, there are some
control variables, the first being the concentrations of both the reactants that
we’re using. The next really important control
is the view that we use in order to determine the time it takes for the cross to
disappear. Our view needs to be straight down
directly over the conical flask. This ensures that our experiment is
repeatable. This is because for each
measurement, we’re looking through exactly the same depth of water and glass.
If we were to change the direction
of the view we took in order to determine when the cross had disappeared, we might
start getting different results. And that’s not what we want. What you should see if you do this
is that as the temperature of the solution increases, the time taken for our cross
to disappear decreases. This means that as the temperature
increases, the rate of our reaction also increases.
Now let’s have a look at a
different experiment. This time, let’s investigate the
effect of a catalyst on the decomposition of hydrogen peroxide. One potential catalyst for this
reaction is manganese dioxide, MnO2. Remember that catalysts are
regenerated at the end of our reaction. So this is why it’s not a reactant
or a product. Once again, let’s look at the
states each of our reactants and products is in. Pure hydrogen peroxide is a liquid,
as is the water product. And of course the oxygen is a
gas. It’s going to be difficult to
measure anything to do with the two liquids. So this time, let’s look at the
gas.
There are several ways we can
measure the production of a gas, one of which is using a gas syringe. What you’ll need is a setup a bit
like this, with a conical or Erlenmeyer flask, a stopper, and a gas syringe attached
by some tubing. The hydrogen peroxide and catalyst
are placed in the conical flask. This is then sealed with a bung and
the gas syringe is attached. As the reaction progresses, oxygen
gas is formed. Because the system is sealed, the
gas only has one place it can go, up the tubing and into the gas syringe. Gradually, as the oxygen pushes its
way into the gas syringe, the plunger on the syringe will move. This will allow you to take
measurements of the volume of gas produced over time. What you’ll end up with is a graph
showing the amount of oxygen gas produced at different time intervals.
In this example, the independent
variable is the catalyst. We can perform multiple versions of
this experiment with no catalyst, a small amount of catalyst, or maybe a large
amount of catalyst. Our dependent variable, the thing
that we measure, is the volume of oxygen gas produced. Things that you’ll want to control
are the volume of the hydrogen peroxide, the temperature, and how quickly you
stopper the conical flask. There are, of course, other ways to
measure the production of a gas. A setup like the one shown is an
equally valid method, though perhaps slightly more fiddly to set up. As the gas is generated in this
example, it travels down the tube and up into the upturned measuring cylinder. As the gas accumulates at the top,
it forces the water down. Using the amount of water
displaced, we can work out what volume of gas is produced.
We’ve seen a few different ways for
measuring the rate of various reactions. However, this is not an exhaustive
list. There are many other different ways
as well, for example, using colorimetry to measure the change in the color of a
solution. But since we can’t explore every
method in this video, let’s move on to looking at some questions.
Which of the following is not a
viable unit for a reaction rate? (A) kg over s, (B) g over s, (C) h
over s, (D) M over min, or (E) t over d.
Let’s start by working out what all
of these symbols stand for. (A) stands for kilograms per
second. (B) is similar, grams per
second. (C) is hours per second. (D) is molars per minute,
remembering that molar means moles per liter, sometimes written as moles per
decimeter cubed. And (E) is tons per day. To work out what a sensible unit
for reaction rate is, let’s remind ourselves what we mean by rate of reaction.
The rate of a reaction is the rate
that to the reactants are turning into the products. To measure the rate of reaction, we
could measure the loss of our reactants or we could measure the formation of our
products. Either way, we’re using a similar
equation. We can calculate the rate of
reaction by doing the mass or amount of our product formed or the mass or amount of
our reactant lost divided by time.
So the units for our mass or amount
will be things like kilograms, grams, moles, et cetera. And the units of time will be
seconds, minutes, hours, et cetera. So we’re looking for units along
the lines of kilograms per second, grams per minute, moles per hour, et cetera. We can see that (A), (B), (D), and
(E) all follow this pattern, while (C) does not. So this is our correct answer.
Let’s summarize the key points. Reaction rates can be calculated by
measuring reactant loss or product formation, giving us units such as these. To set up a suitable experiment for
measuring rate of reaction, consider the states of your reactants or products. This can help you pick the most
appropriate setup.
