Le Châtelier’s Principle (College Board AP® Chemistry): Flashcards
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Define Le Châtelier's principle.
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Cards in this collection (13)
Define Le Châtelier's principle.
If a stress is applied to a system at equilibrium, the reaction shifts in the direction that counteracts the effect of the stress and establishes a new equilibrium.
Why does adding a pure solid or pure liquid to an equilibrium system have no effect on the equilibrium position?
Pure solids and pure liquids are not included in the equilibrium expression — their concentrations are constant and defined as 1. Because they do not appear in the expression, changing their amounts does not alter Q or shift the equilibrium.
True or False?
Adding a catalyst to a system at equilibrium shifts the equilibrium position toward the products.
False.
A catalyst increases the rates of both the forward and reverse reactions equally. It causes the system to reach equilibrium faster but does not shift the equilibrium position or change the value of K.
For a gas-phase equilibrium, how does increasing the pressure (by decreasing the volume) affect the equilibrium position, and why?
The system shifts toward the side with fewer moles of gaseous particles to reduce the total pressure. If both sides have equal moles of gas, there is no shift and the equilibrium position is unaffected.
True or False?
Increasing the temperature of an exothermic equilibrium reaction decreases the value of K.
True.
In an exothermic reaction, heat can be treated as a product. Adding heat (increasing temperature) shifts the equilibrium toward the reactants, reducing the concentration of products relative to reactants. Since K = [products]/[reactants], K decreases. Temperature is the only stress that changes the value of K.
When an aqueous equilibrium is diluted with water, in which direction does the system shift, and why?
Dilution decreases the concentration of all aqueous species, causing Q to differ from K. The system shifts toward the side that produces the greatest number of moles of aqueous particles, restoring Q = K by increasing total particle count to compensate for the reduced concentration.
When a product is added to an equilibrium system, the system shifts in the .......... reaction direction, causing the amount of reactants to .......... .
When a product is added to an equilibrium system, the system shifts in the reverse reaction direction, causing the amount of reactants to increase.
Define reaction quotient (Q).
The numerical value obtained by substituting the current concentrations or partial pressures of all species into the equilibrium expression. Unlike K, Q can be calculated at any point during the reaction, not just at equilibrium.
If Q < Kc for a reversible reaction, in which direction will the reaction proceed, and why?
The reaction proceeds in the forward direction (toward products). When Q < Kc, the ratio of products to reactants is smaller than at equilibrium, so the system produces more products to increase Q until Q = Kc.
True or False?
If Q > Kc, the reaction will shift in the reverse direction to reach equilibrium.
True.
When Q > Kc, there are too many products relative to reactants. The system shifts toward reactants (reverse direction) to reduce the product concentrations until Q = Kc.
How does comparing Q to Kc allow you to predict whether a system is at equilibrium?
If Q = Kc, the system is at equilibrium and no net shift occurs. If Q ≠ Kc, the system shifts in the forward direction when Q < Kc and in the reverse direction when Q > Kc, always moving to restore Q = Kc.
True or False?
The expression for Qc is different from the expression for Kc.
False.
The mathematical expression for Qc is identical to that for Kc — products raised to their stoichiometric powers divided by reactants raised to their stoichiometric powers. The difference is that Kc uses equilibrium concentrations, whereas Qc uses concentrations from any point in the reaction.
If Q does not change with time, the reaction is in a state of .......... and Q = .......... .
If Q does not change with time, the reaction is in a state of equilibrium and Q = Kc.
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