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) Take the square root of the equation2) 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!)

7.4Predicting 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 ofany change in the conditions thataffect 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.