2011 DAT Destroyer #202 GChem

[pandalove89]

Question:

N2 + 3H2 <--- ---> 2NH3

8 moles of N2 and 8 moles of H2 are placed in a 2 liter flask and allowed to come to equilibrium, 2 moles of NH3 are formed. Calculate the Keq.

So the solution says to use an ICE diagram, but is there a faster and easier way to do this? Like the Keq = [Product][Product]/ [Reactant] formula?


Also, are there any tell tale signs of when to actually use the ICE diagram instead of the shortcut?

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[Dental2000]

Im not sure what the Ice diagram is, but you cant use Keq= products/reactants directly

Keq can only be achieved by an experiment the 8 moles of N2 and H2 are just the initial concentrations which can change throughout the course of the reaction

REMEMBER...convert moles to M. always do this to avoid any errors

Hence the answer in DAT destroyer is the only way to do it.

 

[Vaheb]

I remember Chad had answered this before and it was much easier than the way in the Destroyer, even though it is pretty much the same thing except a little shortcut.

"Based upon the stoichiometry, if 2 moles NH3 formed then 1 mole N2 was consumed (and 7 moles are left) and 3 moles H2 were consumed (and 5 moles are left). Accounting for the 2L vessel we can calculate the molarities of each species at equilibrium:

N2: 7moles/2L = 3.5M
H2: 5moles/2L = 2.5M
NH3: 2moles/2L = 1M

We can now substitute these into the Keq expression:

Keq = [NH3]^2/([N2][H2]^3) = (1^2)/((3.5)(2.5^3)) = 0.018"

There you go!

 

[needzmoar]

Question:

N2 + 3H2 <--- ---> 2NH3

8 moles of N2 and 8 moles of H2 are placed in a 2 liter flask and allowed to come to equilibrium, 2 moles of NH3 are formed. Calculate the Keq.

So the solution says to use an ICE diagram, but is there a faster and easier way to do this? Like the Keq = [Product][Product]/ [Reactant] formula?


Also, are there any tell tale signs of when to actually use the ICE diagram instead of the shortcut?

There's really no shortcut. All of the 'shortcuts' will involve doing the 'ICE chart' in your head.

 

[pandalove89]

There's really no shortcut. All of the 'shortcuts' will involve doing the 'ICE chart' in your head.

the guy who responded right before you just gave a shortcut. does this not work?

 

[needzmoar]

the guy who responded right before you just gave a shortcut. does this not work?

Well, it works but what he did was the 'ICE chart' in his head. What I'm trying to say is that the 'Keq formula' that you speak of comes from the ICE table. Once you get really comfortable with it, you can start doing it in your head. Not sure if it really counts as a shortcut.

 

[ammarm]

I remember Chad had answered this before and it was much easier than the way in the Destroyer, even though it is pretty much the same thing except a little shortcut.

"Based upon the stoichiometry, if 2 moles NH3 formed then 1 mole N2 was consumed (and 7 moles are left) and 3 moles H2 were consumed (and 5 moles are left). Accounting for the 2L vessel we can calculate the molarities of each species at equilibrium:

N2: 7moles/2L = 3.5M
H2: 5moles/2L = 2.5M
NH3: 2moles/2L = 1M

We can now substitute these into the Keq expression:

Keq = [NH3]^2/([N2][H2]^3) = (1^2)/((3.5)(2.5^3)) = 0.018"

There you go!

i was wondering when Chad answered this question.. is it in the recent new videos or the old ones.. because i dont remember seeing him solve this question.. but thats a great shortcut thanks