Hi. It's Mr. Andersen and this is chemistry essentials video 35. It's on the rate of reactions.
Or the speed of reactions. Now some reactions we might think of as incredibly slow. In other
words the rusting of this train car is going to take years and years to occur. To fully
complete that reaction if it ever is completed. But if we wanted to speed that up we could
do it by increasing the surface area of that iron. And we do that in a hand warmer. A hand
warmer is essentially a bunch of iron that has been shredded to increase surface area.
And so when you pull it out of a bag it's going to combine with the oxygen and it's
going to create a certain amount of heat. And that happens really really quickly. And
so again, the rate of reactions is going to be different for the reactants and the products.
But we also have a certain amount of control over that. And so in a chemical reaction the
rate is simply the number of reactants that are converted to products in a given time.
And so let's say the red and the blue represent reactants. And we combine those together to
make products. The more quickly that we can do that, the faster our reaction rate is going
to be. And there are a number of things that are going to affect that. The concentration,
in other words the more reactants we have the more quickly they're going to find each
other and the faster that reaction is going to occur. And also the pressure. In other
words, if we decrease the size of that vessel that they're in, then we're going to have
more of those reactions going on. Also it's going to be the phase that it's in. Clearly
if it's in a gas, in an enclosed container we might have faster reactions than we would
if it's a liquid. And then finally environmental factors. Things such as temperature and surface
area are going to affect the rate of the reaction as well. And so if we're looking at two reactions,
the rusting of metal and the combustion of wood in this case, both of those are going
to have reactants. And I have marked those as A. And products, and I've marked those
in C. And so the rate of reaction is going to be represented in this formula right here.
And it's really simple. It's not that complex. It's either going to be the negative change
in the concentration of A. In other words we're going to have a decrease in the amount
of reactants over a given amount of time. Remember delta is just going to be the change.
Or we could measure it as the amount of products that are being created over time. Because
those are essentially going to be the same thing. The more reactants we have being consumed
over a given period of time, the faster that reaction is going to be. And likewise with
the products. And so how do we measure the reactants? Let's say I have a reaction that
I want to measure the reaction rate, how do I actually know how many reactants are in
there? Well a great example of something you could do is to use Beer's law. And so in this
reaction right here what I'm doing is creating a copper sulfate solution. I'm shining light
through it. And what I can do is I can vary the amount of concentration and you can see
that the amount of light of that is being transmitted is going to decrease as we add
more of the concentration. I could then start collecting some sample data. So let me change
it from 50 to 75 to 100 millimoles. In other words let me increase the amount of that copper
sulfate solution. Generally what you would do is start with a stock solution and then
dilute it over time. And you can see that the amount of transmittance rather is changing
over time. Now if I wanted to graph that, let me graph the data that I've just collected,
now right here I've got the concentration on the x-axis. And then the transmittance.
In other words the amount of light that's allowed through that sample and we'll generally
use something like a colorimeter of spectrophotometer to measure that. And now I've got my standard
curve. And so you can see that as I increase the concentration, that transmittance is dropping
off. And it's kind of approaching a limit. So what could I do is I could take a sample
and I could measure the reactants. I could then stop it at different times. And I could
figure out how much of it is actually being consumed. Once I know that my millimoles I
could figure out the time it's taking. I could figure out the rate of the reaction. Now it's
important that you understand what's going on at the molecular level. And so this nice
little phet simulation will help out. And so in this one the yellow and the other two
are going to represent the reactants and there's products. The reactants and products. In other
words it's converting from reactants to products and back again. Now this is working perfectly
because they're lined up perfectly. In other words every every time they hit there's a
perfect reaction. But let me angle that initial shot. Now the reactant doesn't hit the other
reactant perfectly. And so we can't convert it to those products. And so it's really important
that we're lining them up. And so let me now get a number of those reactions. That was
a little bit boring. So let me add a number of different reactants. We're going to call
those reactants A. Now let me add a number of different reactants like this. And so you
can see by increasing the concentration of those reactants it's more likely for them
to hit each other. But again we're not getting that reaction occurring because we don't have
a high enough energy. So now let me increase the temperature. And you can see that now
we're starting to have reactions occur. And if I were to keep track of the amount of those
reactants converting into products over time then I have a pretty good idea of what's going
on with the rate of reaction. So what things could affect this? Well temperature. As we
increase the temperature, the molecules are going faster so they're more likely to run
into each other. More likely to have that reaction. If there's more of them, a higher
concentration or if we squeeze them together, if we have a higher pressure that's going
to increase the rate of their reaction as well. And then finally surface area is incredibly
important. All of these are broken down to the level of the atom. But let's say we just
had a big tablet filled with A. And it hadn't been broken down, just like that big train
car at the beginning, it's going to take a long time for us to breakdown. And the reactions
could only occur on the surface. And that would be a slow rate of reaction. And so did
you learn to interpret the results of an experiment regarding temperature. Could you start by
figuring out the amount of reactants and then measuring it over time? If you could, you
know how to calculate the rate reaction and I hope that was helpful.