S10_Student_Lecture_09

S10_Student_Lecture_09 - Predicting Shifts in...

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[FeSCN 2+ ] Fe 3+ + SCN - FeSCN 2+ K = [Fe 3+ ][SCN - ] After adding SCN - , before reaction, [SCN - ]: Q < K. For Q to = K, some Fe 3+ must react with the SCN - ___________ is formed. We say “the reaction shifts to the _______.” Predicting Shifts in Concentration

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[FeSCN 2+ ] Fe 3+ + SCN - FeSCN 2+ K = [Fe 3+ ][SCN - ] What happens to [SCN - ] at the new equilibrium? Initial equilibrium After adding SCN - [FeSCN 2+ ] [+, -, =] = K = [Fe 3+ ][SCN - ] [+, -, =][+, -, =] Shifts in Concentration
The effect of added Cl 2 on the PCl 3 -Cl 2 -PCl 5 system. PCl 3 ( g ) + Cl 2 ( g ) PCl 5 ( g ) Note that “new equilibrium” concentrations of reactant and product are __________ after perturbation, K is ___________ PCl 3 ___________ Cl 2 _________, then __________ PCl 5 ___________ Disturbance (addition of Cl 2 ) is not completely eliminated. New equilibrium ______________ but same K!

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Le Chatelier’s Principle: A system at equilibrium will ___________ to equilibrium after a change in condition moves it away. When a system at equilibrium is disturbed, it returns to equilibrium by ___________ the effect of the disturbance.
Le Chatelier’s Principle “If a change in conditions (a “stress”) is imposed on a system at equilibrium, the equilibrium position will shift in a direction that tends to reduce that change in conditions.” A + B C + D + Energy For example: In the reaction above, if more A or B is added you will force the reaction to produce more product, if they are removed, it will force the equilibrium to form more reactants. If C or D is added you will force the reaction to form more reactants, if they are Removed from the reaction mixture, it will force the equilibrium to Form more products. If it is heated, you will get more reactants, and if cooled, more products.

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Move away from K and either increase or decrease Q Q c = C [ ] c D [ ] d A [ ] a B [ ] b a A [ ] + b B [ ] Û c C [ ] + d D [ ]
Meaning of “Q” Q < K - Reaction goes to ______ Q > K - Reaction goes to ______ K = Q at _____________. “Q” is a measure of the progress of a reaction as it heads toward equilibrium.

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: When stress is applied to a system at equilibrium, the position of equilibrium (the concentrations) shifts to remove as much of the stress as possible. Stress
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S10_Student_Lecture_09 - Predicting Shifts in...

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