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, Kc and Kp.

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

Equilibrium constant, Kp = (PC)c (PD)d / (PA)a (PB)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, PT 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, PY = Partial Pressure of Z, PZ

Since equilibrium partial pressure of X, PX 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, PY = 3/7 P

Partial pressure of Z, PZ = 3/7 P

Using the formula on Equilibrium Constant, we will have:

Kp = (PY)(PZ) / (PX)    =   (3/7 P)(3/7 P) / (1/7 P)   =   9/7 P

Answer: B

Are you learning something? :-D

Hope you do!

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PS: Do leave me your thoughts in the comments section below. I would love to hear from you.

Many people talked about teaching Chemistry, I simply LOVE it. I am a passionate Chemistry Coach based in Singapore, Southeast-Asia and aspire to be one of the most dynamic, powerful and humorous speakers in Asia. My 16+ years of coaching experience has equipped me to understand the true reasons why students are not able to perform well in Chemistry, and allow me to structure my teaching methodology to cater to different levels of learners. If you have found this post useful, please share it with your friends. I would really appreciate it!

2 Responses to Chemical Equilibrium: Question on Equilibrium Constant, Kp
  1. Anon-y-mouse
    April 21, 2011 | 11:19 am

    Nice job explaining this out.

    Keep it up ;)

  2. maisie
    September 22, 2012 | 11:01 pm

    Hi sir,
    the “heated at constant pressure” in the qn refers to the total equilibrium pressure and not the initial pressure?
    Thanks

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