Equilibrium concentrations

[Function]

Hello

 

Now that I get the basics of a chemical equilibrium, I wanted to remake an excercise from my book, which we solved in class:

 

In a closed space with a volume of 2 litres, 18,4 g N2O4 is added. As a consequence of heating this up to 50°C, half of the molecules is being transferred into NO2.

a) Calculate the equilibrium constant for this analytic reaction.

b) What are the concentrations when the volume is now changed to 1 litre?

 

a) K = 0,22 (this is not the question I'm having problems with)

 

b) Volume is being decreased [math]\Rightarrow[/math] equilibrium shifts to the side with the fewest amount of molecules [math]\Rightarrow[/math] shift to N2O4.

 

According to me:

 

Start with 0,22 moles of N2O4 (just as in (a)) and 0 moles of 2NO2.

As the equilibrium shifts to N2O4, to get the equilibrium concentration of N2O4, I would add [math[x[/math] to the number of N2O4-particles, resulting in a total of N2O4[math]+x[/math] moles of N2O4.

 

As the equilibrium shifts away from 2NO2, [math]2x[/math] needs to be substracted from 0 to get the number of 2NO2-particles.

Here is the first problem: one cannot have a negative amount of particles.

 

Second problem is how we resolved this in class: we substracted x from N2O4 and added 2x to 2NO2. Why? If the equilibrium shifts to the left, more particles are formed at the left, no?

 

Thanks.

 

Function

Edited February 26, 2014 by Function