>> host: Welcome back
to "Chemistry Matters,"
and our experiment on smog.
Now that you've completed
your investigation,
let's share our results
and see if we can use the data
to answer the
investigation's question.
Which of these two gases,
nitrogen dioxide
or dinitrogen tetroxide,
is responsible for
the brown color of smog?
Of course, you'll need to
analyze your results
and construct an explanation
that's supported by data.
Let's head back to
our classroom to see
how our students did
with this experiment.
>> professor: All right,
everyone should have
completed their investigation
and have their data organized
and ready to share.
So let's start with you two.
>> student: Well,
we recorded the color
of the gases in the tube at
negative 79 degrees Celsius,
25 degrees Celsius,
and 100 degrees Celsius.
>> student: And we found
that the gas mixture was
a darker shade of brown
at higher temperatures.
But there was always some brown,
even in the dry ice.
So that means there was
more of the brown gas
at higher temperatures.
>> professor: Interesting.
How about you?
How did your experiment
turn out?
>> student: We got
a paint chart like this
for shades of brown,
and compared it to the color
of the gas mixture
at different temperatures.
We observed the color
at 5 different temperatures.
zero degrees Celsius,
25 degrees Celsius,
50 degrees Celsius,
75 degrees Celsius,
and 100 degrees Celsius.
Just like them, we also
found that the gas mixture
was a darker brown
at higher temperatures.
>> professor: Okay, great.
So you both found this trend
where the brown gas was present
at higher temperatures,
and that's definitely
an important part
of our data analysis.
Let's use that trend,
along with the information
that we learned
about equilibrium
and Le Chatelier's Principle
previously, and let's see
if we can identify
which gas is brown.
And this is where
we analyze our data
and explain the trends that
we saw in our conclusions.
And remember, that we want
our conclusions to be
based upon our data
and what we've learned
previously about equilibrium.
So let's look at the reaction
and our data tables
while we discuss
our conclusions.
And remember, you need to
base your arguments
on the data that
you've collected.
So you both found that
the brown gas increases
at higher temperatures.
How can we use that finding,
along with our knowledge
of equilibrium,
to determine which of these
two gases is the brown one?
>> student: Since the reaction
of NO2 to make N2O4
is exothermic,
heat can be thought of
as a product.
>> student: So, when you
add more heat, which is like
adding more product,
the equilibrium makes
more reactant to
reestablish equilibrium.
>> student: And when
the equilibrium shifts
toward the reactant,
we make more NO2.
>> student: Since the gas
mixture turned darker brown
when we heated it,
the reactant, NO2,
must be brown.
>> professor: Excellent
explanation.
So, let's try to
relate temperature to
the equilibrium constant.
Do you think that
the equilibrium constant
for this reaction is gonna be
larger at lower temperatures,
or will it be larger
at high temperatures?
And I'll get you started
on this one.
A good place to begin
would be to write out
our equilibrium expression,
based off of
the balanced equation.
And remember that
an equilibrium constant
is equal to the concentrations
of the products
divided by the concentrations
of the reactants.
>> student: I think I get it.
We just found out that
there's more of the brown gas,
NO2, at higher temperatures.
When you look at
the equilibrium expression,
NO2 is in the denominator.
So, more NO2 means that
the Keq will get smaller
at higher temperatures.
>> professor: That's right.
So, our reaction is exothermic.
So we can think of heat,
or energy added,
as being a product.
And so, when we
increase the temperature,
the equilibrium is
gonna be disturbed.
And Le Chatelier's Principle
predicts that the reaction
is going to shift away
from the disturbance,
and, in this case,
it's gonna make more reactant,
until the equilibrium
can become reestablished.
So, Anna, can you try
and explain what would happen
to the equilibrium
if we were to remove heat,
or, in other words,
cool the reaction?
>> student: Let's see.
When we cool the reaction,
heat's being removed.
And since that's a product
in our reaction,
removing it means
the equilibrium is disturbed.
So the reaction would make
more products and more heat
to replace what was lost.
Is that right?
>> professor: That's
exactly right.
So, let's summarize
our conclusions.
The conversion of brown NO2
to N2O4 was favored
at low temperatures.
And NO2 is a brown gas
that's favored
at high temperatures.
>> student: And that's why we
see smog on hot summer days.
The high temperatures
shift the equilibrium
towards NO2, the brown gas.
>> professor: Excellent!
So you've all done
a wonderful job of
conducting your experiments,
analyzing your data,
and drawing conclusions.
And as a final challenge,
I would like you to try to
represent the effect
of temperature
on this equilibrium
by drawing a visual model.
So your model should show
what happens in this reaction
at the molecular level.
And the model should explain,
with images, why NO2 is
favored at high temperatures,
and why N2O4 is favored
at low temperatures.
And there are no right
or wrong ways to do this.
Draw a model
and use your imagination.
>> host: We'll take a break here
and let you work on the models
our teacher just mentioned.
When you finish
drawing your model,
rejoin us for the last segment
of our equilibrium unit
to compare and discuss
those models.