Finding Equations Given Partial Pressures

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Lillian Xie 1K
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Joined: Fri Sep 25, 2015 3:00 am

Finding Equations Given Partial Pressures

Postby Lillian Xie 1K » Thu Nov 19, 2015 9:07 pm

In textbook problem #89, the question reads as follows:

The following plot shows how the partial pressures of reactant and products vary with time for the decomposition of compound A into compounds B and C. All three compounds are gases. Use this plot to do the following: (a) Write a balanced chemical equation for the reaction. (b) Calculate the equilibrium constant for the reaction.

I've attached the graph, but I'm not sure how I'd go about finding the chemical equation for the reaction with just this diagram. I tried using the formula partial pressure = (mol fraction)*(total pressure), but I didn't get the answer listed in the textbook... Unless I rounded the approximated numbers incorrectly? Help would be much appreciated :)
Attachments
Screen Shot 2015-11-19 at 9.06.28 PM.png
11.89 Textbook Problem Graph
Screen Shot 2015-11-19 at 9.06.28 PM.png (21.17 KiB) Viewed 816 times

Mallika Varkhedi 2E
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

Re: Finding Equations Given Partial Pressures

Postby Mallika Varkhedi 2E » Thu Nov 19, 2015 10:13 pm

I'm not entirely sure about part a), but it might be because of how the curves change relative to each other. In part b), they might be dividing the kPa values by 100(in the solutions manual) because they want the answer in atm and 100 kPa is equal to 1 atm.

Jake Ney lecture 1 discussion 1F
Posts: 26
Joined: Fri Sep 25, 2015 3:00 am

Re: Finding Equations Given Partial Pressures

Postby Jake Ney lecture 1 discussion 1F » Thu Nov 19, 2015 10:30 pm

For A, the total change in pressure is calculated from the graph as 27.5-17.5=10 kPa. For B, the total change in pressure is 10 while for C this value is 5. This gives us a ratio of 2kPa A: 2 kPa B: 1kPa C. We use this ratio to formulate the balanced chemical equation 2A(g) --> 1B(g) + 2C(g) . (We know that A is the reactant because it decreases and that B and C are products because they increase over time). Once we have the balanced chemical equation we can create the equation for the pressure constant Kp=(Pc^2)/(Pa^2)(Pb) and plug in the final Kp values (17.5 for A, 10 for C and 5 for B) to arrive at the equilibrium constant. You may need to convert the Kp values to atm to get the solution manual's answer.


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