Homework question 11.7 help!

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Natalie Yakobian
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Joined: Fri Sep 25, 2015 3:00 am

Homework question 11.7 help!

Postby Natalie Yakobian » Tue Nov 10, 2015 11:26 pm

The following flasks show the dissociation of a diatomic molecule, X2, over time. (a) Which flask represents the point in time at which the reaction has reached equilibrium?(b) What percentage of the X2 molecules have decomposed at equilibrium? (c) Assuming that the initial pressure of X2 was 0.10 bar, calculate the value of K for the decomposition.

For this question I'm confused for how do we tell which flask is the one that has reached equilibrium. Both the 3rd and the 4th flask have 5 moles of X2 and 12 moles of X.

Then for part b I understand that that the idea value of equilbrium would be 6 moles of X2 for every 12 moles of X but since the answer says that flask three is at equilibrium shouldn't the percentage of X2 decomposed at equilibrium 5/11 instead of 6/11.

For part C why can we not do an ICE table ?

Thanks so much.

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Re: Homework question 11.7 help!

Postby Chem_Mod » Wed Nov 11, 2015 9:26 am

Because the series of flasks is shown over time (left to right), so the dissociation starts at the leftmost flask and reaches equilibrium as time progresses to the right. Because there is no further dissociation in flasks 3 and 4, it is likely that equilibrium is reached in these flasks. But flask 3 is earlier in time (to the left) than flask 4, so flask 3 represents the point in time at which the reaction has "just" reached equilibrium.

Part b asks what percentage of X2 has decomposed at equilibrium. So, in flask 1, there are 11 X2 molecules. But in flask 3, there are 5 X2 molecules remained and 6 X2 molecules have decomposed. So, the percentage of X2 molecules has decomposed will be equal to the ratio of the number of X2 molecules has decomposed (which is 6) over the original number of X2 molecules (which is 11) multiplied by 100.

For part c, because we can count the number of X2 (5) and X (12) at equilibrium, we can calculate the mole fraction of X2 and X, which is necessary for determining partial pressures at equilibrium. That is why we do not need the ICE table.

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