AP Chemistry Flashcards: Properties Of The Equilibrium Constant

What this deck covers

This deck focuses on Properties Of 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 does $Q<K$ indicate about the direction the system will shift to reach equilibrium?

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

Flashcard 2: What is $\Delta n$ in the formula $K_p=K_c(RT)^{\Delta n}$ for a gas-phase reaction?

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

Flashcard 3: What is the general equilibrium constant expression $K_p$ 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 gases.

Flashcard 4: Which species are omitted from $K$ expressions for heterogeneous equilibria: pure solids, pure liquids, or both?

Answer: Both pure solids and pure liquids are omitted. Their activities are constant at 1, so they don't affect K.

Flashcard 5: If $\Delta n_{\text{gas}}=0$, what is the relationship between $K_p$ and $K_c$?

Answer: $K_p=K_c$. When gas moles don't change, $(RT)^0=1$.

Flashcard 6: What is the general rule for how $K$ changes when a reaction is reversed?

Answer: $K_{\text{rev}}=\frac{1}{K}$. Reversing flips products and reactants, so K becomes its reciprocal.

Flashcard 7: What is the equilibrium-constant expression for $aA+bB\rightleftharpoons cC+dD$ using partial pressures?

Answer: $K_p=\frac{(P_C)^c(P_D)^d}{(P_A)^a(P_B)^b}$. Same form as $K_c$ but uses partial pressures instead of concentrations.

Flashcard 8: What is the general rule for how $K$ changes when all coefficients are multiplied by $n$?

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

Flashcard 9: What is the general rule for how $K$ changes 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 10: What is the general equilibrium constant expression $K_c$ for $aA+bB\rightleftharpoons cC+dD$?

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

Flashcard 11: Identify the correct $K$ when $K=3.0$ and the balanced equation coefficients are doubled.

Answer: $9.0$. $K_{\text{new}} = 3.0^2 = 9.0$ when coefficients are doubled.

Flashcard 12: What happens to $K$ when the initial concentrations or partial pressures are changed at constant temperature?

Answer: $K$ is unchanged. K depends only on temperature, not on initial conditions.

Flashcard 13: Find $K$ for $2\times$ the reaction if the original reaction has $K=3.0\times10^2$.

Answer: $K_{\text{new}}=9.0\times10^4$. $K_{\text{new}}=(3.0\times10^2)^2=9.0\times10^4$.

Flashcard 14: How are equilibrium constants combined when reactions are added to give an overall reaction?

Answer: $K_{\text{overall}}=K_1\times K_2\times\cdots$. Adding reactions multiplies their equilibrium constants.

Flashcard 15: What is the activity value used for any pure solid or pure liquid in an equilibrium expression?

Answer: $1$. Pure solids and liquids have unit activity by definition.

Flashcard 16: Identify the equilibrium direction if $K=2.0\times10^3$ for a reaction written as products over reactants.

Answer: Product-favored (equilibrium lies to the right). $K>>1$ means products predominate at equilibrium.

Flashcard 17: What is the relationship between $K_p$ and $K_c$ for gases 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 18: How does $K$ change if all coefficients in the balanced reaction are multiplied by $n$?

Answer: $K_{\text{new}}=K^n$. Each species' exponent multiplies by $n$, so $K$ raises to $n$.

Flashcard 19: Which species are omitted from $K$ for a heterogeneous equilibrium: pure solids, pure liquids, or aqueous solutes?

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

Flashcard 20: Identify the correct $K_p$ if $K_c=0.50$, $\Delta n=2$, and $RT=25$ for an ideal-gas reaction.

Answer: $312.5$. $K_p = 0.50 × 25^2 = 312.5$ using $K_p = K_c(RT)^{\Delta n}$

Flashcard 21: Identify the correct $K$ when $K=4.0$ for $A\rightleftharpoons B$ and the reaction is reversed.

Answer: $0.25$. $K_{\text{rev}} = \frac{1}{4.0} = 0.25$ by the reversal rule.

Flashcard 22: What is $\Delta n_{\text{gas}}$ for $K_p=K_c(RT)^{\Delta n_{\text{gas}}}$?

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

Flashcard 23: What is the sign of $\Delta G^\circ$ when $K<1$ at a given temperature?

Answer: $\Delta G^\circ>0$. Since $\ln K < 0$ when $K < 1$, $\Delta G^\circ$ must be positive.

Flashcard 24: What happens to $K$ when a catalyst is added to a system at constant temperature?

Answer: $K$ is unchanged. Catalysts speed up both forward and reverse reactions equally.

Flashcard 25: Find $K_{\text{rev}}$ if $K=5.0\times10^{-4}$ for the forward reaction.

Answer: $K_{\text{rev}}=2.0\times10^3$. $K_{\text{rev}}=\frac{1}{5.0\times10^{-4}}=2.0\times10^3$.

Flashcard 26: What is the sign of $\Delta G^\circ$ when $K>1$ at a given temperature?

Answer: $\Delta G^\circ<0$. Since $\ln K > 0$ when $K > 1$, $\Delta G^\circ$ must be negative.

Flashcard 27: Identify the correct $K$ for the overall reaction if $K_1=2.0\times10^3$ and $K_2=5.0\times10^{-2}$ are added.

Answer: $1.0\times10^2$. $K_{\text{overall}} = (2.0×10^3)(5.0×10^{-2}) = 1.0×10^2$

Flashcard 28: How does $K$ change if the balanced reaction is reversed?

Answer: $K_{\text{rev}}=\frac{1}{K}$. Reversing swaps products and reactants, inverting $K$.

Flashcard 29: Identify the equilibrium direction if $K=4.0\times10^{-6}$ for a reaction written as products over reactants.

Answer: Reactant-favored (equilibrium lies to the left). $K<<1$ means reactants predominate at equilibrium.

Flashcard 30: What is the only common experimental change that can change $K$ for a given reaction?

Answer: Changing temperature. K is a function of temperature only for a given reaction.

Flashcard 31: What does $Q>K$ indicate about the direction the system will shift to reach equilibrium?

Answer: Shifts left (toward reactants). System has too many products, needs more reactants.

Flashcard 32: Find $K$ for $\frac{1}{2}\times$ the reaction if the original reaction has $K=1.6\times10^{-5}$.

Answer: $K_{\text{new}}=4.0\times10^{-3}$. $K_{\text{new}}=(1.6\times10^{-5})^{1/2}=4.0\times10^{-3}$.

Flashcard 33: What is the relationship between $K_p$ and $K_c$ for ideal gases in terms of $\Delta n$?

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

Flashcard 34: How does $K$ change if all coefficients in the balanced reaction are divided by $n$?

Answer: $K_{\text{new}}=K^{\frac{1}{n}}$. Each exponent divides by $n$, so $K$ raises to $\frac{1}{n}$.

Flashcard 35: What is the relationship between $K$ and $\Delta G^\circ$ at a given temperature?

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

Flashcard 36: What does $Q=K$ indicate about the system?

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

Flashcard 37: What is the reaction quotient expression $Q_c$ for $aA+bB\rightleftharpoons cC+dD$?

Answer: $Q_c=\frac{[C]^c[D]^d}{[A]^a[B]^b}$. Same form as $K_c$ but uses actual concentrations.

Flashcard 38: What is the equilibrium-constant expression for $aA+bB\rightleftharpoons cC+dD$ using concentrations?

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

Flashcard 39: What is the value of the activity used for any pure solid or pure liquid in an equilibrium expression?

Answer: $a=1$. By definition, pure solids/liquids have unit activity.

Flashcard 40: Identify the correct $K$ if $\Delta G^\circ=0$ at a given temperature.

Answer: $1$. When $\Delta G^\circ = 0$, then $\ln K = 0$, so $K = 1$.