Calculating Kp for Decomposition of NH4Cl at High Temperature

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Discussion Overview

The discussion revolves around calculating the equilibrium constant Kp for the decomposition of NH4Cl at high temperature, specifically addressing the reaction NH4Cl(s) -> HCl(g) + NH3(g). Participants explore the application of equilibrium concepts, the role of solid reactants in calculations, and the interpretation of pressure and concentration in the context of Kp.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant calculates Kp using concentrations derived from the ideal gas law, expressing uncertainty about the correctness of their solution.
  • Another participant questions the necessity of calculating concentrations when partial pressures are provided, suggesting a direct use of pressure in the Kp calculation.
  • A participant highlights the role of solids in equilibrium expressions, noting that the activity of solids is always 1 and questioning the initial calculations that included solid concentrations.
  • There is confusion regarding the treatment of NH4Cl as a solid and its implications for the Kp calculation, with some participants expressing uncertainty about the definition and application of Kp.
  • One participant acknowledges their misunderstanding and expresses a desire to clarify their knowledge of Kp and equilibrium concepts.
  • A later reply attempts to clarify that if the activity of a solid is 1, then Kp should be calculated based solely on the products.

Areas of Agreement / Disagreement

Participants exhibit a lack of consensus on the proper approach to calculating Kp, with differing opinions on whether to use concentrations or pressures, and how to treat the solid NH4Cl in the equilibrium expression. The discussion remains unresolved regarding the correct methodology.

Contextual Notes

Participants express uncertainty about foundational concepts related to equilibrium constants, particularly regarding the treatment of solids and the definitions of Kp and Kc. There are indications of missing theoretical knowledge that may affect their calculations.

Masschaos
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A 9g sample of solid NH4Cl is heated in a 4L container to 924.4oC and decomposes according to the following equation.
NH4Cl(s) -> HCl(g) + NH3(g)

at equalibrium the partial pressure of NH3 (g) is 1.346 atm. Calculate the equilibrium constant for Kp for this reaction


Homework Equations



Kp = Pproductn/Preactantn

or

Kc equation and Pv = nRT

The Attempt at a Solution



First I used the Ideal gas equation the amount of NH3 in the system.
So n = PV/RT
n = (136383.45 Pa * (4 *10-3 L))/(8.3144*1197.4K)
n = 5.48*10-5

This means the concentration of the NH4Cl is.
n = 9/54.45 = 0.1653
0.1653 - 2(5.48*10-5) = 0.1652

I assumed then that Cl would also have this concentration.
So the Kc of the reaction = ([5.48*10-5*[5.48*10-5)/0.1652 = 1.817 * 10-8

Then Kp = Kc(RT)(delta)n
Kp= 1.817 * 10-8 * (8.3144*1197.4K)1
Kp = 0.00018 = 1.8*10-4

The more I look at this the more I feel there is a much easier solution, is my solution even right?
 
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You have to calculate Kp, you are given pressure, why do you calculate concentrations instead of using pressure directly?

Besides, what is the activity of the solid (hint: it is always identical)? Does your calculation of NH4Cl make any sense in this context?
 
Well, the thing that confused me is that NH4Cl is a solid, so it is not counted in the calculation for Kp. Doesn't that mean it will be [products]/0? Or do you just not divide the product by anything?

What do you mean about the solid? it says its heated, and gas forms so presumably it gets smaller. My calculations would suggest that the reaction doesn't react very far.
I'm not really clear on what Kp is. Pressure are equilibrium?
 
Activity of solids is always 1, period. Whenever you see a solid in reaction equation, just put 1 in the reaction quotient.

How come you are asked to calculate Kp, but you don't know what it is? Please check your book or lecture notes. Seems like you are missing many basic parts of the theory, and that impedes your ability to solve the question.
 
Ah! So I am wrong.
I assumed it was pressure at equilibrium. And after some searching I thought I had come up with the right answer.
Thanks! I'll keep looking.
 
If, the activity of a solid is always 1.
Then would Kp =[Products]
Kp = [ 5.48*10-5] * [ 5.48*10-5]?
 

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