Download The Equilibrium Constant

The Equilibrium Constant
7.3
Opposing Rates and the Law of
Chemical Equilibrium
• The Law of Chemical Equilibrium:
At equilibrium, there is a
constant ratio between the
concentrations of the products
and reactants in any change.
Example
Consider the decomposition reaction of: dinitrogen
tetroxide (N2O4(g)) into nitrogen dioxide (NO2(g)).
- Initial concentration of N2O4(g) is 0.0200 mol/L.
- Initial concentration of No2(g) is ______.
- What do you think will happen?
Example
- Initial rate of the forward reaction, kf , is
_________________.
- Initial rate of reverse reaction, kr , is _____.
- As the reaction proceeds, kf _____________
because _____________________________.
- At the same time, kr _____________ because
_____________________.
What do you think?
Can you come up with a SIMPLE equation
relating kf to kr at equilibrium?
The Equilibrium Constant, Keq
The equilibrium constant, Keq, is the ratio of the
forward rate constant and reverse rate
constant:
Question: if Kf is 3.4 and Kr is 1.7, what is Keq?
- However, usually we don’t know kf or kr.
- We can use the equilibrium equation and the
molar concentrations of the reactants and
products to find Keq.
Concentrations of products are in the NUMERATOR.
Concentrations of the reactants are in the DENOMINATOR.
Sample Problem
Work together to solve the following
problem:
Work on the next 5 problems:
EQUILIBRIUM CONCENTRATIONS
Consider the following reaction:
Suppose the initial concentration of Fe3+ is 0.0064 mol/L
and the initial concentration of SCN- is 0.0010 mol/L.
By measuring the intensity of the colour, we determine
that the concentration of Fe(SCN)2+ is 4.5 x 10-4 mol/L.
WHAT CAN WE INFER ABOUT THE EQUILIBRIUM
CONCENTRATIONS OF THE REACTANTS?
ICE TABLES
Suppose the initial concentration of Fe3+ is 0.0064 mol/L and the
initial concentration of SCN- is 0.0010 mol/L.
By measuring the intensity of the colour, we determine that the
concentration of Fe(SCN)2+ is 4.5 x 10-4 mol/L.
LET’S SET UP AN ICE TABLE FOR THIS
REACTION!!!
TIPS FOR SOLVING EQUILIBRIUM
CONSTANT QUESTIONS
1) If given the initial concentrations of all parts and the change
in concentration of 1+ parts (reactants or products), you can
use stoichiometry to find the change in the other parts.
2) If not given, let x be the change in concentration of the
reactant with the smallest coefficient (to avoid fractions)
3) You will usually need to do one of these steps in order to
solve:
1)
2)
Take the square root of the equation
Use the quadratic formula.
4) Don’t forget the +/- !
5) Use logic to determine which value for x to use!
Qualitatively Interpreting the
Equilibrium Constant
The Kc for
Is
What can we say about a large Kc? Small? Value of
0? Value of 1?
When Kc > 1, __________________ are
favoured, and the equilibrium lies far to the
___________. ________________________
When Kc = 1, ____________________________
When Kc < 1, ___________________ are
favoured, and the equilibrium lies far to the
___________. ________________________.
Using the Approximation Method
Sometimes the change in concentrations is so
small that it can be ignored.
Eg:
If Kc is super small, the term “[initial] – x” is just
[initial].
Ignoring ‘x’
- Values of Kc are not measured with accuracy
better than 5%.
- Divide the initial concentration by the value of
Kc. If:
More than 500, can be ignored.
 between100-500, may be ignored (will not see in
this chapter)
Less than 100, can not be ignored (carry out full
calculation!)
7.4
Predicting the Direction of a Reaction
• Any problems with the homework?
The Reaction Quotient
• Reaction quotient, Qc:
– expression that is identical to the equilibrium
constant expression
– Concentrations not necessarily at equilibrium.
• If Qc = Kc, ________________________.
• If Qc > Kc, ________________________.
– _______________ must be large.
– _______________ must be larger than concentrations at
equilibrium.
– System will attain equilibrium by moving to the _____________.
• Qc < Kc, _____________________________.
– _________________ must be large.
– _________________ must be larger than concentrations at
equilibrium.
– System will attain equilibrium by moving to the _____________.
Work on PPs 26 – 28 on Page 356.
Le Chatelier’s Principle
• Predicts the way that an equilibrium system
responds to change.
• A dynamic equilibrium tends to
respond as to relieve the effect of
any change in the conditions that
affect the equilibrium.
The Common Ion Effect
• A fancy way of saying “the effect of adding
substances already present”
• A “concentration” effect.
Ex//
Effect of adding phosphorus pentachloride gas?
___________________________________
Effect of removing chlorine gas?
_______________________________
The Effect of Temperature
• Reaction will shift to relieve the temperature changes on
a system.
Recall:
∆H > 0 : _____________________________________________________
∆H < 0 : _____________________________________________________
Example:
Left to right: energy is ______________ and converted to chemical
potential energy.
Adding heat: ________________________. Kc will ___________.
Removing heat: _____________________. Kc will ____________.
The Effect of Temperature(2)
Effect of temperature on the position of equilibrium can be
summarized as follows:
If ∆H > 0 (Endothermic):
Increase in temperature shifts equilibrium to the _________________:
More __________. Kc ____________________.
Decrease in temperature shifts equilibrium to the ________________:
More ___________. Kc ______________________.
If ∆H < 0 (Exothermic):
Increase in temperature shifts the equilibrium to the ______________:
More____________. Kc ____________________.
Decrease in temperature shifts equilibrium to the ________________:
More ____________. Kc ____________________.
The Effect of Volume/Pressure
• Recall that
:
As the volume of gas __________, the pressure ___________.
• Ex//
If the volume of the system were to decrease, how would the
system relieve this pressure? (Hint: look at the number of
moles!)
The Effect of Volume/Pressure(2)
Number of molecules on the left side: __________.
Number of molecules on the right side: __________.
The pressure of the system would decrease if it shifted
to the ________________.
Why?
Adding Inert Gases
Ex//
• If oxygen were to be injected into a reaction
vessel containing this reaction, what would
happen? ___________________________
• If helium gas were to be added to the system,
what would happen? ___________________
– This is the same as increasing _______________.
Adding a Catalyst
• A catalyst speeds up the RATE of a reaction by
lowering the ACTIVATION ENERGY.
• Activation energy is lowered the same amount
for the forward and reverse reactions.
• Therefore, a catalyst does not affect the
position of equilibrium, Kc.
PPs 29-33, pg 366.