Practice: Partial pressures of the following equilibrium mixture at 955 K are: 130 torr methane, 92 torr hydrogen sulfide, 167 torr hydrogen gas and 532 torr carbon disulfide. What is the value of Kc at 955 K?
CH4(g) + 2 H2S(g) ⇌ 4 H2(g) + CS2(g)
Equilibrium Constants: Kp vs Kc
Concept #1: Kp vs Kc
Kp is used when dealing with equilibrium units in terms of pressure.
Kc is used when the equilibrium units are in molarity.
Using the above formula allows the conversion between Kp and Kc.
Example #1: Calculate Kp for the following reaction with Kc = 0.77 at 570 K:
2A (g) + B (s) ⇌ CD (g) + 3E (g)
Practice: Consider the hypothetical reaction: ? X(g) + 3 Y(g) ⇌ 3 Z(g), where Kp = 1.16 x 10–3 and Kc = 1.3 at 135°C. Find the value of the coefficient of X.
Concept #2: Value of ∆n
Determination of the value for Δn can help us determine if Kp is greater than, less than or equal to Kc.
Example #2: For the following reactions, identify whether Kp is greater than, less than or equal to Kc.
a) 4 NH3 (g) + 3 O2 (g) ⇌ 2 N2 (g) + 6 H2O (g) _____________
b) 2 HI (g) ⇌ H2 (g) + I2 (g) _____________
c) CO (g) + 3 H2 (g) ⇌ CH4 (g) + H2O (g) _____________
Practice: For which reaction(s) will Kp = Kc?
a) N2(g) + O2(g) ⇌ 2 NO(g)
b) CaCO3(s) ⇌ CaO(s) + CO2(g)
c) 2 CH4(g) ⇌ C2H2(g) + 3 H2(g)
d) H2O(g) + CO(g) ⇌ H2(g) + CO2(g)
Practice: Select the correct choice below for the reaction: PCl5(g) ⇌ PCl3(g) + Cl2(g)
