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# Le Chatelier’s Principle PowerPoint PPT Presentation

Le Chatelier’s Principle. When a system is in equilibrium and a stress is added to the system, the system will shift in order to alleviate itself of the stress in order to return to equilibrium Stresses: 1. concentration changes 2. temperature changes 3. pressure changes.

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Le Chatelier’s Principle

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### Le Chatelier’s Principle

• When a system is in equilibrium and a stress is added to the system, the system will shift in order to alleviate itself of the stress in order to return to equilibrium

• Stresses: 1. concentration changes

2. temperature changes

3. pressure changes

### Concentration Changes

• Adding a substance shifts the equilibrium away from the added substance in order to use the excess amount up and to reestablish equilibrium

A + B  CAdd A, shift right (away from A)

Add B, shift right (away from B)

Add C, shift left (away from C)

• Removing a substance shifts the equilibrium toward the substance in order to replenish the amount removed and to reestablish equilibrium

A + B  CRemove A, shift left (toward A)

Remove B, shift left (toward B)

Remove C, shift right (toward C)

### Temperature Changes

• Based upon whether the reaction is endothermic (takes in heat) or exothermic (gives off heat)

• Follow the same rules as concentration –

• Add heat, shift away from the side heat is located on

• Remove heat, shift toward the side heat is located on

A + B  C + heatAdd heat, shift left (away)

Remove heat, shift right (toward)

A + B + heat  CAdd heat, shift right (away)

Remove heat, shift left (toward)

### Pressure Changes

• ONLY affects GASES

• Use coefficients to determine the total # of gases of products and reactants

• Increase pressure, shift towards to the side with the fewest moles of gas

• Decrease pressure, shift towards the side with the greatest moles of gas

A(g) + B(g)  C (g)

1 mole product gas vs. 2 moles of reactant gas

Increase pressure, shift right (fewest moles)

Decrease pressure, shift left (greatest moles)

### Additional Stuff

• Adding a catalyst has no affect on the equilibrium system

• Adding an inert gas (gas that doesn’t react) or a gas not in the equilibrium system, has no affect on the equilibrium

• The only change that will affect the value of K is a temperature change.

### Example

N2(g) + 3H2(g)  2NH3(g) + 92 kJ/mol

What affect does each of the following have on the equilibrium system? What direction will the equation shift?

• Adding H2shift right

• Removing NH3shift right

• Decreasing volume V, P, shift right (fewest mols)

• Decreasing pressureshift left (greater mols)

• Increasing temperature shift left

• Adding a catalystno affect