Le Chatelier’s Principle and Dynamic Equilibrium

Objective to understand how chemists use stresses to control

the amount of product formed

Equilibrium

Chemical Equilibrium chemical equilibrium a dynamic state

where the concentrations of all reactants and products remain constant.

Dynamic Marked by continuous activity or

change.

What is Dynamic equilibrium? When the rate of formation of the

reactants = rate of formation of the products.

Dynamic Equilibrium When reactions occur at such

rates that the composition of the mixture does not change with time. Reactions do in fact occur, sometimes vigorously, but to such an extent that changes in composition cannot be observed.

Jar with lid

There are more molecules evaporating than condensing in the open jar. It is not at dynamic equilibrium.

Instant Question #1

In which jar do the liquid molecules stop turning into gas molecules?

Both just (a)Just (b)Neither

Instant Question #2

In which jar do the are the gas molecules turning into liquid molecules?

Both just (a)Just (b)Neither

Shifting equilibrium

What would happen to the [H2O(g)] (in the covered jar) if the temperature were increased? You would get more H2O(g)

Shifting equilibrium

What would happen to the amount of H2O( (in the covered jar) if the pressure were increased by making the volume smaller? You would get more H2O(L)

Adding More H2O(g)

Adding more H2O(g) would (at first) make more gas molecules but they would soon turn into liquid molecules

Question #3What would happen to the

pressure if the temperature were increased?

A. It would go upB. It would go downC. It would stay the same

StressWhat changes can cause the equilibrium

to shift? (1) Changing the temperature.(2) Changing the concentration. (3)Changing the pressure (volume)  Collectively, what are these factors

referred to as Stresses.

Le Chatelier’s Principle When a stress is imposed on a system

at equilibrium, the position of the equilibrium shifts in a direction that tends to reduce the effect of that stress.

Question #4Which would cause more “C” to start

forming according to the equation below?A + B C + D + heat

(a) adding more D(b) removing A or B(c) removing D(d) adding heat

Using the Collision Theory, explain why adding MnO2 to H2O2 makes the rate of the reaction increase.

H2O2 molecules must to collide with each other with enough force to break bonds.

In the presence of MnO2 the molecules absorb onto the catalyst’s surface making the H2O2 bonds weaker

Questions #5 and 65. True or False: A catalyst speeds up a

chemical reaction by increasing the concentration of the reactants.

6. Determine which is the catalyst1st A + B AB, then AB + C AC + BWhich is the catalyst “A” “B” or “C”

The Water GameMake a graph like so ….

0 1 2 3 4 5 6 7 8 9 10

100

80

60

40

20

0

VOL

transfers

The Water GameGet two 100 ml graduated cylinders. Put

100 ml of water into one of them. Label it “A” and the other (empty one) label it “B”

Also get two empty 250 ml beakers. Label one “A” and the other “B”

A Micro pipette and a paper towel.

The Water GameGet three colored pencils or crayons

(one red and one blue and one green) from the supply.

Using Red to represent the volume in graduated cylinder “A”, put a point on your graph.

Do the same using blue to represent the volume in “B”

The Water GameAfter zero transfers, your graph should

look like this.

0 1 2 3 4 5 6 7 8 9 10

100

80

60

40

20

0

The Water GameNow take ½ of the water in grad “A” and

put it into beaker “A”Then take ¼ of the water in grad “B” and

put it into beaker “B”. Yes, ¼ of 0.0 = 0Make sure you have the volumes correct.Now put the water in beaker “A” in Grad

“B” and the water in beaker “B” in grad “A”

The Water GameRecord the volume in the grads.After adding the water to the grads, using

red to represent the volume in graduated cylinder “A”, put a point on your graph.

Do the same using blue to represent the volume in “B”

Repeat six more times after each transfer, make a mark on your graph showing the volumes of “A” and “B”

The Water game If you have not yet figured it out, this

represents a reversible reaction.Define a reversible reaction where

reactant “A” turns into reactant “B” Write an equation representing this

reaction Identify the forward and reverse reactions.

The Water GameCalculate the volume of “B”/”A” at each

point along your graph. Identify with a vertical green line, the point

on your graph where the forward and reverse reaction are equal to each other.

What is “B”/”A” at this point? This ratio is called the equilibrium constant.

For chemical reactions it stays constant unless the temperature is changed.

The Water GameNow add 30.0 ml of water to Grad “A” this

represents a “stress”. Calculate the “B”/”A” at this point and graph it onto your graph.

Repeat the water game 3 more times then recalculate the “B”/”A”

Without actually doing the lab, show on your graph what would happen if 30 ml of water was now added to “B”

The Graph

Final Graph

CalculationsSuppose the “B”/”A” = 50, what would the

values of “A” and “B” equal?The equation would be: B/(100 –B) =50

Graphs

So the size of the equilibrium constant tells us how far the reaction goes to completion

The Equilibrium ExpressionWrite the equilibrium expression for the

equations below.H2 + Cl2 2HClH2 + O2 2H2O

Calculate the Constant If A <=> B [A] at equilibrium = .005 and [B] at equilibrium = .02 Calculate the equilibrium constant