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Le Chatelier’s Principle

SCH 4U. Le Chatelier’s Principle. What happens when we disturb chemical systems at equilibrium?. Demo: Cobalt Complex Equilibrium. [Co(H 2 O) 6 ]Cl 2 + heat [ CoCl 2 (H 2 O) 2 ] + 4 H 2 O pink blue. Analogy. Le Chatelier’s Principle:.

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Le Chatelier’s Principle

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  1. SCH 4U Le Chatelier’s Principle What happens when we disturb chemical systems at equilibrium?

  2. Demo: Cobalt Complex Equilibrium [Co(H2O)6]Cl2 + heat [ CoCl2(H2O)2] + 4 H2O pinkblue

  3. Analogy...

  4. Le Chatelier’s Principle: When a chemical system at equilibrium is disturbed by a change in a property, the systems adjusts in a way that opposes the change ...eventually establishing a new equilibrium!

  5. Three Factors Affecting Equilibrium: • Concentration Change • Temperature Change • Total Pressure Change

  6. 1. Concentration Changes aA + bBcC + dD Adding a reactant = more available to react = favours forward rxn (“equil shifts right”) With time, product concentration(s) increase Eventually new equilibrium established...

  7. ... with different reactant/product concentrations from initial equilibrium NOTE: Initial and final K values are the same!

  8. aA + bBcC + dD Removing (decreasing conc of) a product also favours forward rxn (shifts equil right) Remember: system shifts to try to counteract the disturbance!

  9. e.g. 2 CO2(g) 2 CO(g) + O2(g)

  10. aA + bBcC + dD What about... ....decreasing a reactant concentration? ....increasing a product concentration?

  11. Common Ion Effect Add an ion to solution that already contains that ion... ...equilibrium shifts away from added ion

  12. Industrial Example 3 NO2(g) + H2O(l) 2 HNO3(aq) + NO(g) desired product remove what about removing water?

  13. Biological Example Hb(aq) + O2 HbO2(aq) Decreases as blood circulates through body Increases as blood goes into lungs

  14. 2. Temperature Changes reactants + E products Endothermic: Exothermic: reactants products + E If we heat/ cool the system, equilibrium shifts to minimize the change...

  15. If we cool the system... If we heat up the system... Equilibrium shifts in direction that produces heat (favours exothermic) Equilibrium shifts in direction that absorbs heat (favours endothermic)

  16. Temperature Change & K NOTE: The value of K changes with temp. (only constant at specific temperature) Predict how K will change if... ...we heat up an endothermic (fwd) rxn? ...we cool down an endothermic rxn? And what about exothermic (fwd) reaction? → Equil. shifts right, K increases → Equil. shifts left, K decreases

  17. Temperature Change Example

  18. Remember this? N2O4(g) + energy 2 NO2(g)

  19. 3. Pressure (Volume) Change Recall Boyle’s Law: Remember, pressure is caused by gas molecules striking the walls of a container, so you change pressure by changing the number of collisions (change volume)

  20. ONLY affects GASES! • Increasing total pressure of system shifts equilibrium to side of balanced equation showing fewest number of gas molecules • If P (↑), and concentrations change (i.e. increase) then V (↓)

  21. 2 SO3(g) ↔ 2 SO2(g) + O2(g)

  22. Note If both sides of the equation have the same number of molecules of gas, any change in pressure or volume will have no effect on equilibrium i.e. H2(g) + I2(g) 2 HI(g)

  23. So what doesn’t affect equilibrium? Addition of a catalyst • A catalyst speeds up the rate of a reaction • Lower the activation energy, which increases the rate of reaction, of both the forward and reverse reactions • Catalysts do NOT affect position of equilibrium, only time taken

  24. So what doesn’t affect equilibrium? Adding Inert Gases • Inert gases do NOT react with other gases (i.e. NOT part of the equilibrium system) • Presence of inert gas changes the probability of successful collisions for both the reactants and products equally, resulting in NO shift in equilibrium

  25. Le Chatelier’s Summary

  26. Homework • Pg 457 #1 – 4 • Pg 459 #2 – 4 • Read pgs 461 – 462 on the Haber Process Haha, just kidding!

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