GC #214 DAT Destroyer

[Muggs]

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N2 + 3H2 <--> 2NH3

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

I'm not following the solution. It does an ICE chart, which I get, but not the calculation that follows afterwards.

Can someone explain why the ICE for 2NH3 (product) is
I: 0
C: +2x
Equilibrium: 2/2 = 1M

Therefore, 2x =1 , x = 0.5

How can they conclude with the above statement?

 

[kc1469]

As the way you wrote it, I am assuming you understand the ICE chart; so i'll just explain the "2/2" at equilibrium. He converted it to molarity by dividing the moles produced by the volume of the gas, which as stated in the question, is 2 Liters. As a general rule, always convert to Molarity for equilibrium questions if possible, makes the calculations much easier.

 

[Muggs]

As the way you wrote it, I am assuming you understand the ICE chart; so i'll just explain the "2/2" at equilibrium. He converted it to molarity by dividing the moles produced by the volume of the gas, which as stated in the question, is 2 Liters. As a general rule, always convert to Molarity for equilibrium questions if possible, makes the calculations much easier.

thanks. i'm still confused on the 2x = 1. From the ICE charts that I've done, the equilibrium value has always been the initial + change.

1. is the reason the molarity for the NH3 is given in the equilibrium because that is when it is formed? and same reasoning goes for the two reactants; the reason you solve their two molarities in the initial part of the ICE chart is because you start off with them?

2. given that you got the 1M for NH3, how can you make the 2x = 1 statement?

 

[kc1469]

I'll just do a full explanation; pick up from wherever you feel comfortable.
The question states that intially 8 moles of N2 and 8 moles of H2 are placed in a flask and it is allowed to come to eq. (i.e. the gases have reacted). Generally, we would not know the exact amount reacted unless we had the equilibrium constant, K. HOWEVER, here, they tell us the amount that is formed, so indirectly we know the amount that was reacted.
In other words, since 2 moles of NH3 formed, and there was 0 moles initially, we know two moles were created and that the value of 2x equals 2 (in terms of moles. Why we use the variable 2x? Because from the given reaction, we know the amount of NH3 that will form when the reactants react, (i.e. two moles will form PER 1 mol N2 and 3 mol H2). So what does the value of x stand for in an ICE chart; it stands for ratio of 1.
In other words, if the amount formed for NH3 was 2moles, then 2x=2 moles and 1x=1 mole. NOW, that is not to say that x always equals 1 mol. It stand for the ratio. If we were told that 4 moles of NH3 were produced, we would say that x=2, and that 2x=4.

Now, in regards to your question. You learned that x always equals the TOTAL change. That's fine, you just defined x differently. If we define it your way, it still does not change the outcome. If x= the total change, as you learned it, then x=2 and (x/2)=1. Once again, this is in moles Since the liters are constant, we are indifferent between using moles or molarity.
As a precautionary note, doing it the way you originally learned is much more difficult and is usually not the way it is taught.

If I am not being clear, or you need further explanation, just let me know!