Calculating Equilibrium Concentrations - AP Chemistry

1. For the reaction $CH_4(g) + 2O_2(g) \rightleftharpoons CO_2(g) + 2H_2O(g)$, write the $K_c$ expression.

   $K_c = \frac{[CO_2][H_2O]^2}{[CH_4][O_2]^2}$. Products over reactants, each raised to their stoichiometric coefficients.

2. Calculate $K_c$ for $H_2 + I_2 \rightleftharpoons 2HI$ with $[H_2] = 1M$, $[I_2] = 1M$, $[HI] = 2M$.

   $K_c = \frac{4}{1}$. $K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{2^2}{1 \times 1} = 4$.

3. State the effect of adding a reactant on the equilibrium position.

   Shifts right to form more products. Le Chatelier's principle: system responds by consuming the added reactant.

4. What is the effect of a catalyst on the equilibrium constant $K$?

   No effect. Catalysts only affect reaction rate, not equilibrium position or $K$.

5. Identify the equilibrium expression for the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$.

   $K_c = \frac{[NH_3]^2}{[N_2][H_2]^3}$. Products over reactants, each raised to their stoichiometric coefficients.

6. State the relationship between $K_p$ and $K_c$ for gaseous reactions.

   $K_p = K_c(RT)^{\Delta n}$. Where $\Delta n$ is moles of gaseous products minus gaseous reactants.

7. What happens to $K_c$ of an exothermic reaction when temperature increases?

   $K_c$ decreases. Higher temperature opposes exothermic reactions, decreasing $K_c$.

8. Find $K_c$ for the reaction $2A \rightleftharpoons B + C$ if $[A] = 0.5M$, $[B] = 1M$, $[C] = 1M$.

   $K_c = \frac{1 \times 1}{0.5^2}$. $K_c = \frac{[B][C]}{[A]^2} = \frac{1 \times 1}{0.5^2} = 4$.

9. What is the effect of adding an inert gas at constant volume on the equilibrium of a reaction?

   No effect. Inert gas doesn't participate and doesn't change partial pressures.

10. For the reaction $C(s) + CO_2(g) \rightleftharpoons 2CO(g)$, write the $K_c$ expression.

    $K_c = [CO]^2/[CO_2]$. Solid carbon is excluded, only gaseous species are included.

11. Given $K_c = 10$ for $A \rightleftharpoons B$, find $K_c$ for $2A \rightleftharpoons 2B$.

    $K_c = 10^2$. When reaction is doubled, equilibrium constant is squared.

12. What happens to the equilibrium position when pressure is increased for the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$?

    Shifts towards $NH_3$. Higher pressure favors the side with fewer gas molecules (2 vs 4).

13. What happens to the equilibrium position when pressure is increased for the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$?

    Shifts towards $NH_3$. Higher pressure favors the side with fewer gas molecules (2 vs 4).

14. What is the equilibrium constant expression for $2H_2O_2(aq) \rightleftharpoons 2H_2O(l) + O_2(g)$?

    $K_c = [O_2]$. Liquids are excluded from equilibrium expressions, only gases included.

15. State the $K_c$ expression for the reaction $C_2H_4(g) + H_2(g) \rightleftharpoons C_2H_6(g)$.

    $K_c = \frac{[C_2H_6]}{[C_2H_4][H_2]}$. Products over reactants, each raised to their stoichiometric coefficients.

16. State the unit for $K_c$ for the reaction $aA + bB \rightleftharpoons cC + dD$ if $c+d = a+b$.

    Unitless. When total moles of products equals total moles of reactants.

17. For $2NO_2(g) \rightleftharpoons N_2O_4(g)$, what happens to $K_c$ if temperature is increased?

    $K_c$ decreases. Higher temperature opposes exothermic reactions, decreasing $K_c$.

18. For the reaction $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$, find $K_c$ if $[HI] = 2M$, $[H_2] = 1M$, $[I_2] = 1M$.

    $K_c = 4$. $K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{2^2}{1 \times 1} = 4$.

19. For the reaction $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$, find $K_c$ if $[HI] = 2M$, $[H_2] = 1M$, $[I_2] = 1M$.

    $K_c = 4$. $K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{2^2}{1 \times 1} = 4$.

20. What is the equilibrium expression for $H_2(g) + S(s) \rightleftharpoons H_2S(g)$?

    $K_c = [H_2S]$. Solids are excluded from equilibrium expressions, only gases included.

21. For the reaction $CH_4(g) + 2O_2(g) \rightleftharpoons CO_2(g) + 2H_2O(g)$, write the $K_c$ expression.

    $K_c = \frac{[CO_2][H_2O]^2}{[CH_4][O_2]^2}$. Products over reactants, each raised to their stoichiometric coefficients.

22. If $K_c = 1.5$ for the reaction $A \rightleftharpoons B$, what is the value of $K_c$ for $B \rightleftharpoons A$?

    $K_c = \frac{1}{1.5}$. For the reverse reaction, $K_c$ becomes the reciprocal of the original.

23. State the relationship between $K_p$ and $K_c$ for gaseous reactions.

    $K_p = K_c(RT)^{\Delta n}$. Where $\Delta n$ is moles of gaseous products minus gaseous reactants.

24. What is the expression for the equilibrium constant $K_c$ for the reaction $aA + bB \rightleftharpoons cC + dD$?

    $K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$. Products in numerator raised to their coefficients, reactants in denominator.

25. For the reaction $PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)$, what is the effect of decreasing temperature?

    Shifts towards $PCl_5$. Lower temperature favors the exothermic direction (heat-producing side).

26. For the reaction $2NO(g) + O_2(g) \rightleftharpoons 2NO_2(g)$, what is the $K_p$ expression?

    $K_p = \frac{P_{NO_2}^2}{P_{NO}^2 P_{O_2}}$. Products over reactants using partial pressures, raised to coefficients.

27. For the reaction $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$, find $K_c$ if $[HI] = 2M$, $[H_2] = 1M$, $[I_2] = 1M$.

    $K_c = 4$. $K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{2^2}{1 \times 1} = 4$.

28. Calculate $K_c$ for $H_2 + I_2 \rightleftharpoons 2HI$ with $[H_2] = 1M$, $[I_2] = 1M$, $[HI] = 2M$.

    $K_c = \frac{4}{1}$. $K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{2^2}{1 \times 1} = 4$.

29. State the $K_c$ expression for the reaction $C_2H_4(g) + H_2(g) \rightleftharpoons C_2H_6(g)$.

    $K_c = \frac{[C_2H_6]}{[C_2H_4][H_2]}$. Products over reactants, each raised to their stoichiometric coefficients.

30. Identify the equilibrium expression for $CO(g) + Cl_2(g) \rightleftharpoons COCl_2(g)$.

    $K_c = \frac{[COCl_2]}{[CO][Cl_2]}$. Products over reactants, each raised to their stoichiometric coefficients.

31. Find $K_c$ for $A \rightleftharpoons 2B$ if $[A] = 1M$, $[B] = 0.5M$ at equilibrium.

    $K_c = \frac{0.5^2}{1}$. $K_c = \frac{[B]^2}{[A]} = \frac{0.5^2}{1} = 0.25$.

32. What is the effect of pressure decrease on equilibrium of $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$?

    Shifts left. Lower pressure favors the side with more gas molecules (4 vs 2).

33. State the unit for $K_c$ for the reaction $aA + bB \rightleftharpoons cC + dD$ if $c+d = a+b$.

    Unitless. When total moles of products equals total moles of reactants.

34. For the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$, what is the expression for $K_p$?

    $K_p = \frac{P_{SO_3}^2}{P_{SO_2}^2 P_{O_2}}$. Products over reactants using partial pressures, raised to coefficients.

35. State the effect of adding a reactant on the equilibrium position.

    Shifts right to form more products. Le Chatelier's principle: system responds by consuming the added reactant.

36. Identify the equilibrium expression for $CO(g) + Cl_2(g) \rightleftharpoons COCl_2(g)$.

    $K_c = \frac{[COCl_2]}{[CO][Cl_2]}$. Products over reactants, each raised to their stoichiometric coefficients.

37. If $K_c = 1.5$ for the reaction $A \rightleftharpoons B$, what is the value of $K_c$ for $B \rightleftharpoons A$?

    $K_c = \frac{1}{1.5}$. For the reverse reaction, $K_c$ becomes the reciprocal of the original.

38. For $N_2O_4(g) \rightleftharpoons 2NO_2(g)$, how does increasing pressure affect equilibrium?

    Shifts towards $N_2O_4$. Higher pressure favors the side with fewer gas molecules (1 vs 2).

39. What is the effect of adding an inert gas at constant volume on the equilibrium of a reaction?

    No effect. Inert gas doesn't participate and doesn't change partial pressures.

40. For $N_2O_4(g) \rightleftharpoons 2NO_2(g)$, how does increasing pressure affect equilibrium?

    Shifts towards $N_2O_4$. Higher pressure favors the side with fewer gas molecules (1 vs 2).

41. For the reaction $2NO(g) + O_2(g) \rightleftharpoons 2NO_2(g)$, what is the $K_p$ expression?

    $K_p = \frac{P_{NO_2}^2}{P_{NO}^2 P_{O_2}}$. Products over reactants using partial pressures, raised to coefficients.

42. For the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$, what is the expression for $K_p$?

    $K_p = \frac{P_{SO_3}^2}{P_{SO_2}^2 P_{O_2}}$. Products over reactants using partial pressures, raised to coefficients.

43. What is the equilibrium expression for $H_2(g) + S(s) \rightleftharpoons H_2S(g)$?

    $K_c = [H_2S]$. Solids are excluded from equilibrium expressions, only gases included.

44. How does a decrease in the concentration of $O_2$ affect equilibrium in $2SO_2 + O_2 \rightleftharpoons 2SO_3$?

    Shifts left. Removing reactant shifts equilibrium to replace it via reverse reaction.

45. Determine the equilibrium constant $K_c$ for the reaction $2NO_2(g) \rightleftharpoons N_2O_4(g)$ given $[NO_2] = 0.5M$, $[N_2O_4] = 0.2M$.

    $K_c = 0.8$. $K_c = \frac{[N_2O_4]}{[NO_2]^2} = \frac{0.2}{0.5^2} = 0.8$.

46. What is the effect of increasing temperature on the equilibrium position of an exothermic reaction?

    Shifts towards reactants. Higher temperature opposes exothermic reactions, favoring the reverse direction.

47. Calculate $K_c$ for $H_2 + I_2 \rightleftharpoons 2HI$ with $[H_2] = 1M$, $[I_2] = 1M$, $[HI] = 2M$.

    $K_c = \frac{4}{1}$. $K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{2^2}{1 \times 1} = 4$.

48. State the effect of a temperature decrease on an endothermic reaction's equilibrium position.

    Shifts towards reactants. Lower temperature opposes endothermic reactions, favoring reactant formation.

49. Identify the expression for $K_c$ for $H_2O(l) \rightleftharpoons H^+(aq) + OH^-(aq)$.

    $K_w = [H^+][OH^-]$. Auto-ionization of water uses $K_w$ notation for the equilibrium constant.

50. If $[N_2] = 2M$, $[H_2] = 1M$, $[NH_3] = 0.5M$, find $Q_c$ for $N_2 + 3H_2 \rightleftharpoons 2NH_3$.

    $Q_c = \frac{0.5^2}{2 \times 1^3}$. $Q_c = \frac{[NH_3]^2}{[N_2][H_2]^3} = \frac{0.5^2}{2 \times 1^3} = \frac{0.25}{2}$.

51. For the endothermic reaction $A \rightleftharpoons B$, what is the effect of increasing temperature?

    Shifts towards $B$. Higher temperature favors endothermic reactions (heat-absorbing direction).

52. Find $K_c$ for the reaction $2A \rightleftharpoons B + C$ if $[A] = 0.5M$, $[B] = 1M$, $[C] = 1M$.

    $K_c = \frac{1 \times 1}{0.5^2}$. $K_c = \frac{[B][C]}{[A]^2} = \frac{1 \times 1}{0.5^2} = 4$.

53. What is the effect of adding a product on the position of equilibrium?

    Shifts left to form more reactants. Le Chatelier's principle: system responds by consuming the added product.

54. Identify the expression for $K_c$ for $H_2O(l) \rightleftharpoons H^+(aq) + OH^-(aq)$.

    $K_w = [H^+][OH^-]$. Auto-ionization of water uses $K_w$ notation for the equilibrium constant.

55. Which factor does not affect the value of the equilibrium constant $K$?

    Concentration of reactants or products. $K$ depends only on temperature, not concentrations at equilibrium.

56. Which factor does not affect the value of the equilibrium constant $K$?

    Concentration of reactants or products. $K$ depends only on temperature, not concentrations at equilibrium.

57. State the effect of adding a reactant on the equilibrium position.

    Shifts right to form more products. Le Chatelier's principle: system responds by consuming the added reactant.

58. For $2NO_2(g) \rightleftharpoons N_2O_4(g)$, what happens to $K_c$ if temperature is increased?

    $K_c$ decreases. Higher temperature opposes exothermic reactions, decreasing $K_c$.

59. What is the effect of pressure decrease on equilibrium of $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$?

    Shifts left. Lower pressure favors the side with more gas molecules (4 vs 2).

60. What is the effect of a catalyst on the equilibrium constant $K$?

    No effect. Catalysts only affect reaction rate, not equilibrium position or $K$.

61. For the reaction $C(s) + CO_2(g) \rightleftharpoons 2CO(g)$, write the $K_c$ expression.

    $K_c = [CO]^2/[CO_2]$. Solid carbon is excluded, only gaseous species are included.

62. For $N_2O_4(g) \rightleftharpoons 2NO_2(g)$, how does increasing pressure affect equilibrium?

    Shifts towards $N_2O_4$. Higher pressure favors the side with fewer gas molecules (1 vs 2).

63. Identify the expression for $K_c$ for $H_2O(l) \rightleftharpoons H^+(aq) + OH^-(aq)$.

    $K_w = [H^+][OH^-]$. Auto-ionization of water uses $K_w$ notation for the equilibrium constant.

64. What is the equilibrium constant expression for $2H_2O_2(aq) \rightleftharpoons 2H_2O(l) + O_2(g)$?

    $K_c = [O_2]$. Liquids are excluded from equilibrium expressions, only gases included.

65. What is the equilibrium expression for $H_2(g) + S(s) \rightleftharpoons H_2S(g)$?

    $K_c = [H_2S]$. Solids are excluded from equilibrium expressions, only gases included.

66. What is the effect of increasing temperature on the equilibrium position of an exothermic reaction?

    Shifts towards reactants. Higher temperature opposes exothermic reactions, favoring the reverse direction.

67. Identify the equilibrium expression for the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$.

    $K_c = \frac{[NH_3]^2}{[N_2][H_2]^3}$. Products over reactants, each raised to their stoichiometric coefficients.

68. Which factor does not affect the value of the equilibrium constant $K$?

    Concentration of reactants or products. $K$ depends only on temperature, not concentrations at equilibrium.

69. Find $K_c$ for the reaction $2A \rightleftharpoons B + C$ if $[A] = 0.5M$, $[B] = 1M$, $[C] = 1M$.

    $K_c = \frac{1 \times 1}{0.5^2}$. $K_c = \frac{[B][C]}{[A]^2} = \frac{1 \times 1}{0.5^2} = 4$.

70. If $[N_2] = 2M$, $[H_2] = 1M$, $[NH_3] = 0.5M$, find $Q_c$ for $N_2 + 3H_2 \rightleftharpoons 2NH_3$.

    $Q_c = \frac{0.5^2}{2 \times 1^3}$. $Q_c = \frac{[NH_3]^2}{[N_2][H_2]^3} = \frac{0.5^2}{2 \times 1^3} = \frac{0.25}{2}$.

71. Identify the equilibrium expression for the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$.

    $K_c = \frac{[NH_3]^2}{[N_2][H_2]^3}$. Products over reactants, each raised to their stoichiometric coefficients.

72. What is the effect of a catalyst on the equilibrium constant $K$?

    No effect. Catalysts only affect reaction rate, not equilibrium position or $K$.

73. What is the equilibrium constant expression for $2H_2O_2(aq) \rightleftharpoons 2H_2O(l) + O_2(g)$?

    $K_c = [O_2]$. Liquids are excluded from equilibrium expressions, only gases included.

74. State the $K_c$ expression for the reaction $C_2H_4(g) + H_2(g) \rightleftharpoons C_2H_6(g)$.

    $K_c = \frac{[C_2H_6]}{[C_2H_4][H_2]}$. Products over reactants, each raised to their stoichiometric coefficients.

75. Find $K_c$ for $A \rightleftharpoons 2B$ if $[A] = 1M$, $[B] = 0.5M$ at equilibrium.

    $K_c = \frac{0.5^2}{1}$. $K_c = \frac{[B]^2}{[A]} = \frac{0.5^2}{1} = 0.25$.

76. Determine the equilibrium constant $K_c$ for the reaction $2NO_2(g) \rightleftharpoons N_2O_4(g)$ given $[NO_2] = 0.5M$, $[N_2O_4] = 0.2M$.

    $K_c = 0.8$. $K_c = \frac{[N_2O_4]}{[NO_2]^2} = \frac{0.2}{0.5^2} = 0.8$.

77. If $K_c = 1.5$ for the reaction $A \rightleftharpoons B$, what is the value of $K_c$ for $B \rightleftharpoons A$?

    $K_c = \frac{1}{1.5}$. For the reverse reaction, $K_c$ becomes the reciprocal of the original.

78. What is the effect of increasing temperature on the equilibrium position of an exothermic reaction?

    Shifts towards reactants. Higher temperature opposes exothermic reactions, favoring the reverse direction.

79. For the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$, what is the expression for $K_p$?

    $K_p = \frac{P_{SO_3}^2}{P_{SO_2}^2 P_{O_2}}$. Products over reactants using partial pressures, raised to coefficients.

80. Given $K_c = 10$ for $A \rightleftharpoons B$, find $K_c$ for $2A \rightleftharpoons 2B$.

    $K_c = 10^2$. When reaction is doubled, equilibrium constant is squared.

81. Find $K_c$ for $A \rightleftharpoons 2B$ if $[A] = 1M$, $[B] = 0.5M$ at equilibrium.

    $K_c = \frac{0.5^2}{1}$. $K_c = \frac{[B]^2}{[A]} = \frac{0.5^2}{1} = 0.25$.

82. State the effect of a temperature decrease on an endothermic reaction's equilibrium position.

    Shifts towards reactants. Lower temperature opposes endothermic reactions, favoring reactant formation.

83. What is the effect of pressure decrease on equilibrium of $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$?

    Shifts left. Lower pressure favors the side with more gas molecules (4 vs 2).

84. State the effect of a temperature decrease on an endothermic reaction's equilibrium position.

    Shifts towards reactants. Lower temperature opposes endothermic reactions, favoring reactant formation.

85. For the reaction $2NO(g) + O_2(g) \rightleftharpoons 2NO_2(g)$, what is the $K_p$ expression?

    $K_p = \frac{P_{NO_2}^2}{P_{NO}^2 P_{O_2}}$. Products over reactants using partial pressures, raised to coefficients.

86. How does a decrease in the concentration of $O_2$ affect equilibrium in $2SO_2 + O_2 \rightleftharpoons 2SO_3$?

    Shifts left. Removing reactant shifts equilibrium to replace it via reverse reaction.

87. For $A \rightleftharpoons B + C$, if $K_c = 2$, what is $K_c$ for $B + C \rightleftharpoons A$?

    $K_c = \frac{1}{2}$. For the reverse reaction, $K_c$ becomes the reciprocal of the original.

88. What is the expression for the equilibrium constant $K_c$ for the reaction $aA + bB \rightleftharpoons cC + dD$?

    $K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$. Products in numerator raised to their coefficients, reactants in denominator.

89. For the endothermic reaction $A \rightleftharpoons B$, what is the effect of increasing temperature?

    Shifts towards $B$. Higher temperature favors endothermic reactions (heat-absorbing direction).

90. For the reaction $PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)$, what is the effect of decreasing temperature?

    Shifts towards $PCl_5$. Lower temperature favors the exothermic direction (heat-producing side).

91. Given $K_c = 10$ for $A \rightleftharpoons B$, find $K_c$ for $2A \rightleftharpoons 2B$.

    $K_c = 10^2$. When reaction is doubled, equilibrium constant is squared.

92. What happens to the equilibrium position when pressure is increased for the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$?

    Shifts towards $NH_3$. Higher pressure favors the side with fewer gas molecules (2 vs 4).

93. For the reaction $PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)$, what is the effect of decreasing temperature?

    Shifts towards $PCl_5$. Lower temperature favors the exothermic direction (heat-producing side).

94. For $A \rightleftharpoons B + C$, if $K_c = 2$, what is $K_c$ for $B + C \rightleftharpoons A$?

    $K_c = \frac{1}{2}$. For the reverse reaction, $K_c$ becomes the reciprocal of the original.

95. State the unit for $K_c$ for the reaction $aA + bB \rightleftharpoons cC + dD$ if $c+d = a+b$.

    Unitless. When total moles of products equals total moles of reactants.

96. What is the effect of adding a product on the position of equilibrium?

    Shifts left to form more reactants. Le Chatelier's principle: system responds by consuming the added product.

97. What is the effect of adding an inert gas at constant volume on the equilibrium of a reaction?

    No effect. Inert gas doesn't participate and doesn't change partial pressures.

98. What happens to $K_c$ of an exothermic reaction when temperature increases?

    $K_c$ decreases. Higher temperature opposes exothermic reactions, decreasing $K_c$.

99. What happens to $K_c$ of an exothermic reaction when temperature increases?

    $K_c$ decreases. Higher temperature opposes exothermic reactions, decreasing $K_c$.

100. For the reaction $C(s) + CO_2(g) \rightleftharpoons 2CO(g)$, write the $K_c$ expression.

     $K_c = [CO]^2/[CO_2]$. Solid carbon is excluded, only gaseous species are included.

101. For the endothermic reaction $A \rightleftharpoons B$, what is the effect of increasing temperature?

     Shifts towards $B$. Higher temperature favors endothermic reactions (heat-absorbing direction).

102. For $A \rightleftharpoons B + C$, if $K_c = 2$, what is $K_c$ for $B + C \rightleftharpoons A$?

     $K_c = \frac{1}{2}$. For the reverse reaction, $K_c$ becomes the reciprocal of the original.

103. What is the expression for the equilibrium constant $K_c$ for the reaction $aA + bB \rightleftharpoons cC + dD$?

     $K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$. Products in numerator raised to their coefficients, reactants in denominator.

104. For $2NO_2(g) \rightleftharpoons N_2O_4(g)$, what happens to $K_c$ if temperature is increased?

     $K_c$ decreases. Higher temperature opposes exothermic reactions, decreasing $K_c$.

105. Determine the equilibrium constant $K_c$ for the reaction $2NO_2(g) \rightleftharpoons N_2O_4(g)$ given $[NO_2] = 0.5M$, $[N_2O_4] = 0.2M$.

     $K_c = 0.8$. $K_c = \frac{[N_2O_4]}{[NO_2]^2} = \frac{0.2}{0.5^2} = 0.8$.

106. For the reaction $CH_4(g) + 2O_2(g) \rightleftharpoons CO_2(g) + 2H_2O(g)$, write the $K_c$ expression.

     $K_c = \frac{[CO_2][H_2O]^2}{[CH_4][O_2]^2}$. Products over reactants, each raised to their stoichiometric coefficients.

107. Identify the equilibrium expression for $CO(g) + Cl_2(g) \rightleftharpoons COCl_2(g)$.

     $K_c = \frac{[COCl_2]}{[CO][Cl_2]}$. Products over reactants, each raised to their stoichiometric coefficients.

108. What is the effect of adding a product on the position of equilibrium?

     Shifts left to form more reactants. Le Chatelier's principle: system responds by consuming the added product.

109. How does a decrease in the concentration of $O_2$ affect equilibrium in $2SO_2 + O_2 \rightleftharpoons 2SO_3$?

     Shifts left. Removing reactant shifts equilibrium to replace it via reverse reaction.

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