- [Instructor] The equilibrium constant is symbolized
by the letter K, and equilibrium constant tells us
about the relative concentrations of reactants
and products at equilibrium.
Let's say we have a hypothetical reaction
where reactants A and B turn into products C and D.
And in the balanced equation,
the lowercase letters are the coefficients.
So we have a lowercase a, a lowercase b,
lowercase c and lowercase d as coefficients
in our balanced equation.
If we were to write an equilibrium constant expression
for this hypothetical reaction,
we'd start by writing the equilibrium constant K
and then we have a subscript c here
because we're dealing with concentrations
in our equilibrium constant expression.
And the equilibrium constant Kc is equal to,
and in the numerator, we have the concentrations
of our two products multiplied together.
And the concentration of each product
is raised to the power of the coefficient.
In the denominator,
we have the concentrations of the two reactants
multiplied by each other and raised to the power,
each concentration is raised to the power
of the coefficient in the balanced equation.
It's important to emphasize that the concentrations
that we're plugging
into our equilibrium constant expression
are equilibrium concentrations.
And when we plug in our equilibrium concentrations
into our equilibrium constant expression,
we get a value for the equilibrium constant K.
And K is constant for a particular reaction
at a certain temperature.
Let's write an equilibrium constant expression
for the following reaction,
which shows the synthesis of ammonia
from nitrogen and hydrogen,
and everything is in the gaseous state.
We start by writing the equilibrium constant Kc,
c because we're dealing with concentrations,
and we start with our product, which is ammonia.
So we write the concentration of ammonia
and we raise the concentration of ammonia
to the power of the coefficient
in the balanced equation, which is a two.
So this is the concentration of ammonia to the second power.
Then, in the denominator, we think about our reactants.
So we have nitrogen.
So we write the concentration of nitrogen.
And since the coefficient is a one in the balanced equation,
that'd be the concentration of nitrogen to the first power
multiplied by the concentration of our other reactants,
which is hydrogens.
We write in here H2.
And because there's a coefficient of three
in the balanced equation,
we raise the concentration of hydrogen to the third power.
For gases, it's often more convenient
to measure partial pressures
instead of measuring concentrations.
So let's say that A, B, C and D are all gases.
We could write an equilibrium constant expression
using partial pressures instead of concentrations.
And if we did that, instead of writing Kc,
we would write Kp where p stands for pressure.
And Kc and Kp usually have different values from each other.
So if we go back to our previous reaction
where everything was in the gaseous state,
we could write a Kp expression.
So we would write Kp is equal to,
we think about products over reactants.
So this would be the partial pressure of our product,
ammonia, raised to the second power
divided by the partial pressure of nitrogen
raised to the first power times the partial pressure
of hydrogen raised to the third power.
For the synthesis of ammonia,
everything was in the gaseous state.
And when all substances, reactants and products
are in the same phase,
we call this a homogeneous equilibrium.
When the substances are in different phases,
we call it a heterogeneous equilibrium.
For example, in the decomposition of calcium carbonate
to turn into calcium oxide and carbon dioxide,
calcium carbonate is a solid and calcium oxide is a solid,
but carbon dioxide is a gas.
So we have substances in different phases.
When we write an equilibrium constant expression
for a heterogeneous equilibrium,
we leave pure solids and pure liquids
out of the equilibrium constant expression.
So if we write an equilibrium constant expression
for the decomposition of calcium carbonate,
let's write a Kc expression first here.
So we write Kc is equal to,
and we think about products over reactants.
For products, we have carbon dioxide in the gaseous state.
So it's okay to include that
in our equilibrium constant expression.
So we write the concentration of CO2.
And since the coefficient is a one in the balanced equation,
this would be the concentration of CO2
raised to the first power.
Our other products is a solid.
So we're gonna leave that
out of our equilibrium constant expression.
And for our reactant calcium carbonate, that's also a solid,
so that's also left out of our expression.
If we were to write a Kp expression here,
we would include the partial pressure of our gas,
which is carbon dioxide.
So this would be the partial pressure of carbon dioxide
to the first power.
And once again, we would leave the two solids
out of our equilibrium constant expression.
The reason why we leave pure solids
and pure liquids out of equilibrium constant expressions
for heterogeneous equilibria is because the concentration
of a pure solid or a pure liquid remains constant over time.
So it doesn't help us to include it
in our equilibrium expression.
Finally, let's talk about the reaction quotient,
which is symbolized by the letter Q.
A Q expression has the same form
as an equilibrium constant expression.
And Q tells us the relative concentrations of reactants
and products at any moment in time.
And just like we could write a Kc or a Kp expression,
we could write a Qc or a Qp expression.
Let's go back to our reaction for the synthesis of ammonia
from nitrogen gas and hydrogen gas.
Notice how the Qc expression has the same form
as the Kc expression.
The difference is, for the Kc expression,
all of our concentrations are equilibrium concentrations.
So I could put an eq here
for the concentrations of ammonia, nitrogen, and hydrogen.
So for the Kc expression,
it's only equilibrium concentrations,
but for the Qc expression,
it's the concentrations at any moment in time.
So that moment in time might be at equilibrium
or it might not be at equilibrium.
If Qc is equal to KC, the reaction is at equilibrium,
but if Qc is greater than Kc, or if Qc is less than Kc,
the reaction is not at equilibrium.