Le Chatelier Principle 2016.notebook

Le Chatelier Principle 2016.notebook April 19, 2016

Le Chatelier's Principle

Aim: How does a reaction respond to changes that disturb equilibrium?


Le Chatelier's Principle

A system at equilibrium has constant concentrations, equal reaction rates and is HAPPY. It wants to stay at equilibrium.

Any change or stress on a system at equilibrium will cause the reaction to shift in the direction that removes the stress in order to re‐establish equilibrium. Reactions want to be at equilibrium and have equal forward and reverse rates.

Reactions can move towards the forward (right) or reverse (left) directions.


Stresses that can be introduced:

1) Changes in concentration2) Changes in temperature3) Changes in pressure (only affects gases)


Shifting a Reaction

When equilibrium is disturbed, the reaction will either shift forward or reverse to fix itself. The direction it shifts depends on the stress and how equilibrium must be re‐established.

• Reactants are always used up and their concentration decreases as the products are produced and their concentration increases.


Concentration Changes

When increasing concentration the reaction will shift in the direction that uses up the substance thats been added as a reactant.

When decreasing concentration the reaction will shift to replace the substance whose concentration was decreased.

2 H2 + O2 → 2 H2O a. Increase the hydrogen concentration

2 H2 + O2 → 2 H2O b. Decrease the hydrogen concentration

2 H2 + O2 → 2 H2O c. Increase the water concentration


Temperature Changes

Increasing temperature causes the reaction to shift in the endothermic direction to absorb the heat added.

Decreasing temperature causes the reaction to shift in the exothermic direction to replace the energy removed.

2 C + H2 + 227.4 kJ ↔ C2H2 d. Increasing the temperature

e. Decreasing the temperature


Pressure ChangesAffects gases only!!!

Increasing pressure causes a shift in the direction with less molecules to remove the added pressure.

Decreasing pressure causes a shift in the direction with more molecules to replace the pressure that was decreased.

N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) a. Increasing the pressure

b. Decreasing the pressure


Adding a Catalyst

• Adding a catalyst WILL NOT AFFECT EQUILIBRIUM.

• The catalyst increases the forward and reverse rates by the same amount, therefore they remain equal.

Equilibrium has EQUAL RATES and CONSTANT CONCENTRATION.


Stress Direction of Shift of Equilibrium

A (g) + B (g) C (g) + heat

Increased concentration of A or BIncreased concentration of CIncreased pressureDecreased pressureIncreased temperatureDecreased temperature


StressDirection of Shift of Equilibrium2 A (g) + B (g) + heat 2 C (g) + 2 D(g)

Increased concentration of A or BIncreased concentration of C or DIncreased pressureDecreased pressureIncreased temperatureDecreased temperature


2 SO2 (g) + O2 (g) <> 2SO3 + 47 kcal

1. If the concentration of SO3 decreased, the concentration of SO2 ________________

2. If the pressure is decreased, concentration of O2 ________________

3. If the concentration of O2 decreased, the concentration of SO2 ________________

4. If the temperature is decreased, the concentration of O2 ________________

5. If the concentration of SO2 is increased, the concentration of SO3 ________________

6. If the concentration of SO3 is increased, the concentration of O2 ________________

7. If the pressure is increased, the concentration of SO3 ________________

8. If the temperature is increased, the concentration of SO3 ________________


H2 (g) + I2 (g) + 12.4 kcal <> 2 HI (g)

10. If the concentration of H2 increased, the concentration of HI ________________

11. If the pressure is decreased, concentration of H2 ________________

12. If a catalyst is added the concentration HI ________________

13. If the concentration of I2 is decreased, the concentration of HI ________________

14. If the concentration of H2 is increased, the concentration of I2 ________________

15. If the pressure is increased, the concentration of HI ________________

16. If the temperature is increased, the concentration of HI ________________

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Le Chatelier Principle 2016.notebook