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Chem 2 - Chemical Equilibrium X: Le Chatelier's Principle and Temperature Changes

Chem 2 - Chemical Equilibrium X: Le Chatelier's Principle and Temperature Changes

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Chem 2 - Chemical Equilibrium X: Le Chatelier's Principle and Temperature Changes

  1. 1. Chemical Equilibrium (Pt. 10) Le Chatelier’s Principle and Temperature Changes By Shawn P. Shields, Ph.D. This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
  2. 2. “Stressing” a Chemical System Chemical reactions at equilibrium like to stay at equilibrium. Any factor that disrupts the equilibrium state is considered a “stress.”
  3. 3. Stresses and Le Chatelier’s Principle A stress is some change to the system, which can include • Adding/removing product or reactant • Changing the pressure on a system • Changing the volume of a system • Changing the temperature
  4. 4. Le Chatelier’s Principle When a chemical reaction at equilibrium is stressed, it will shift the equilibrium point in the direction that counteracts the stress applied.
  5. 5. When the reaction temperature changes, the equilibrium constant K no longer has the same value, but the value for Q remains the same. The equilibrium shifts so that Q = K at the new temperature. Temperature Changes as a Stress
  6. 6. The easiest way to think about this is to describe “heat” as a reactant or product. An endothermic reaction absorbs heat from the surroundings (H) in the forward direction. Heat is a reactant Temperature Changes and H
  7. 7. An exothermic reaction releases heat into the surroundings (H) in the forward direction. Heat is a product So how can we use this concept? Temperature Changes and H
  8. 8. Suppose we increase the temperature… Which way will the equilibrium shift? 2 NO2 N2O4 Temperature Changes as a Stress Hrxn =  57 kJ/mol
  9. 9. Suppose we increase the temperature… Which way will the equilibrium shift? Temperature Changes as a Stress Hrxn =  57 kJ/mol 2 NO2 N2O4 + heat
  10. 10. If the temperature goes up, we are adding heat (a product), so the reaction shifts toward reactants (in reverse) 2 NO2 N2O4 Temperature Changes as a Stress Hrxn =  57 kJ/mol 2 NO2 N2O4 + heat
  11. 11. Now, reverse the reaction and decrease the temperature… Which way will the equilibrium shift? Decreasing the Temperature- Endothermic Reactions N2O4 2 NO2 Hrxn = + 57 kJ/mol
  12. 12. If the temperature goes down, we are removing heat, so the reaction shifts toward reactants (in reverse). Reducing Temperature N2O4 2 NO2 Hrxn = + 57 kJ/mol Heat + N2O4 2 NO2
  13. 13. The temperature is increased… Which direction will the equilibrium shift? Mini Quiz A + B 2 C Hrxn = + 105 kJ/mol
  14. 14. If the temperature is increased, we are adding heat, so the reaction shifts toward products (in the forward direction). Mini Quiz Solution A + B 2 C Hrxn = + 105 kJ/mol Heat + A + B 2 C

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