Changes in Equilibrium systems

Le Châtelier’s Principle& The Haber ProcessLearning Goals:

I will understand Le Chatelier’s Principle in terms of what shift will occur when ‘stress’ is added to a chemical system

I will understand the Haber process, it’s economical importance and how Le Chatelier’s Principle plays a key role in understanding optimal conditions for ammonia production

Le Chatelier’s Principle•When a system at equilibrium is subjected

to a stress, the equilibrium will tend to shift in the direction to relieve the stress.

•What kind of “stress” could a chemical reaction be subjected to??▫Change in concentration▫Change in temperature▫Change in pressure

Change in pressure affects which state of matter?

▫Adding a catalyst? NO! Not a stress.

What are the effects of each stress?

)(2)(3)( 322 gNHgHgN

•Change in concentration – system shifts to get back to the same ratio of reactants and products.

▫Add more N2?

▫Add NH3?

▫Take out NH3?

HEATgSOgOgSO )(2)()(2 322

•Change in temperature – system shifts so that heat is equalized.

▫Raise the temperature

▫Lower the temperature

▫NOTE: Lowering the temp. may shift the eq. in the direction of the products, but if the temp. is lowered TOO much, what happens to the rxn. rate?

•Change in Pressure – System shifts to equalize the number of moles of gas.

▫Raise the pressure

▫Lower the pressure

▫NOTE: This is a famous reaction – the Haber process. It is extremely important in the agricultural industry to make fertilizers… you’ll learn about this next

)(2)()(3 322 gNHgNgH

•Change amount of Catalyst?▫NO! This does not cause a shift in

equilibrium. It speeds the reaction in BOTH directions, so that it reaches equilibrium faster.

Example # 1N2 + 3 H2 2 NH3

a) Which way will the equilibrium shift if more H2 is added to this reaction at equilibrium:

b) Which way will the equilibrium shift if some NH3 is removed from the reaction when it is at equilibrium?

Example #22 SO2 + O2 2 SO3 + heat

a) Which way will the equilibrium shift if the system temperature goes up (heat is added):

b) Which way will the equilibrium shift if the temperature is decreased?

Example # 3The container holding the following reaction (already at equilibrium) has its volume suddenly increased. Which way will the equilibrium shift to compensate?

H2 + Cl2 2 HCl

Example #4The system below is already at equilibrium when a catalyst is added to the system. What happens to the position of the equilibrium? Does it shift right, left, or no change?

PCl3 + Cl2 PCl5

Homework

•Pg 446 Q 1, 3

The Reaction Quotient, “Q”

What is “Q”?•Q is a value we can use to determine if a

reaction is at equilibrium.

•If a reaction is NOT at equilibrium, we can predict which direction (LEFT or RIGHT) the reaction will shift in order to reach equilibrium by comparing the value of “Q” to the value of “Keq”

Comparing Q and Keq

•If Q<Keq: system must shift right toward products to reach equilibrium because product-to-reactant ratio is too low

•If Q=Keq: system is at equilibrium

•If Q>Keq: system must shift left toward reactants to reach equilibrium because product-to-reactant ratio is too high

How Do We Get the Value of “Q”?

•It is calculated exactly like Keq, EXCEPT the concentrations we plug in are NOT equilibrium concentrations.▫They are just the concentrations of

reactants and products at whatever time Q is calculated.

Example #1: at 472oC, Keq = .105 N2(g) + 3H2(g) 2NH3(g)

•2 minutes after this reaction starts, you want to know if it’s at equilibrium so you measure the concentrations and find:

a)What is the value of Q?b)In order to reach equilibrium, will this

reaction shift left or right?

[N2] = .0020M

[H2] = .10M

[NH3] = .15M

These are notnecessarilyEquilibrium

concentrations

Example #2: Keq = 170 for CoCl2(g) Co(g) + Cl2(g)

•If [Co] = [Cl2] = .15M and [CoCl2] = 1.1x10-3, is the reaction at equilibrium? If not, which way will it shift to reach equilibrium?

Homework

•Pg 459 Q1, 2