Chemistry: Equilibrium Constants from Concentration Tables
Using equilibrium and ICE tables to calculate Kc
Chemistry: Equilibrium Constants from Concentration Tables
Using equilibrium and ICE tables to calculate Kc
Chemistry - Grade 9-12
- 1
For the reaction A(g) ⇌ B(g), the equilibrium concentrations are [A] = 0.25 M and [B] = 0.75 M. Write the Kc expression and calculate Kc.
Products go in the numerator and reactants go in the denominator.
The expression is Kc = [B]/[A]. Substituting the values gives Kc = 0.75/0.25 = 3.0. - 2
For the reaction H2(g) + I2(g) ⇌ 2HI(g), the equilibrium concentration table is: [H2] = 0.020 M, [I2] = 0.020 M, and [HI] = 0.160 M. Calculate Kc.
The expression is Kc = [HI]^2/([H2][I2]). Substituting the values gives Kc = (0.160)^2/(0.020 × 0.020) = 0.0256/0.000400 = 64. - 3
For the reaction N2O4(g) ⇌ 2NO2(g), the equilibrium concentrations are [N2O4] = 0.12 M and [NO2] = 0.30 M. Calculate Kc.
Use the coefficient 2 as an exponent on [NO2].
The expression is Kc = [NO2]^2/[N2O4]. Substituting the values gives Kc = (0.30)^2/0.12 = 0.090/0.12 = 0.75. - 4
For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), the equilibrium concentrations are [SO2] = 0.10 M, [O2] = 0.20 M, and [SO3] = 0.40 M. Calculate Kc.
The expression is Kc = [SO3]^2/([SO2]^2[O2]). Substituting the values gives Kc = (0.40)^2/((0.10)^2 × 0.20) = 0.16/0.0020 = 80. - 5
For the reaction PCl5(g) ⇌ PCl3(g) + Cl2(g), the initial concentrations are [PCl5] = 0.500 M, [PCl3] = 0 M, and [Cl2] = 0 M. At equilibrium, [PCl5] = 0.200 M. Use the concentration changes to find the equilibrium concentrations of PCl3 and Cl2, then calculate Kc.
The mole ratio is 1:1:1, so the amount of PCl5 lost equals the amount of each product formed.
PCl5 decreases by 0.300 M, so PCl3 and Cl2 each increase by 0.300 M. At equilibrium, [PCl3] = 0.300 M and [Cl2] = 0.300 M. The expression is Kc = ([PCl3][Cl2])/[PCl5], so Kc = (0.300 × 0.300)/0.200 = 0.45. - 6
For the reaction A(g) + 2B(g) ⇌ C(g), the equilibrium concentrations are [A] = 0.10 M, [B] = 0.20 M, and [C] = 0.80 M. Calculate Kc.
The expression is Kc = [C]/([A][B]^2). Substituting the values gives Kc = 0.80/(0.10 × (0.20)^2) = 0.80/0.0040 = 200. - 7
For the reaction CO(g) + Cl2(g) ⇌ COCl2(g), the equilibrium concentrations are [CO] = 0.050 M, [Cl2] = 0.100 M, and [COCl2] = 0.350 M. Calculate Kc.
Each coefficient is 1, so no concentration is squared.
The expression is Kc = [COCl2]/([CO][Cl2]). Substituting the values gives Kc = 0.350/(0.050 × 0.100) = 0.350/0.00500 = 70. - 8
For the reaction 2NOBr(g) ⇌ 2NO(g) + Br2(g), the initial concentrations are [NOBr] = 0.60 M, [NO] = 0 M, and [Br2] = 0 M. At equilibrium, [NOBr] = 0.20 M. Use an ICE table to calculate Kc.
If 2x of NOBr is used, then 2x of NO forms and x of Br2 forms.
NOBr decreases by 0.40 M. Since 2NOBr produces 2NO and 1Br2, [NO] increases by 0.40 M and [Br2] increases by 0.20 M. The equilibrium concentrations are [NOBr] = 0.20 M, [NO] = 0.40 M, and [Br2] = 0.20 M. The expression is Kc = ([NO]^2[Br2])/[NOBr]^2, so Kc = ((0.40)^2 × 0.20)/(0.20)^2 = 0.032/0.040 = 0.80. - 9
For the reaction H2(g) + CO2(g) ⇌ H2O(g) + CO(g), the equilibrium concentrations are [H2] = 0.15 M, [CO2] = 0.25 M, [H2O] = 0.35 M, and [CO] = 0.45 M. Calculate Kc.
The expression is Kc = ([H2O][CO])/([H2][CO2]). Substituting the values gives Kc = (0.35 × 0.45)/(0.15 × 0.25) = 0.1575/0.0375 = 4.2. - 10
For the reaction CaCO3(s) ⇌ CaO(s) + CO2(g), the equilibrium concentration of CO2 is 0.020 M. Write the Kc expression and calculate Kc.
Do not include pure solids or pure liquids in an equilibrium constant expression.
Pure solids are not included in the Kc expression. The expression is Kc = [CO2], so Kc = 0.020. - 11
For the reaction A(g) ⇌ 2B(g), the equilibrium concentrations are [A] = 0.40 M and [B] = 0.20 M. Calculate Kc for the forward reaction. Then calculate Kc for the reverse reaction, 2B(g) ⇌ A(g).
For the forward reaction, Kc = [B]^2/[A] = (0.20)^2/0.40 = 0.040/0.40 = 0.10. For the reverse reaction, Kc is the reciprocal, so Kc = 1/0.10 = 10. - 12
For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), the equilibrium concentrations are [N2] = 0.50 M, [H2] = 0.20 M, and [NH3] = 0.30 M. Calculate Kc.
The coefficient 3 on H2 becomes an exponent of 3 in the denominator.
The expression is Kc = [NH3]^2/([N2][H2]^3). Substituting the values gives Kc = (0.30)^2/(0.50 × (0.20)^3) = 0.090/(0.50 × 0.0080) = 0.090/0.0040 = 22.5. - 13
For the reaction A(g) + 2B(g) ⇌ 3C(g), the equilibrium concentrations are [A] = 0.250 M, [B] = 0.400 M, and [C] = 0.500 M. Calculate Kc.
The expression is Kc = [C]^3/([A][B]^2). Substituting the values gives Kc = (0.500)^3/(0.250 × (0.400)^2) = 0.125/(0.250 × 0.160) = 0.125/0.0400 = 3.125. - 14
For the reaction N2O4(g) ⇌ 2NO2(g), the initial concentration of N2O4 is 0.100 M and the initial concentration of NO2 is 0 M. At equilibrium, [NO2] = 0.080 M. Find [N2O4] at equilibrium, then calculate Kc.
Use the 1:2 mole ratio between N2O4 and NO2.
Because 2NO2 forms for every 1N2O4 used, forming 0.080 M NO2 means 0.040 M N2O4 was consumed. The equilibrium concentration of N2O4 is 0.100 M - 0.040 M = 0.060 M. The expression is Kc = [NO2]^2/[N2O4], so Kc = (0.080)^2/0.060 = 0.0064/0.060 = 0.107. - 15
For the reaction PCl3(g) + Cl2(g) ⇌ PCl5(g), the initial concentrations are [PCl3] = 0.40 M, [Cl2] = 0.40 M, and [PCl5] = 0 M. At equilibrium, [PCl5] = 0.25 M. Use an ICE table to calculate Kc.
All coefficients are 1, so the same concentration change applies to each substance.
If 0.25 M PCl5 forms, then 0.25 M PCl3 and 0.25 M Cl2 are consumed. The equilibrium concentrations are [PCl3] = 0.15 M, [Cl2] = 0.15 M, and [PCl5] = 0.25 M. The expression is Kc = [PCl5]/([PCl3][Cl2]), so Kc = 0.25/(0.15 × 0.15) = 0.25/0.0225 = 11.1.