Video Transcript
In this video, we will learn to
describe how fast or how slowly reactions happen and look at things like the
temperature and how they affect the rate of reactions.
All chemical reactions start with
reactants and finish with products. All chemical reactions involve at
least some changes in chemical bonds and the positions of atoms, ions, electrons,
and nuclei. The modern world depends on a huge
number of chemical reactions actually happening. But what is equally important is
how fast or slow these reactions happen.
And sometimes, even if a reaction
can happen, we don’t necessarily want it to happen. For instance, it’s better if
rusting is very slow because we want to hold on to bits of ion because its strength
allowed us to do very useful things. On the other hand, our bodies use
enzymes to break down toxins incredibly quickly. If these toxins aren’t broken down
fast enough, we can get very ill or even die. But thankfully, chemistry is in
some ways very reliable and dependable.
When reactants turn into products,
we can see lots of things changing, and we can measure many more. But some of these are more useful
than others. For instance, if we wanted to
measure how quickly you could eat a portion of french fries, we could do it in a few
ways. Well, the easiest way would be
simply to time how long it takes to eat the entire portion. Or we could express it by counting
the number of fries we eat in the same time. And then we could simplify that
number to see how long it takes to eat one fry on average. And another way we could do it
would be to look at the mass of the portion and see how much mass we consume per
second.
When we calculate rates like this
and simplify them, what we’re doing is coming up with an average rate, which is
equal to the change in mass or amount commonly divided by the time taken. We can use this average rate to
compare one meal to another. And here, we can plot how amount
changes over time. We can do exactly the same with the
mass, although they don’t necessarily match up because one fry doesn’t necessarily
have the same mass as another. But either way, changes in amount
and mass seem like a good way to assess a rate. Instead, we could try measuring how
smell declined with time or even take photos to the plate and see how much yellow
there was. We could even try to assess
remaining hunger.
Some of these metrics are very
helpful and will allow us to quantitatively compare one meal to another. But others are quite challenging
and present problems that would be difficult or even impossible to solve. It would help if we didn’t have to
rely on something as personal as hunger or as difficult to measure as smell. This is why we’ll most often see
reaction rates expressed as changes in mass over time or changes in amount over
time. Some chemical reactions can be
incredibly slow, like the conversion of organic matter to coal and oil. Or they can be fast enough for us
to observe at a reasonable rate, like a candle burning. Or they can be so fast; they’re an
explosion, like the detonation of a hydrogen balloon. But what do we really mean when we
talk about the rate of a specific reaction?
Let’s imagine we’re burning some
coal. Under ideal circumstances, the
reaction of coal and oxygen in the air produces carbon dioxide. We can assess the rate of this
reaction by monitoring changes in mass or amount. And in this case, we’re seeing a
reduction in the mass of the coal of three grams per second. If this were the case, then we’ll
be consuming eight grams of oxygen per second as well, although I won’t go into the
calculation for this in this video. Instead, we’re looking at how the
numbers relate to one another.
If the reaction was working
perfectly and we could measure precisely, we’d also measure we were producing 11
grams of carbon dioxide per second. The law of conservation of mass
reminds us that the mass of a closed system cannot change if we’re dealing only with
chemical reactions. So it makes sense that the rate
that the mass of reactants goes down is going to be equal to the rate that the mass
of the products goes up. Now, what about amount in
moles? Here, for simplicity’s sake, I’m
going to assume that coal is pure carbon, and I’m going to use the rounded molar
masses for the elements to work out the amounts in moles.
Three grams of carbon is equivalent
to 0.25 moles of carbon. So if we’re consuming three grams
of carbon per second, we’re consuming 0.25 moles of carbon per second. Eight grams of O2 molecules is also
equivalent to 0.25 moles of O2 molecules. So consuming eight grams per second
of O2 is equivalent to consuming 0.25 moles per second of O2. And we can do the same for our
product carbon dioxide and demonstrate that producing 11 grams of carbon dioxide per
second is equivalent to producing 0.25 moles of carbon dioxide per second. Using amount rather than mass gives
a certain advantages because we can relate the rate of production or consumption of
a component to its stoichiometric coefficient and molar ratios to other
substances.
We could express the ratio or rates
like this, paying attention to whether the rate is negative, in which case we’re
consuming, or positive, in which case we’re producing. But the convention is to just have
the stoichiometric coefficients in a ratio. And remember that the rates for
reactants are going to be negative and the rates for products are going to be
positive. In this situation, it seems more
helpful to talk about the rates in moles per second. But in industry, talking about mass
per time is often more helpful. We could also do a similar
reaction, where instead of producing carbon dioxide, we burn coal inefficiently to
produce carbon monoxide. If this happens perfectly to plan,
we’ll need twice as much carbon as before if we’re using the same amount of
oxygen.
Just like with regular reactions
that go to completion, if we’ve got a component we’re only providing at a certain
rate, we can consider it the limiting reactant. So here, we’re only consuming eight
grams of oxygen per second. So we’ll be using six grams of
carbon per second. And therefore, we must be producing
14 grams of carbon monoxide every second. And if we convert the mass rates to
mole rates, this is what we get. If you look closely, you should be
able to see a pattern. We can see that the rate we consume
a reactant or produce a product is in proportion to its stoichiometry. This makes sense since if we’re
using twice as much carbon per reaction, we’ll consume the carbon twice as fast.
But all this is based on what we
can measure, what do we actually understand about what’s going on and how can we use
that knowledge to our advantage. The first question we need to
answer about reactions and reaction rate is this: why do reactions happen in the
first place? Collision theory suggests that
reactions will only happen when particles collide with each other. If particles never touch, they
can’t possibly react. But not every collision is going to
lead to a successful reaction. In fact, the air around us is
filled with particles that are colliding and not reacting. The other necessary component is
that there’s enough energy in the collision in order to break bonds and rearrange
particles.
But even then, if the collision has
the right energy, it may not be successful because the particles are not arranged
properly. If they aren’t in the right
orientation, the energy won’t be transferred into breaking the right bonds. This means that only when particles
collide with enough energy in the right orientation will a reaction occur. We call the minimum energy needed
for a reaction to occur the activation energy. Although it’s not measured in this
way, we can think of the activation energy as the energy required to break just the
right bonds that allow the new bonds in the products to form. Armed with this information, let’s
see how we can put it to use by changing reaction rates.
Collision theory suggests there’s a
few things that could be different to make a reaction faster. If we can find a way of making
collisions happen more frequently, the reaction should be faster. And if we can find a way of
increasing the energy of those collisions, the proportion of successful collisions
should increase. So the rate of the reaction should
go up. And lastly, if we can find some way
to do it, if we can lower the activation energy, then we increase the proportion of
collisions that will be successful. In this video, I’m only going to
look at the effect of temperature on reaction rate, but I’ll tell you a few others
as well.
We can take the typical example of
a gas. If we heat up the gas, the
particles will move more quickly. The faster particles are moving,
the more often they will collide. And since they’re moving faster,
their collisions will be more energetic. If we do the opposite and cool the
system down, the particles will move more slowly. This will result in less frequent
lower-energy collisions. Although we generally depict this
scenario with gases, heating up solids and liquids will also increase the rate at
which they react. So broadly speaking, increasing the
temperature will increase the rate of reactions by providing for more frequent
collisions with higher energy.
And while we won’t be going into
the mechanics in this video, here are a few other factors that you could change to
increase a reaction rate. For reactions involving at least
one gas, increasing the gas pressure will increase the frequency of collisions and
increase the reaction rate. And we see the same effect for
solutes in solution. If we increase their concentration,
the rate of reaction will increase. The more of them there are, the
more frequent collisions will be. And for reactions involving solids,
it’s better that we have lots of small pieces with a high surface area than one big
piece with a small surface area. Because the higher the surface
area, the more frequently collisions will happen on the surface and the faster the
rate of the reaction will be.
There are some circumstances where
this is useful for liquids as well. And the last thing to try would be
to add a catalyst. These are specific chemicals for
specific reactions that, if they’re added, will help provide an alternate reaction
route with a lower activation energy. Catalysts do react, but by the time
the reaction is over, the catalyst is regenerated. So we see the same reactants and
the same products overall. Before we move on to some practice,
let’s have a quick look at the units for reaction rate you might come across and how
to convert between them.
For mass, the most common units are
grams and kilograms. 1000 grams is equivalent to one
kilogram. You can remember this because the k
in kilogram indicates ten to the three or 1000 times. So to convert from a value in grams
to the equivalent value in kilograms, we multiply by one kilogram per 1000
grams. And to do the reverse, we simply
multiply by 1000 grams per kilogram. And we can use the exact same
techniques when we’re converting between rates in grams per second and rates in
kilograms per second. The slightly more tricky thing to
convert is the time between units of seconds, minutes, and hours because they will
be in the denominator.
We know that 60 seconds is
equivalent to one minute, 60 minutes is equivalent to one hour, and therefore 3600
seconds is also equivalent to one hour. So let’s imagine we have a rate
expressed in grams per second. To get the equivalent in grams per
minute, we just multiply by 60 seconds per one minute. We need the unit second on the
bottom of one fraction and at the top of the other so that they cancel. And what we get is a value that’s
60 times bigger with units of grams per minute. This makes sense since if we’re
making something at so many grams per second, we’re going to produce much more of it
per minute.
So to convert between per second
and per minute, we multiply by 60 seconds per one minute or one minute per 60
seconds. And to convert between per minute
and per hour, we multiply by 60 minutes per hour or one hour per 60 minutes. And if we want to make the jump
between per hour and per second all in one go, we multiply by one hour per 3600
seconds or 3600 seconds per hour.
Let’s take, for example, converting
3.6 kilograms per hour into the equivalent grams per second. As the first step, we can convert
the kilograms per hour into grams per hour by multiplying by 1000 grams per
kilogram. This gets us 3600 grams per
hour. And then we can convert to grams
per second by multiplying by one hour per 3600 seconds, giving us one gram per
second. And now, it’s time for some
practice.
When excess hydrochloric acid
reacts with calcium carbonate, carbon dioxide is produced. If five grams of calcium carbonate
is consumed in three minutes and 20 seconds, what is the average rate of
reaction?
Hydrochloric acid, HCl, is
obviously an acid. And we can see that calcium
carbonate, CaCO3, is a carbonate. What might spring to mind is the
classic reaction between an acid and a carbonate producing a salt, carbon dioxide,
and water. And indeed, the question tells us
that carbon dioxide is one of the products. It’s not necessary to answer the
question, but I’m going to draw out the chemical equation. Here, we have HCl reacting with
solid calcium carbonate. I’ve assumed it’s solid because
calcium carbonate isn’t soluble in water. Our products are the soluble salt,
calcium chloride; the gas, CO2; and H2O in its liquid form. And we’ll need two HCl in order to
balance it.
The question also tells us that
we’re consuming five grams of calcium carbonate in three minutes and 20 seconds. Our job is to use this information
to calculate the average rate of reaction. When we’re dealing with masses, the
average rate of our reaction is equal to the change in mass divided by the time
taken. But we’ve been given the time taken
in a mixture of units, minutes and seconds. So we should convert this all to
minutes or all to seconds. I’m going to do it all to
seconds.
There are 60 seconds in one minute,
so our time taken is equal to three minutes multiplied by 60 seconds per minute add
the 20 seconds which is equal to 180 seconds plus 20 seconds, which is 200
seconds. So our average rate with respect to
the calcium carbonate is five grams divided by 200 seconds, which works out as 0.025
grams of calcium carbonate per second.
Now, let’s have a look at the key
points. A reaction rate is simply the rate
reactants convert into products. Reaction rate is commonly measured
by mass per time — for instance, grams per second — or amount per time — for
instance, moles per second. And one of the simplest ways that
we can assess the rate of a reaction is to measure the average rate, which is the
change in mass or amount of a reactant or product divided by the time taken. And reaction rates are generally
affected by changes in temperature, pressure, concentration, surface area, and the
presence of a catalyst.
