Reaction Quotient and Le Chatelier's Principle - AP Chemistry

1. What does $Q_c = K_c$ signify?

   The system is at equilibrium. When $Q_c$ equals $K_c$, forward and reverse reaction rates are equal.

2. Define Le Chatelier's Principle.

   A system at equilibrium will adjust to counteract imposed changes. The system responds to minimize the disturbance and restore balance.

3. What does $Q_c > K_c$ suggest about reaction direction?

   The reaction will proceed in the reverse direction. When $Q_c > K_c$, excess products convert back to reactants.

4. What is the effect of removing a product from a reaction at equilibrium?

   The equilibrium shifts to the right to form more products. System compensates by producing more of the removed product.

5. State what happens to equilibrium if a catalyst is added.

   The rate increases, but equilibrium position is unchanged. Catalysts affect kinetics but not the equilibrium position.

6. How does adding an inert gas at constant volume affect equilibrium?

   There is no effect on the position of equilibrium. Inert gas doesn't change partial pressures of reactants or products.

7. How does removing a reactant affect the position of equilibrium?

   The equilibrium shifts to the left, forming more reactants. System produces more reactant to replace what was removed.

8. What is the effect of temperature decrease on the $K_c$ of an endothermic reaction?

   $K_c$ decreases with a decrease in temperature. Lower temperature opposes endothermic reactions, decreasing $K_c$.

9. What defines the direction of shift when pressure is increased in a gaseous reaction?

   Shifts to the side with fewer moles of gas. Higher pressure favors the side with fewer gas molecules.

10. What effect does increasing pressure have on a gaseous equilibrium?

    The equilibrium shifts towards the side with fewer moles of gas. Higher pressure favors the side with fewer gas molecules.

11. Calculate $Q_c$ for $2A \rightleftharpoons B$ if $[A] = 0.3$, $[B] = 0.2$.

    $Q_c = \frac{[B]}{[A]^2} = \frac{0.2}{0.3^2} = 2.22$. Substitute the given concentrations into the $Q_c$ expression.

12. How does increasing reactant concentration affect equilibrium position?

    Shifts right, forming more products. More reactant drives the equilibrium forward to consume it.

13. What happens to equilibrium when a product concentration is decreased?

    The equilibrium shifts to the right to produce more product. System compensates by producing more of the removed product.

14. What is the effect of adding a strong acid to a weak acid equilibrium?

    The equilibrium shifts to the left, decreasing dissociation. Added acid increases $H^+$ concentration, suppressing weak acid dissociation.

15. What is the effect of temperature increase on the equilibrium constant for exothermic reactions?

    The equilibrium constant, $K_c$, decreases. Higher temperature opposes exothermic reactions, reducing $K_c$.

16. What is the value of $Q_c$ if $[A] = 2$, $[B] = 3$, $[C] = 1$, $[D] = 4$ for $A + 2B \rightleftharpoons C + D$?

    $Q_c = \frac{[C][D]}{[A][B]^2} = \frac{1 \times 4}{2 \times 3^2} = \frac{2}{9}$. Substitute concentrations into $Q_c$ expression and calculate.

17. Calculate $Q_c$ for a reaction if $[A] = 1.0$, $[B] = 2.0$, $[C] = 1.5$ for $A + B \rightleftharpoons C$.

    $Q_c = \frac{[C]}{[A][B]} = \frac{1.5}{1.0 \times 2.0} = 0.75$. Apply the $Q_c$ formula using the given concentration values.

18. What is the effect of increasing volume on a gaseous equilibrium?

    The equilibrium shifts to the side with more moles of gas. Increased volume decreases pressure, favoring more gas molecules.

19. Identify the effect of adding more product to an equilibrium system.

    The equilibrium shifts to the left, producing more reactants. Excess product drives the equilibrium backward to consume it.

20. State the expression for $Q_c$ for $3A \rightleftharpoons 2B + C$.

    $Q_c = \frac{[B]^2[C]}{[A]^3}$. Products in numerator, reactants in denominator with stoichiometric coefficients.

21. Which direction does equilibrium shift when a reactant concentration decreases?

    The equilibrium shifts to the left, forming more reactants. System compensates by producing more of the depleted reactant.

22. How does temperature decrease affect $K_c$ in endothermic reactions?

    $K_c$ decreases with a decrease in temperature. Lower temperature opposes endothermic reactions, decreasing $K_c$.

23. What happens to equilibrium when an aqueous solution is diluted?

    The equilibrium shifts to the side with more solute particles. Dilution favors the side that produces more dissolved particles.

24. How does increasing the concentration of a reactant affect equilibrium?

    The equilibrium shifts to the right, forming more products. System responds by consuming the excess reactant to form products.

25. Calculate $Q_c$ for $A + B \rightleftharpoons 2C$ if $[A] = 0.5$, $[B] = 0.5$, $[C] = 0.1$.

    $Q_c = \frac{[C]^2}{[A][B]} = \frac{0.1^2}{0.5 \times 0.5} = 0.04$. Substitute given concentrations into the $Q_c$ formula.

26. How does the presence of a catalyst alter the rate of reaction?

    A catalyst increases the rate of reaction without shifting equilibrium. Catalysts only affect reaction kinetics, not thermodynamic equilibrium.

27. What happens to $K_c$ in an exothermic reaction if temperature decreases?

    $K_c$ increases with a decrease in temperature. Lower temperature favors exothermic reactions, increasing $K_c$.

28. What is the effect of a catalyst on the position of equilibrium?

    A catalyst does not affect the position of equilibrium. Catalysts speed up both forward and reverse reactions equally.

29. How does the addition of an inert gas at constant pressure affect equilibrium?

    It changes the equilibrium position depending on moles of gas. At constant pressure, inert gas causes volume expansion affecting equilibrium.

30. How does decreasing temperature affect an exothermic reaction at equilibrium?

    The equilibrium shifts to the right, favoring product formation. Lower temperature favors the exothermic direction (forward).

31. What does $Q_c < K_c$ indicate about reaction direction?

    The reaction will proceed forward to reach equilibrium. When $Q_c < K_c$, more products must form to reach equilibrium.

32. What happens when $Q_c$ is less than $K_c$?

    The reaction proceeds forward to reach equilibrium. When $Q_c < K_c$, forward reaction proceeds to reach equilibrium.

33. How does decreasing pressure affect a reaction with equal moles of gas on both sides?

    There is no effect on the position of equilibrium. Pressure changes don't affect equilibrium when gas moles are equal.

34. Identify the change in equilibrium with an increase in product pressure.

    The equilibrium shifts to the left, forming more reactants. Increased product concentration drives the reaction backward.

35. Define the reaction quotient, $Q_c$.

    The ratio of product concentrations to reactant concentrations at any point. Measures current concentration ratio, not necessarily at equilibrium.

36. What is the effect of temperature increase on an endothermic reaction at equilibrium?

    The equilibrium shifts to the right, favoring product formation. Higher temperature favors the endothermic direction (forward).

37. State the reaction quotient expression for $2A + B \rightleftharpoons 3C + D$.

    $Q_c = \frac{[C]^3[D]}{[A]^2[B]}$. Products in numerator with coefficients as exponents, reactants in denominator.

38. What happens to $K_c$ with temperature change in an exothermic reaction?

    $K_c$ decreases with an increase in temperature. Higher temperature opposes exothermic reactions, decreasing $K_c$.

39. What effect does temperature have on $K_c$ for endothermic reactions?

    $K_c$ increases with an increase in temperature. Higher temperature favors endothermic reactions, increasing $K_c$.

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