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# Kp and Kc

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Sections
Intro to Chemical Equilibrium
Equilibrium Constant (K)
Equilibrium Constant Calculations
Kp and Kc
Using Hess's Law To Determine K
Calculating K For Overall Reaction
Le Chatelier's Principle
ICE Charts
Reaction Quotient
###### 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: 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)

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)