AP Chemistry Flashcards: Reaction Quotient And Equilibrium Constant

What this deck covers

This deck focuses on Reaction Quotient And Equilibrium Constant, 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 $2NO_2(g) \rightleftharpoons N_2O_4(g)$, write the correct $Q_p$ expression.

Answer: $Q_p = \frac{P_{N_2O_4}}{(P_{NO_2})^2}$. Products over reactants with stoichiometric exponents.

Flashcard 2: Given $K = 50$ and current $Q = 200$ at the same $T$, which direction will the reaction shift to reach equilibrium?

Answer: Shift left (toward reactants). Since $Q > K$, reaction must decrease $Q$ by forming reactants.

Flashcard 3: For $N_2(g)+3H_2(g) \rightleftharpoons 2NH_3(g)$, what is $\Delta n_{\text{gas}}$?

Answer: $\Delta n_{\text{gas}} = 2-4 = -2$. Products have 2 moles gas, reactants have 4 moles gas.

Flashcard 4: What is the correct equilibrium constant name for an expression written with concentrations: $K_c$ or $K_p$?

Answer: $K_c$. Subscript 'c' indicates concentration-based equilibrium constant.

Flashcard 5: What happens to $K$ if all coefficients in the balanced equation are multiplied by $n$?

Answer: $K_{new}=K^n$. Each concentration term gets raised to the new coefficient.

Flashcard 6: Identify the correct reaction direction if $Q > K$ for the current mixture.

Answer: The reaction proceeds in reverse (toward reactants). More reactants must form to decrease $Q$ until it equals $K$.

Flashcard 7: What is the exponent on each term in $Q$ or $K$ relative to the balanced chemical equation?

Answer: Each exponent equals its stoichiometric coefficient. Reflects how many moles participate in the reaction.

Flashcard 8: For $A(s) + 2B(g) \rightleftharpoons C(g)$, what is the correct $K_p$ expression?

Answer: $K_p = \frac{P_C}{(P_B)^2}$. Solid A is omitted; only gas pressures appear in $K_p$.

Flashcard 9: If $K_c=5.0\times10^{-3}$, which side is favored at equilibrium: reactants or products?

Answer: Reactants are favored. $K<<1$ means equilibrium lies far to the left.

Flashcard 10: Which species are omitted from $Q$ and $K$ expressions: pure solids and pure liquids or aqueous solutes?

Answer: Pure solids and pure liquids are omitted. Their activities equal 1, so they don't affect the equilibrium expression.

Flashcard 11: State the formula for $Q_c$ for $aA + bB \rightleftharpoons cC + dD$ using molar concentrations.

Answer: $Q_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$. Products over reactants, each raised to its stoichiometric coefficient.

Flashcard 12: What is $\Delta n_{\text{gas}}$ in $K_p = K_c(RT)^{\Delta n_{\text{gas}}}$ for a balanced gas reaction?

Answer: $\Delta n_{\text{gas}} = \sum \nu_{\text{products,gas}}-\sum \nu_{\text{reactants,gas}}$. Change in moles of gas: products minus reactants.

Flashcard 13: For $CaCO_3(s)\rightleftharpoons CaO(s)+CO_2(g)$, what is the correct $K_p$ expression?

Answer: $K_p=P_{CO_2}$. Solids omitted; only gas pressure appears.

Flashcard 14: Identify the correct reaction direction if $Q < K$ for the current mixture.

Answer: The reaction proceeds forward (toward products). More products must form to increase $Q$ until it equals $K$.

Flashcard 15: What is the correct equilibrium constant name for an expression written with partial pressures: $K_c$ or $K_p$?

Answer: $K_p$. Subscript 'p' indicates pressure-based equilibrium constant.

Flashcard 16: Which variable(s) change the numerical value of $K$ for a given reaction: concentration, pressure, catalyst, or temperature?

Answer: Only temperature changes $K$. Concentrations, pressure, and catalysts don't affect $K$'s value.

Flashcard 17: Which species are omitted from $Q$ and $K$: pure solids, pure liquids, solutes, or gases?

Answer: Pure solids and pure liquids are omitted. Their activities equal 1, so they don't affect the ratio.

Flashcard 18: What is the relationship between $K$ and $Q$ at equilibrium for a reaction mixture?

Answer: At equilibrium, $Q = K$. System reaches equilibrium when reaction quotient equals equilibrium constant.

Flashcard 19: State the relationship between $K$ values when two reactions are added to give an overall reaction.

Answer: $K_{\text{overall}} = K_1K_2$. When reactions add, their equilibrium constants multiply.

Flashcard 20: State the expression for $K$ of the reverse reaction in terms of $K$ for the forward reaction.

Answer: $K_{\text{reverse}} = \frac{1}{K_{\text{forward}}}$. Products and reactants swap positions, inverting the ratio.

Flashcard 21: What is the expression for $Q_p$ for $aA(g)+bB(g)\rightleftharpoons cC(g)+dD(g)$ using partial pressures?

Answer: $Q_p=\frac{(P_C)^c(P_D)^d}{(P_A)^a(P_B)^b}$. Uses partial pressures instead of concentrations.

Flashcard 22: What happens to $K$ if the balanced equation is reversed?

Answer: $K_{rev}=\frac{1}{K}$. Reversing flips the fraction.

Flashcard 23: State the formula for $Q_p$ for $aA(g)+bB(g) \rightleftharpoons cC(g)+dD(g)$ using partial pressures.

Answer: $Q_p = \frac{(P_C)^c(P_D)^d}{(P_A)^a(P_B)^b}$. Uses partial pressures instead of concentrations for gas-phase species.

Flashcard 24: State the expression for $K$ when the entire balanced equation is multiplied by a factor $n$.

Answer: $K_{\text{new}} = K^n$. Each concentration term gets raised to the power $n$.

Flashcard 25: Identify the correct comparison rule: if $Q>K$, which direction does the reaction shift to reach equilibrium?

Answer: Shifts left (toward reactants). System has too many products relative to equilibrium.

Flashcard 26: State the formula relating $K_p$ and $K_c$ using $\Delta n_{gas}$ and $R$ and $T$.

Answer: $K_p=K_c(RT)^{\Delta n_{gas}}$. Converts between pressure and concentration units.

Flashcard 27: Identify the correct comparison rule: if $Q<K$, which direction does the reaction shift to reach equilibrium?

Answer: Shifts right (toward products). System needs more products to reach equilibrium.

Flashcard 28: What is the exponent on each term in a $Q$ or $K$ expression relative to the balanced chemical equation?

Answer: Each exponent equals its stoichiometric coefficient. Reflects how many moles of each species participate in the reaction.

Flashcard 29: What is $\Delta n_{gas}$ in $K_p=K_c(RT)^{\Delta n_{gas}}$ for $aA(g)\rightleftharpoons bB(g)$?

Answer: $\Delta n_{gas}=b-a$. Moles of gas products minus moles of gas reactants.

Flashcard 30: What is the definition of $K$ in terms of $Q$ for a reaction at equilibrium?

Answer: $K=Q$ at equilibrium. At equilibrium, reaction quotient equals equilibrium constant.

Flashcard 31: What is the correct conclusion if $Q = K$ for a reaction mixture at a fixed temperature?

Answer: The system is at equilibrium. No net change occurs; forward and reverse rates are equal.

Flashcard 32: If $K_c=2.0\times10^{4}$, which side is favored at equilibrium: reactants or products?

Answer: Products are favored. $K>>1$ means equilibrium lies far to the right.

Flashcard 33: Identify the correct comparison rule: if $Q=K$, what is the system's status?

Answer: At equilibrium (no net shift). Forward and reverse rates are equal.

Flashcard 34: Calculate $Q_c$ for $A\rightleftharpoons B$ if $[A]=0.50$ and $[B]=2.0$.

Answer: $Q_c=4.0$. $Q_c=\frac{[B]}{[A]}=\frac{2.0}{0.50}=4.0$

Flashcard 35: State the relationship between $K_p$ and $K_c$ for gases in terms of $\Delta n_{\text{gas}}$.

Answer: $K_p = K_c(RT)^{\Delta n_{\text{gas}}}$. Converts between pressure and concentration units; $R = 0.08206$ L·atm/(mol·K).

Flashcard 36: For $2NO_2(g)\rightleftharpoons N_2O_4(g)$, what is the correct $Q_p$ expression?

Answer: $Q_p=\frac{P_{N_2O_4}}{(P_{NO_2})^2}$. Products over reactants with stoichiometric exponents.

Flashcard 37: For $H_2(g)+I_2(g)\rightleftharpoons 2HI(g)$, what is $\Delta n_{gas}$?

Answer: $\Delta n_{gas}=0$. 2 moles gas products minus 2 moles gas reactants.

Flashcard 38: Identify the shift for $A\rightleftharpoons B$ if $K_c=10$ and $Q_c=4$.

Answer: Shifts right (toward products). $Q<K$ means more products needed for equilibrium.

Flashcard 39: What is the expression for $Q_c$ for $aA+bB\rightleftharpoons cC+dD$ using concentrations?

Answer: $Q_c=\frac{[C]^c[D]^d}{[A]^a[B]^b}$. Products over reactants, each raised to its coefficient.

Flashcard 40: For $H_2(g)+I_2(g)\rightleftharpoons 2HI(g)$, what is the relationship between $K_p$ and $K_c$?

Answer: $K_p=K_c$. When $\Delta n_{gas}=0$, $(RT)^0=1$.