Chemical Equilibrium: Question on Equilibrium Constant, Kp

In the previous blogpost, we have discussed on the concepts of chemical equilibrium in a reversible reaction.

We have also discussed on the formula to determine Equilibrium Constant, K_c and K_p.

For Reversible Reactions involving Gases, we should use the following formula:

Equilibrium constant, K_p = (P_C)^c (P_D)^d / (P_A)^a (P_B)^b

Let’s apply the formula to a question that was given by my students from a local Junior College (JC).

Now, at equilibrium, reactant (X) and products (Y and Z) will be all present together.

Since the question stated that the pressure is constant (value is P) and the temperature is also constant, we can assume that the total pressure, P_T of the gases at equIlibrium is P.

By stoichiometry ratio comparison of equation

X <=> Y + Z;

Since Mole of Y = Mole of Z at equilibrium, therefore

Partial Pressure of Y, P_Y = Partial Pressure of Z, P_Z

Since equilibrium partial pressure of X, P_X is 1/7 P, it means that the other 6/7 P of pressure must be shared equally among Y and Z.

Thus,

Partial pressure of Y, P_Y = 3/7 P

Partial pressure of Z, P_Z = 3/7 P

Using the formula on Equilibrium Constant, we will have:

K_p = (P_Y)(P_Z) / (P_X)    =   (3/7 P)(3/7 P) / (1/7 P)   =   9/7 P

Answer: B

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Hope you do!

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Related Articles:

• Chemical Equilibrium: Reversible Reaction Introduction
• Ideal Gas Law: Exam-Based Question
• Ideal Gas Law: Exam-Based Question on pV = nRT
• Ideal Gaw Law: Challenging Question
• Key Definitions & Formulae in Atoms, Molecules and Stoichiometry

Sean Chua

Winners Education Centre Pte. Ltd.

Many know me as the Ten Year Series book author for JC A-Level H2 Chemistry and O-Level Pure Chemistry. My 19+ years of coaching experience (since 1999) with more than 1500 students from 180+ JCs and Secondary Schools has allowed me to understand the true reasons why students are not able to perform well in Chemistry. Most importantly, my strength lies in using everyday analogies (even grandmothers can understand!) to simplify abstract concepts. Also, my teaching methodology has been designed to be fun, enjoyable and effective for my students.

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