AP Chemistry Flashcards: Calculating The Equilibrium Constant

What this deck covers

This deck focuses on Calculating The 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: What is $\Delta n_{\text{gas}}$ for $2\text{SO}_2(g)+\text{O}_2(g)\rightleftharpoons^2\text{SO}_3(g)$?

Answer: $\Delta n_{\text{gas}}=-1$. $2$ moles gas products minus $3$ moles gas reactants equals $-1$.

Flashcard 2: What is $K_p$ from $K_c=0.50$ at $T=300\,K$ for $N_2O_4(g)\rightleftharpoons 2NO_2(g)$? Use $R=0.0821$.

Answer: $K_p=0.50\,(0.0821\cdot 300)^1=12$. $\Delta n=2-1=1$; multiply $K_c$ by $(RT)^1$.

Flashcard 3: Calculate $K_p$ for $2A(g)\rightleftharpoons B(g)$ if $P_A=0.50\,\text{atm}$ and $P_B=2.0\,\text{atm}$ at equilibrium.

Answer: $K_p=8.0$. $K_p = P_B/(P_A)^2 = 2.0/(0.50)^2 = 2.0/0.25 = 8.0$

Flashcard 4: What is $K$ for the reverse reaction if $K=4.0\times10^{-3}$ for the forward reaction?

Answer: $K_{\text{reverse}}=2.5\times10^2$. $K_{reverse} = 1 ÷ (4.0×10^{-3}) = 2.5×10^2$

Flashcard 5: State the relationship between $K_p$ and $K_c$ using $\Delta n_{\text{gas}}$.

Answer: $K_p=K_c(RT)^{\Delta n_{\text{gas}}}$. Relates pressure and concentration constants via ideal gas law.

Flashcard 6: What is the relationship between $\Delta G^\circ$, $K$, $R$, and $T$?

Answer: $\Delta G^\circ=-RT\ln K$. Links thermodynamic favorability to equilibrium position.

Flashcard 7: Identify the correct $K_c$ for $\text{N}_2(g)+3\text{H}_2(g)\rightleftharpoons^2\text{NH}_3(g)$.

Answer: $K_c=\frac{[\text{NH}_3]^2}{[\text{N}_2][\text{H}_2]^3}$. Products over reactants with coefficients: $2$ for NH₃, $3$ for H₂.

Flashcard 8: What is $K$ for the reaction if the given reaction has $K=3.0$ and all coefficients are doubled?

Answer: $K_{\text{new}}=9.0$. Doubling coefficients squares $K$: $3.0^2 = 9.0$

Flashcard 9: What is $\Delta n_{\text{gas}}$ for $K_p=K_c(RT)^{\Delta n_{\text{gas}}}$?

Answer: $\Delta n_{\text{gas}}=\sum \nu_{\text{prod,gas}}-\sum \nu_{\text{react,gas}}$. Change in moles of gas: product coefficients minus reactant coefficients.

Flashcard 10: What is the numerical value of $K$ when $\Delta G^\circ=0$ at that temperature?

Answer: $K=1$. At equilibrium, forward and reverse rates are equal.

Flashcard 11: What is the value of $K$ for any reaction written as $X\rightleftharpoons X$?

Answer: $K=1$. Same species on both sides cancel out, leaving unity.

Flashcard 12: What is the equilibrium constant expression $K_c$ for $2NO_2(g)\rightleftharpoons N_2O_4(g)$?

Answer: $K_c=\frac{[N_2O_4]}{[NO_2]^2}$. Products over reactants with stoichiometric exponents.

Flashcard 13: Which species are omitted from $K$ expressions: pure solids, pure liquids, gases, or aqueous solutes?

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

Flashcard 14: What is the relationship between $K_c$ and $K_p$ using $\Delta n$ and $R,T$?

Answer: $K_p=K_c(RT)^{\Delta n}$. Relates concentration and pressure equilibrium constants.

Flashcard 15: Which temperature unit must be used in $K_p=K_c(RT)^{\Delta n}$ calculations?

Answer: $T$ must be in kelvin $(\text{K})$. Absolute temperature required for ideal gas calculations.

Flashcard 16: What is $K_c$ for $CaCO_3(s)\rightleftharpoons CaO(s)+CO_2(g)$?

Answer: $K_c=[CO_2]$. Solids omitted; only gas concentration appears.

Flashcard 17: Find $K_c$ if $K_p=2.0$, $\Delta n_{\text{gas}}=-2$, $T=400\,\text{K}$, and $R=0.0821$.

Answer: $K_c=2.15\times10^3$. $K_c = 2.0 ÷ (0.0821 × 400)^{-2} = 2.0 × 1079 = 2.15×10^3$

Flashcard 18: Identify the correct $K_c$ for $\text{CaCO}_3(s)\rightleftharpoons\text{CaO}(s)+\text{CO}_2(g)$.

Answer: $K_c=[\text{CO}_2]$. Solids omitted; only gaseous CO₂ appears in the expression.

Flashcard 19: What is the general expression for $K_p$ for $aA+bB\rightleftharpoons cC+dD$ (gases)?

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

Flashcard 20: Find $K_p$ if $K_c=0.50$, $\Delta n_{\text{gas}}=1$, $T=300\,\text{K}$, and $R=0.0821$.

Answer: $K_p=12.3$. $K_p = 0.50 × (0.0821 × 300)^1 = 0.50 × 24.63 = 12.3$

Flashcard 21: Which species are omitted from $K$ for a heterogeneous equilibrium involving pure solids and liquids?

Answer: Pure solids and pure liquids are omitted (activity $=1$). Their concentrations remain constant during reaction.

Flashcard 22: What is $K_c$ for $CH_3COOH(aq)\rightleftharpoons H^+(aq)+CH_3COO^-(aq)$?

Answer: $K_c=\frac{[H^+][CH_3COO^-]}{[CH_3COOH]}$. Weak acid dissociation with products over reactant.

Flashcard 23: Find $K_\text{overall}$ if reaction 2 is the reverse of a step with constant $K_2$ and reactions are added.

Answer: $K_\text{overall}=K_1\left(\frac{1}{K_2}\right)$. Reversing step 2 inverts its $K$ before multiplying.

Flashcard 24: What happens to $K$ when all coefficients in the reaction are multiplied by $n$?

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

Flashcard 25: What is the general expression for $K_p$ for $aA+bB\rightleftharpoons cC+dD$?

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

Flashcard 26: Identify $K_c$ for $2SO_2(g)+O_2(g)\rightleftharpoons 2SO_3(g)$.

Answer: $K_c=\frac{[SO_3]^2}{[SO_2]^2[O_2]}$. Products over reactants with stoichiometric powers.

Flashcard 27: What is $\Delta n$ (used in $K_p=K_c(RT)^{\Delta n}$) for gas-phase equilibria?

Answer: $\Delta n=\text{mol gas products}-\text{mol gas reactants}$. Change in moles of gas from reactants to products.

Flashcard 28: What is the general expression for $K_c$ for $aA+bB\rightleftharpoons cC+dD$?

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

Flashcard 29: What is the general expression for $K_c$ for $aA+bB\rightleftharpoons cC+dD$?

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

Flashcard 30: What is the equilibrium constant expression $K_p$ for $N_2(g)+3H_2(g)\rightleftharpoons 2NH_3(g)$?

Answer: $K_p=\frac{(P_{NH_3})^2}{P_{N_2}(P_{H_2})^3}$. Partial pressures raised to stoichiometric coefficients.

Flashcard 31: What is $K_p$ for $CaCO_3(s)\rightleftharpoons CaO(s)+CO_2(g)$?

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

Flashcard 32: What is $K$ for an overall reaction made by adding reactions with constants $K_1$ and $K_2$?

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

Flashcard 33: Calculate $K_c$ if at equilibrium $[A]=0.20\,M$, $[B]=0.30\,M$, $[C]=0.40\,M$ for $A+B\rightleftharpoons C$.

Answer: $K_c=\frac{0.40}{(0.20)(0.30)}=6.7$. Substitute equilibrium concentrations into $K_c$ expression.

Flashcard 34: Calculate $K_p$ if $P_{CO}=2.0\,atm$, $P_{H_2}=3.0\,atm$, $P_{CH_3OH}=1.5\,atm$ for $CO+2H_2\rightleftharpoons CH_3OH$.

Answer: $K_p=\frac{1.5}{(2.0)(3.0)^2}=0.083$. Substitute equilibrium pressures into $K_p$ expression.

Flashcard 35: What is the effect on $K$ when a reaction is reversed?

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

Flashcard 36: What happens to $K$ when the reaction is reversed?

Answer: $K_{\text{reverse}}=\frac{1}{K_{\text{forward}}}$. Reversing swaps products and reactants, inverting the fraction.

Flashcard 37: What is the effect on $K$ when all coefficients are multiplied by $n$?

Answer: $K_\text{new}=K^n$. Multiplying coefficients raises $K$ to that power.

Flashcard 38: Identify the correct $K_c$ for $\text{CH}_3\text{COOH}(aq)\rightleftharpoons\text{H}^+(aq)+\text{CH}_3\text{COO}^-(aq)$.

Answer: $K_c=\frac{[\text{H}^+][\text{CH}_3\text{COO}^-]}{[\text{CH}_3\text{COOH}]}$. Weak acid dissociation: products over undissociated acid.

Flashcard 39: Calculate $K_c$ for $A\rightleftharpoons^2B$ if $[A]=0.50\,\text{M}$ and $[B]=0.20\,\text{M}$ at equilibrium.

Answer: $K_c=0.080$. $K_c = [B]^2/[A] = (0.20)^2/0.50 = 0.040/0.50 = 0.080$

Flashcard 40: What is $K$ for the overall reaction if two steps have constants $K_1$ and $K_2$ and are added?

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