Equilibrium:Le Chatelier’s Principle

AP ChemistryAmar Patel

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Purpose

To see the effect of changes of concentration on an equilibrium system

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The equation of our equilibrium system is:

Cu(H2O)52+

(aq) + 4Cl-

(aq) CuCl⇌ 4

2-(aq)

+ 5H2O(aq)

- This is the net ionic equation

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Safety

• Wear goggles at all times• Use stoppers when shaking test tubes• Do NOT directly smell or touch the chemicals• Do NOT ingest the chemicals• Be careful with glass test tubes

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The equation of our equilibrium system is:

Cu(H2O)52+

(aq) + 4Cl-

(aq) CuCl⇌ 4

2-(aq)

+ 5H2O(aq)

- This is the net ionic equation

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Cu(H2O)52+

(aq) + 4Cl-

(aq) CuCl⇌ 4

2-(aq)

+ 5H2O(aq)

• Le Chatelier’s Principle: When a stress is placed on a system at

equilibrium, the system will react (shift) in a direction to relieve the stress

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Cu(H2O)52+

(aq) + 4Cl-

(aq) CuCl⇌ 4

2-(aq)

+ 5H2O(aq)

What is the Kc expression?

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Stress 1: Concentration

Cu(H2O)52+

(aq) + 4Cl-

(aq) CuCl⇌ 4

2-(aq)

+ 5H2O(aq)

What is the Kp?

[CuCl42- ][H2O]5

[Cu(H2O)52+][Cl-]4

Kc =

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Cu(H2O)52+

(aq) + 4Cl-

(aq) CuCl⇌ 4

2-(aq)

+ 5H2O(aq)

Predict the equilibirum shift and effect on [CuCl42- ].

1. Adding Cu(H2O)52+

2. Adding 4Cl-

3. Adding H2O4. Removing Cu(H2O)5

2+

5. Removing CuCl42-

6. Adding Ne gas7. Adding AgNO3

1. Right; Increase2. Right; Increase3. Left; Decrease4. Left; Decrease5. Right; Increase**6. No effect – Why?7. Left; Decrease – Why?

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Stress 2: Temperature

New equilibrium system is:

Co(H2O)62+

(aq) + 6Cl-

(aq) CoCl⇌ 6

2-(aq)

+ 6H2O(aq)

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Is the reaction endothermic or exothermic?

Co(H2O)62+

(aq) + 6Cl-

(aq) CoCl⇌ 6

2-(aq)

+ 6H2O(aq)

We can treat heat/energy as a reactant or product.Think about the direction of the shift.

Is heat a reactant or product?

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Temperature

• The shift will occur to correct the stress (removal or addition of heat).

• Remember that K is temperature dependent so K will change with changes in temperature.– Endothermic rxn with heat input • Heat is a reactant. • By adding heat, the rxn shifts to the product side• K (products divided by reactants) increases

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Stress 3: Pressure/Volume

New equilibrium system is:

PCl3(g) + Cl2(aq) PCl⇌ 5

(g)

Remember Boyle’s Law?What is the relationship between P and V?Inverse relation – As P increases, V decreases

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Pressure/Volume• If there is a increase in pressure (decrease in

volume), the system will try to reduce the pressure by shifting to decrease the number of gas molecules

PCl3(g) + Cl2(aq) PCl⇌ 5

(g)

Left has 2 molecules, right has 1 molecule so the reaction will shift to the right

• If both sides of rxn have same number of molecules, change in P has NO EFFECT.

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Examples

1. CO (g) + H2O(g) CO⇌ 2(g) + H2 (g)

a.) What is the Kc expression?

b.) If [CO2 ] increases, then [H2O] _______

c.) If [H2] decreases, then [CO] ________

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CO (g) + H2O(g) CO⇌ 2(g) + H2 (g)

[CO2 ][H2]

[CO][H2O] Kc =

b.) If [CO2 ] increases, then [H2O] _______

c.) If [H2] decreases, then [CO] ________

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Examples2. Fe(s) + H2O(g) Fe⇌ 3O4 (S) + H2 (g) ∆H⁰ = -155 kJ

a.) Balance the equation.

b.) Write the Kc expression for the equation.

How will each of the following affect the concentration of the H2 (increase, decrease, or unchanged) in an equilibrium mixture of the reaction at 100⁰C :

c.) Raise the temperature to 250⁰

d.) Remove the amount of Fe3O4 (S)

e.) Add a small amount of H2O(g)

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3 Fe(s) + 4 H2O(g) Fe⇌ 3O4 (S) + 4 H2 (g) ∆H⁰ = -155 kJ

[H2]4 [H2O]4

Kc =

How will each of the following affect the concentration of the H2 (increase, decrease, or unchanged) in an equilibrium mixture of the reaction at 100⁰C :

c.) Raise the temperature to 250⁰

d.) Remove the amount of Fe3O4 (S) NO CHANGE

e.) Add a small amount of H2O(g)

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