Le Chatelier’s Principle

04252012 G U E R R E R O & A Q U I N O EG2

To predict the effects of changing the equilibrium

system’s conditions.

To interpret the results based on the Le Chatelier’s

principle.

the o b j e c t i v e s

Chemical Equilibrium is the continuous formation of products, and formation of reactants from the

products. In a chemical equilibrium, rates of the forward and backward reactions are equal. The

concentrations of the reactants and products remain constant

.This dynamic equal state can be interpreted as:

A + B ↔ C

the c o n c e p t

“If a dynamic equilibrium is disturbed by changing the conditions, the position of

equilibrium moves to counteract the change and re-establish equilibrium.”

The Principle of Henry Louis le Chatelier(French chemist, 1850-1936)

Le Chatelier’s Principle

Le Chatelier’s principle lets us determine as to which direction a reaction is going towards.

Knowledge in chemical equilibrium is used in

manufacturing technologies to ensure efficiency of products

s i g n i f i c a n c e

d i s t u r b i n g a chemical equilibrium1. Change in concentration

Addition of reactants will produce more products (forward), addition of products will produce more reactants (backward)

2. Change in temperature

Increase in temperature will push forward a endothermic reaction and push backwards if it is exothermic

3. Change in pressure or volume

Increase in pressure has the same effect as a decrease in volume in gases and results to a push forward to a side with lesser gas molecules

Equilibrium Constant

Keq is the equilibrium constant. It is the ratio of the concentration of products and reactants raised to their coefficients in a reaction at equilibrium.

For hypothetical reaction A + B C + D

Keq = [moles of C]c [moles of D]d

[Moles of A]a [moles of B]b

graphing rxn’s at equilibrium

M E T H O D O L O G YIn a test tube: 1 mL 2.0M Cu(NO3)2

+ 1.0 mL 2.0M NH4OH +5 mL distilled water. 

Spot plate depressions (5 drops of previous solution) + 5 drops of 0.1M of 

A: Cu(NO3)2B: NH4OHC: K4FECN6D: HClE: NaOHF: NaNO3

D

ECA

S B

Compare with depression S (H2O + Cu(NO3)2)

as the reference IG

F H

In a small test tube: 1mL previous solution obtained + 0.5mL water. Place in cold bath. Transfer 10 drops of this in

position G. Compare with S.

On position H: 5 drops 0.1M K2CrO4

+ dropwise 6M HCl (stir every addition)

until color changes. Observe.

On position I: 5 drops 0.1M K2CrO4 ]+ 0.1 Pb(NO3)2 until

ppt is complete + HCl dropwise

until a change is observed

Add dropwise 3M HCl until another color change.

Observe.

Position observations direction of reaction

S Cloudy blueReference (in equilibrium)

A Cloudy blue darker blue Forward

B Cloudy blue darker blue Forward

C Cloudy blue yellowish brown ppt

Backward

D Cloudy blue light blue (lightest)

Backward

E Cloudy blue light blue Backward

F Same as S (no change) also cloudy blue

No Shift

Pic 1: S – E, Pic 2: F- IG Cloudy blue dark blue Forward

the results

With the solution obtained in the first step,

1 mL 2.0M Cu(NO3)2 + 1.0 mL 2.0M NH4OH +5 mL distilled water. 

There is equilibrium. This reaction can be put as:

Cu2+ (aq) + 4NH3 (aq) ↔ Cu(NH3)42+

(aq)

Aqueous Cupric ion is sky blue, ammonia is colorless, and the Copper (II) Ammine

Complex product is cloudy blue in color

d i s c u s s i o n

In position A, there is an increase in reactant concentration, by adding Cu(NO3)2

thus the reaction shifts to the right and favors formation of products. There are more effective collisions because Cu(NO3)2 dissociates into

Cu2+ ions which is a reactant. Thus, a forward shift.

In position B, the same concept is applied. There is an increase in reactant concentration, by adding NH4OH thus the reaction shifts to the

right and favors formation of products. There are more effective collisions because NH4OH increases the amount of NH3 reactants that

will push the reaction forward.

That is why depressions A and B have a more darker solution color compared to S.

d i s c u s s i o n

In position C, a yellowish-browsh precipitate was formed after adding K4Fe(CN)6. This

dissociated into K+ and Fe(CN)6 4-. This

ferrocyanide ion reacted with the Cupric ion, consuming the copper ions in the reactants.

This causes the reaction to push backward so as to attain the loss in reactants.

The color is caused by the Cu2Fe(CN)6 product.

d i s c u s s i o n

In position D, addition of HCl dissociated into H+ ions. This ion reacts with NH3 to form NH4,

decreasing the reactant. In order to relieve the stress, the reaction shift backward and thus a

lighter solution.

In position E, addition of NaOH dissociated to OH- and reacted with Cu2

+, and just like in position D, it shifted backward, a lighter solution.

D I S C U S S I O N S

D I S C U S S I O N: Temperature Change

Altering of temperature changes the equilibrium constant

In reactions, we can treat heat, ∆H, as a chemical reagent:• Endothermic: Reactants + ∆H ↔ product• Exothermic: Reactants ↔ product + ∆H

Increase in temperature: forward shift for endothermic, backward shift for exothermic.

“Increase in temperature, would be as if we increased the amount of a reactant, or a product, to the equilibrium system. The equilibrium

shifts to the direction that consumes the excess reactant (or product), in this case, heat.”

In position G, the solution became darker because the temperature was

decreased and the reaction is an exothermic process. The shift is

forward, causing the solution to turn darker.

d i s c u s s i o n

For the part B of the experiment, there is equilibrium. This reaction

can be put as:

2CrO42- + 2H+ ↔ Cr2 O7

2+ + H2O

Chromate ion (2CrO4 2- )is yellow, Dichromate is orange.

In position H, addition of HCl turned the potassium chromate

tuned from yellow to orange. a forward reaction

The addition of NaOH turned the solution lighter and almost the same as the original one. a

backward reaction

d i s c u s s i o n

Adding HCl turned the solution acidic because the HCl dissociated into H+ and Cl-. The hydrogen ion reacted with Chromate ion (CrO4 2-), forming the Dichromate ion (Cr2O7 2-) which was the orange solution. The dichromate ion is the predominant species in the solution because the hydrogen ion dissociation. Thus, turned the solution from yellow to orange.

When adding NaOH to the solution, the OH- ions dissociated from it and reacted with the H+, which consumes the reactant and forms more water, a product. This cause a backward shift to relieve the excess in products. Therefore chromate ion (Cr2O4 2-) is the predominant in the basic medium. This backward reaction cause the orange color to return to its yellow color again.

d i s c u s s i o n

D I S C U S S I O N

In position I, addition of if Pb(NO3)2 to K2CrO4, a yellow precipitate formed. This equilibrium is:

K2CrO4 2- + Pb 2+ PbCrO4

Potassium chromate is yellow, and the Lead (II) Chromate is the yellow precipitate

Addition of HCl turned the solution from yellow to orange, lessening the precipitate. The H+ ions of HCl reacted with the

Chromate ion and formed Dichromate, thus a color change. The precipitate lessened because of chromate was consumed and

shifted backward to the reactant side.

C O N C L U S I O N S & R E C O M M E N D A T I O N S

• Equilibrium is a state in chemical reactions where the rate of formation of products is equal to the rate of formation of reactants.

• In accordance to Le Chatelier’s principle, when there is a stress that is applied on this system. This stress shift the reaction towards the formation of products or reactants, depending on which relieves

the stress. This principle was applied in the experiment.

• Change in concentration: Increase in products or decrease in reactants shifts reaction backwards, to form more reactants.

C O N C L U S I O N S & R E C O M M E N D A T I O N S

Errors in the experiment may be due to the inaccuracy in obtaining reagents or manipulating experimental

variables such as changes in temperature.

It is recommended to study the degrees of color differences in each test tube and note which is

darkest and lightest among them.

Chang, R. (2010) Chemistry 10th Edition. McGraw Gill Publishing. New York.

Le May, Brown. Chemistry: The Central Science. 2012.

http://www.chemguide.co.uk/physical/equilibria/lechatelier.html

http://dwb.unl.edu/calculators/activities/ChemEquilibrium.html

http://www.pinkmonkey.com/studyguides/subjects/chem/chap11/c1111301.asp

http://www.ourstory.info/library/3-FF/SLF/prof2.html

Chemistry 18.1 Lab Manual

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