>> host: Wildfires destroy
approximately five million
acres of land in
the United States each year,
consuming everything
in their path,
at a speed of about
one mile every four minutes.
These out-of-control
combustions are one of nature's
fastest chemical reactions.
Of course, fast chemical
reactions are not
always destructive.
If we know how to control them,
they can be very useful.
For example, space travel
is made possible by
harnessing the tremendous
power of the combustion
of hydrogen gas
and solid rocket fuel.
And not all chemical
reactions are fast.
Rusting of iron can take place
over many, many years.
Why are some chemical reactions
fast and some slow?
What factors do these
chemical reactions depend on?
Can we speed up a slow reaction,
or slow down a fast one?
These are just a few of
the questions we'll explore
in "Chemistry Matters", Unit 9.
Kinetics and Gases.
♪♪
>> host: Welcome to
"Chemistry Matters," Unit 9,
Kinetics and Gases,
where we'll investigate the
effects of molecular motion
on the rates of reaction
and properties of gases.
Most of you probably know
that kinetic energy is
the energy of motion,
so it should be no surprise
that kinetics is
the study of motion.
Specifically, in chemistry,
kinetics is the study
of reaction rates.
The rate of a chemical reaction
is simply a measure of
how fast a product is made
or a reactant is consumed.
Rates of reaction
are expressed as the change
in concentration of a reactant
or product over a specific
period of time.
As we learned in Unit 7,
molarity is the most common
unit of concentration.
Units of time depend on
the rate of reaction,
and vary from seconds,
to minutes, to hours.
So just like the rate
of a moving car usually
has units of miles per hour,
the rate of a chemical reaction
typically has units
of molarity per second.
Scientists explain
different rates of reaction
with the Collision Theory.
This theory states that
particles must collide
in order to react.
But not just any type
of collision will result
in a chemical reaction.
In order for a chemical
reaction to occur,
collisions must occur with
enough energy for a reaction.
Additionally, collisions must
occur with the reactants
properly oriented
relative to each other.
We call collisions that result
in a successful reaction
effective collisions.
Here, you see examples
of ineffective
and effective collisions.
In an ineffective collision,
there's not enough energy,
and the reactants are not
properly oriented to each other,
so there's no change
to the reactants.
A successful chemical reaction
is kind of like
a good game of pool.
Imagine you've just
set up the pool balls
in a perfect triangle,
and you're ready to
make the break shot.
You aim for the ball
front and center.
If your cue ball hits
with the correct orientation
and energy,
the 15 billiard balls
will scatter across
the pool table.
Some of them will
drop into holes.
But if the cue ball
hits in the wrong place
or with low energy,
the 15 billiard balls
will barely move.
To further explore
reaction rate,
let's go out to our classroom
and join our students.
>> professor: While we don't
have as much control
over a chemical reaction
as we do in a game of pool,
we can affect
the rate of a reaction
by altering the conditions
under which it occurs.
In order to adjust
the rate of reaction,
we must first understand
the factors that affect rate.
Chemists often use
an energy diagram
to represent
a chemical reaction.
The energy of the reactants
is shown to the left,
and the energy of the products
is shown to the right.
The top of the energy hill
in between the reactants
and the products represents
the activation energy,
which is the minimum amount
of energy needed for reactants
to convert to products.
Activation energy supplies
the energy needed
to break existing bonds
in the reactants.
We'll explore activation energy
further in Unit 10.
For now, think of
activation energy as a hill
the reactants must climb
in order to transform
into products.
If reactants do not collide
with enough kinetic energy,
or activation energy,
they won't react
to form a product.
If, on the other hand,
reactants collide
with adequate energy
to break the existing bonds,
a product will form.
Based on what you've learned
about the Collision Theory
so far, can you think of a way
to speed up the reaction?
>> student: Couldn't you just
give the reaction more energy?
>> professor: That's right.
Particles have to collide
with enough energy to react.
How could we give
a reaction system more energy?
>> student: By heating it up.
>> professor: Good.
Heating the reaction
would give the reactants
more kinetic energy.
More kinetic energy
means faster movement
and more energetic collisions.
Does anyone else
have any other ideas?
Dori.
>> student: Since more
frequent collisions mean
a faster rate of reaction,
could you put the reactants
into a smaller container,
so they're more crowded
and they collide frequently?
>> professor: That's
a creative idea, Dori.
Let's think through
a little bit more.
Let's say I have
a 250-milliliter beaker
that's holding 100 milliliters
of an aqueous solution.
Now, if I were to pour
the 100 milliliters
into a smaller container...
will the components of the
mixture be closer together?
>> student: No, I guess not.
>> professor: Good.
Why not?
>> student: Because you can't
squeeze 100 milliliters
of a solution into
a smaller space.
>> student: But if it's a gas,
you can.
Think about tires.
They're filled with
high air pressure, right?
>> professor: Great example.
>> student: Yeah, I don't think
liquids and solids
can be compressed.
But I'll bet gases react faster
when they're put
into a smaller space.
>> professor: That's
absolutely right, Dori.
So you weren't incorrect
with your suggestion
of putting reactants
into a smaller container.
We just have to clarify that
only gases can be compressed
into a smaller volume.
We'll explore rates of reaction
with gases later.
Right now, we'll investigate
aqueous reactions.
There are ways to make
reactants dissolved in water
collide more frequently.
Can anyone think of
how we might do that,
aside from heating the solution?
I'll give you a hint.
The solution I'm holding
contains only a few milligrams
of table salt, in about
100 milliliters of water.
The particles
dissolved in the water
collide with each other,
but probably not very often.
Is there anything that
we can do to make them
collide more frequently?
>> student: Add more salt.
>> professor: That's right.
If we add more
salt particles...
they're more likely to
collide with each other.
The same way you're more
likely to bump into somebody
in a crowded room.
The frequency of collisions
will increase
as concentration increases,
and so will the
rate of reaction.
So, we've come up with
two methods to increase
the frequency of collisions,
and, therefore,
the rate of a reaction.
Add energy by heating...
and increase the concentration
of the reactants.
Those are the two
most important ways to
increase the rate of reaction.
There are a couple more methods
that I can think you'll discover
if you conduct your own
investigation.
We're going to investigate
the chemical reaction
that the ingredients
in an Alka-Seltzer tablet
undergo when dissolved in water.
This is the reaction that
occurs inside the stomach
when people take Alka-Seltzer
for an upset stomach.
Alka-Seltzer is a medicine
used to neutralize
excessive stomach acid.
Each tablet contains
citric acid
and sodium bicarbonate,
which is simply the chemical
name for baking soda.
When a tablet of Alka-Seltzer
is dissolved in water,
the citric acid
and the baking soda
react with each other
to produce carbon dioxide
and sodium citrate.
Sodium citrate is a weak base
that neutralizes stomach acid.
Monitoring the carbon dioxide
produced is a good way to follow
the rate of this reaction.
If we were to conduct
this reaction
in a closed container,
the carbon dioxide generated
would create pressure.
And you can think
of your stomach as
a partially closed system.
When you take the Alka-Seltzer,
as the carbon dioxide gas
is formed in your stomach,
the pressure builds up,
probably causing you to burp.
We're going to investigate
this reaction
using a film canister
as our closed reaction vessel.
I need everyone to
put on their goggles.
I have water in
this film canister.
When I add an Alka-Seltzer
tablet and then quickly
put the lid on,
pressure will build up.
Until...
>> student: Wow!
>> student: Sweet!
>> student: That scared me.
>> professor: Now, how long
do you think it took for
the canister lid to pop off?
>> student: Just a few seconds.
>> student: It didn't take
very long at all.
>> professor: Well, if we
measure the time it takes
for the lid of the canister
to pop off,
we'll be able to compare
the rate of this reaction
under different conditions.
The faster the rate of reaction,
the faster carbon dioxide
will be generated,
and the sooner
the lid will pop off.
So let's get started
on this lab.
You'll begin by
designing an experiment
to determine which conditions
speed up or slow down
the reaction rate.
>> host: This is a good time
to take a break
and let you design
your own experiment,
along with our students.
Your objective in this
experiment is to determine
the set of conditions
that gives you the highest
rate of reaction.
In other words,
what conditions cause
the lid of the canister
to pop off fastest?
You've already learned that
temperature and concentration
are factors that affect
the rate of reaction,
but there are additional
factors that can also
affect reaction rate.
See if you can determine
those factors.
Since you're going to
design your own experiment,
the first thing
you'll need to do
is to develop a hypothesis
that can be tested
by experimentation.
Once you've generated
your hypothesis,
sketch a model
of what's happening
on a molecular level,
based on that hypothesis.
Remember, a model is
a simplified representation
of a complex system that
helps explain a phenomenon.
The model should demonstrate
the effect that changing
conditions have on
the reactants and products.
Once you've developed
your hypothesis and a model,
you'll design your own
experimental procedure,
have it approved
by your teacher,
and then conduct
your experiment.
Make sure you include
a list of types and quantities
of the materials you'll use
in your experiment.
This will be a step-by-step
procedure, including
safety precautions,
and a data table
that will be used to
collect observations
and measurements.
Be sure to collect data
in a way that ensures
the relationships between
your variables will be
clearly identified.
That's a lot, but you can
find a document containing
these instructions in the
Chemistry Matters Toolkit.
When you're done
planning your experiment,
go to the next video
on the Unit 9 playlist,
where we'll head back
to our classroom to see
how our students did
with their hypotheses, models,
and experimental procedures.