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.

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Le Chatelier’s Principle

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Le chatelier s principle

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

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

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

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

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

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


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