AP Chemistry Flashcards: Calculating Equilibrium Concentrations

What this deck covers

This deck focuses on Calculating Equilibrium Concentrations, giving you a quick way to review the definitions, rules, and examples that matter most for AP Chemistry.

How to use these flashcards

Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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