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AP Chemistry Flashcards: Calculating Equilibrium Concentrations

Study Calculating Equilibrium Concentrations in AP Chemistry with focused flashcards that help you recognize the idea, recall the key rule, and apply it in practice-style prompts.

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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.

AP Chemistry Flashcards: Calculating Equilibrium Concentrations

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QUESTION

Find KcK_cKc​ for A⇌2BA \rightleftharpoons 2BA⇌2B if [A]=1M[A] = 1M[A]=1M, [B]=0.5M[B] = 0.5M[B]=0.5M at equilibrium.

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ANSWER

Kc=0.521K_c = \frac{0.5^2}{1}Kc​=10.52​. Kc=[B]2[A]=0.521=0.25K_c = \frac{[B]^2}{[A]} = \frac{0.5^2}{1} = 0.25Kc​=[A][B]2​=10.52​=0.25.

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Flashcard 1: Find KcK_cKc​ for A⇌2BA \rightleftharpoons 2BA⇌2B if [A]=1M[A] = 1M[A]=1M, [B]=0.5M[B] = 0.5M[B]=0.5M at equilibrium.

Answer: Kc=0.521K_c = \frac{0.5^2}{1}Kc​=10.52​. Kc=[B]2[A]=0.521=0.25K_c = \frac{[B]^2}{[A]} = \frac{0.5^2}{1} = 0.25Kc​=[A][B]2​=10.52​=0.25.

Flashcard 2: For the reaction 2NO(g)+O2(g)⇌2NO2(g)2NO(g) + O_2(g) \rightleftharpoons 2NO_2(g)2NO(g)+O2​(g)⇌2NO2​(g), what is the KpK_pKp​ expression?

Answer: Kp=PNO22PNO2PO2K_p = \frac{P_{NO_2}^2}{P_{NO}^2 P_{O_2}}Kp​=PNO2​PO2​​PNO2​2​​. Products over reactants using partial pressures, raised to coefficients.

Flashcard 3: How does a decrease in the concentration of O2O_2O2​ affect equilibrium in 2SO2+O2⇌2SO32SO_2 + O_2 \rightleftharpoons 2SO_32SO2​+O2​⇌2SO3​?

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

Flashcard 4: 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 5: For the endothermic reaction A⇌BA \rightleftharpoons BA⇌B, what is the effect of increasing temperature?

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

Flashcard 6: 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 7: For 2NO2(g)⇌N2O4(g)2NO_2(g) \rightleftharpoons N_2O_4(g)2NO2​(g)⇌N2​O4​(g), what happens to KcK_cKc​ if temperature is increased?

Answer: KcK_cKc​ decreases. Higher temperature opposes exothermic reactions, decreasing KcK_cKc​.

Flashcard 8: What is the equilibrium constant expression for 2H2O2(aq)⇌2H2O(l)+O2(g)2H_2O_2(aq) \rightleftharpoons 2H_2O(l) + O_2(g)2H2​O2​(aq)⇌2H2​O(l)+O2​(g)?

Answer: Kc=[O2]K_c = [O_2]Kc​=[O2​]. Liquids are excluded from equilibrium expressions, only gases included.

Flashcard 9: What is the effect of pressure decrease on equilibrium of N2(g)+3H2(g)⇌2NH3(g)N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)N2​(g)+3H2​(g)⇌2NH3​(g)?

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

Flashcard 10: Calculate KcK_cKc​ for H2+I2⇌2HIH_2 + I_2 \rightleftharpoons 2HIH2​+I2​⇌2HI with [H2]=1M[H_2] = 1M[H2​]=1M, [I2]=1M[I_2] = 1M[I2​]=1M, [HI]=2M[HI] = 2M[HI]=2M.

Answer: Kc=41K_c = \frac{4}{1}Kc​=14​. Kc=[HI]2[H2][I2]=221×1=4K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{2^2}{1 \times 1} = 4Kc​=[H2​][I2​][HI]2​=1×122​=4.

Flashcard 11: What is the equilibrium expression for H2(g)+S(s)⇌H2S(g)H_2(g) + S(s) \rightleftharpoons H_2S(g)H2​(g)+S(s)⇌H2​S(g)?

Answer: Kc=[H2S]K_c = [H_2S]Kc​=[H2​S]. Solids are excluded from equilibrium expressions, only gases included.

Flashcard 12: For the reaction PCl5(g)⇌PCl3(g)+Cl2(g)PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)PCl5​(g)⇌PCl3​(g)+Cl2​(g), what is the effect of decreasing temperature?

Answer: Shifts towards PCl5PCl_5PCl5​. Lower temperature favors the exothermic direction (heat-producing side).

Flashcard 13: 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 14: For N2O4(g)⇌2NO2(g)N_2O_4(g) \rightleftharpoons 2NO_2(g)N2​O4​(g)⇌2NO2​(g), how does increasing pressure affect equilibrium?

Answer: Shifts towards N2O4N_2O_4N2​O4​. Higher pressure favors the side with fewer gas molecules (1 vs 2).

Flashcard 15: Identify the equilibrium expression for CO(g)+Cl2(g)⇌COCl2(g)CO(g) + Cl_2(g) \rightleftharpoons COCl_2(g)CO(g)+Cl2​(g)⇌COCl2​(g).

Answer: Kc=[COCl2][CO][Cl2]K_c = \frac{[COCl_2]}{[CO][Cl_2]}Kc​=[CO][Cl2​][COCl2​]​. Products over reactants, each raised to their stoichiometric coefficients.

Flashcard 16: If Kc=1.5K_c = 1.5Kc​=1.5 for the reaction A⇌BA \rightleftharpoons BA⇌B, what is the value of KcK_cKc​ for B⇌AB \rightleftharpoons AB⇌A?

Answer: Kc=11.5K_c = \frac{1}{1.5}Kc​=1.51​. For the reverse reaction, KcK_cKc​ becomes the reciprocal of the original.

Flashcard 17: For the reaction H2(g)+I2(g)⇌2HI(g)H_2(g) + I_2(g) \rightleftharpoons 2HI(g)H2​(g)+I2​(g)⇌2HI(g), find KcK_cKc​ if [HI]=2M[HI] = 2M[HI]=2M, [H2]=1M[H_2] = 1M[H2​]=1M, [I2]=1M[I_2] = 1M[I2​]=1M.

Answer: Kc=4K_c = 4Kc​=4. Kc=[HI]2[H2][I2]=221×1=4K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{2^2}{1 \times 1} = 4Kc​=[H2​][I2​][HI]2​=1×122​=4.

Flashcard 18: State the KcK_cKc​ expression for the reaction C2H4(g)+H2(g)⇌C2H6(g)C_2H_4(g) + H_2(g) \rightleftharpoons C_2H_6(g)C2​H4​(g)+H2​(g)⇌C2​H6​(g).

Answer: Kc=[C2H6][C2H4][H2]K_c = \frac{[C_2H_6]}{[C_2H_4][H_2]}Kc​=[C2​H4​][H2​][C2​H6​]​. Products over reactants, each raised to their stoichiometric coefficients.

Flashcard 19: What happens to KcK_cKc​ of an exothermic reaction when temperature increases?

Answer: KcK_cKc​ decreases. Higher temperature opposes exothermic reactions, decreasing KcK_cKc​.

Flashcard 20: For the reaction C(s)+CO2(g)⇌2CO(g)C(s) + CO_2(g) \rightleftharpoons 2CO(g)C(s)+CO2​(g)⇌2CO(g), write the KcK_cKc​ expression.

Answer: Kc=[CO]2/[CO2]K_c = [CO]^2/[CO_2]Kc​=[CO]2/[CO2​]. Solid carbon is excluded, only gaseous species are included.

Flashcard 21: Find KcK_cKc​ for the reaction 2A⇌B+C2A \rightleftharpoons B + C2A⇌B+C if [A]=0.5M[A] = 0.5M[A]=0.5M, [B]=1M[B] = 1M[B]=1M, [C]=1M[C] = 1M[C]=1M.

Answer: Kc=1×10.52K_c = \frac{1 \times 1}{0.5^2}Kc​=0.521×1​. Kc=[B][C][A]2=1×10.52=4K_c = \frac{[B][C]}{[A]^2} = \frac{1 \times 1}{0.5^2} = 4Kc​=[A]2[B][C]​=0.521×1​=4.

Flashcard 22: Identify the equilibrium expression for the reaction N2(g)+3H2(g)⇌2NH3(g)N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)N2​(g)+3H2​(g)⇌2NH3​(g).

Answer: Kc=[NH3]2[N2][H2]3K_c = \frac{[NH_3]^2}{[N_2][H_2]^3}Kc​=[N2​][H2​]3[NH3​]2​. Products over reactants, each raised to their stoichiometric coefficients.

Flashcard 23: Given Kc=10K_c = 10Kc​=10 for A⇌BA \rightleftharpoons BA⇌B, find KcK_cKc​ for 2A⇌2B2A \rightleftharpoons 2B2A⇌2B.

Answer: Kc=102K_c = 10^2Kc​=102. When reaction is doubled, equilibrium constant is squared.

Flashcard 24: What is the expression for the equilibrium constant KcK_cKc​ for the reaction aA+bB⇌cC+dDaA + bB \rightleftharpoons cC + dDaA+bB⇌cC+dD?

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

Flashcard 25: What happens to the equilibrium position when pressure is increased for the reaction N2(g)+3H2(g)⇌2NH3(g)N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)N2​(g)+3H2​(g)⇌2NH3​(g)?

Answer: Shifts towards NH3NH_3NH3​. Higher pressure favors the side with fewer gas molecules (2 vs 4).

Flashcard 26: 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 27: For the reaction 2SO2(g)+O2(g)⇌2SO3(g)2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)2SO2​(g)+O2​(g)⇌2SO3​(g), what is the expression for KpK_pKp​?

Answer: Kp=PSO32PSO22PO2K_p = \frac{P_{SO_3}^2}{P_{SO_2}^2 P_{O_2}}Kp​=PSO2​2​PO2​​PSO3​2​​. Products over reactants using partial pressures, raised to coefficients.

Flashcard 28: What is the effect of a catalyst on the equilibrium constant KKK?

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

Flashcard 29: For the reaction CH4(g)+2O2(g)⇌CO2(g)+2H2O(g)CH_4(g) + 2O_2(g) \rightleftharpoons CO_2(g) + 2H_2O(g)CH4​(g)+2O2​(g)⇌CO2​(g)+2H2​O(g), write the KcK_cKc​ expression.

Answer: Kc=[CO2][H2O]2[CH4][O2]2K_c = \frac{[CO_2][H_2O]^2}{[CH_4][O_2]^2}Kc​=[CH4​][O2​]2[CO2​][H2​O]2​. Products over reactants, each raised to their stoichiometric coefficients.

Flashcard 30: Determine the equilibrium constant KcK_cKc​ for the reaction 2NO2(g)⇌N2O4(g)2NO_2(g) \rightleftharpoons N_2O_4(g)2NO2​(g)⇌N2​O4​(g) given [NO2]=0.5M[NO_2] = 0.5M[NO2​]=0.5M, [N2O4]=0.2M[N_2O_4] = 0.2M[N2​O4​]=0.2M.

Answer: Kc=0.8K_c = 0.8Kc​=0.8. Kc=[N2O4][NO2]2=0.20.52=0.8K_c = \frac{[N_2O_4]}{[NO_2]^2} = \frac{0.2}{0.5^2} = 0.8Kc​=[NO2​]2[N2​O4​]​=0.520.2​=0.8.