Chemical Equilibrium: find concentration from 2 solutions (ICE table)

In summary, the conversation is discussing a problem involving choosing between two solutions in which the values of 0.65 and 0.39 are being plugged into (1-2x). The question arises as to why these values are not being plugged into K=x/(1-2x)^2 instead, since 2 moles of NO2 were given. The expert explains that the correct solution is to plug the values into K=x/(1-2x)^2, as suggested by the person asking the question. However, x=0.65 would result in a negative concentration of NO2, which is impossible. Therefore, the correct answer is to plug the values into (1-2x) as previously stated.
  • #1
doridoridori
2
0
Homework Statement
I've recently encountered this problem https://www.physicsforums.com/threads/need-major-help.117630/
In case the link doesn't open, here's the problem itself:
9. At a certain temperature, T, K for the reaction below is 7.5 liters/mole.

2NO2 <===> N2O4

If 2.0 moles of NO2 are placed in a 2.0-liter container and permitted to react at this temperature, what will be the concentration of N2O4 at equilibrium?

a) 0.39 moles/liter

b) 0.65 moles/liter

c) 0.82 moles/liter

d) 7.5 moles/liter

e) none of these
Relevant Equations
Super stuck on this. All of the deets are given in the question I linked. I'm still getting used to this forum so don't mind please if I accidentally mess something.
Also this is the equation itself:

7.5=x/(1-4x-4x^2)
30x^2-31x+7.5=0
x=0.39(approx.)
x=0.65(also approx.)
Hello! I've recently encountered this problem https://www.physicsforums.com/threads/need-major-help.117630/ and solved it and I'm stuck at choosing between 2 solutions. I don't understand why do we need to plug in 0.39 and 0.65 to (1-2x) and NOT to (1-2x)^2. I mean, we were given 2NO2, not just NO2. I see that in ICE table the result is [1-2x] but then why Keq=N2O4/(NO2)^2 and not just NO2?
 
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  • #2
Those are both correct solutions to your equation, but x=0.65 corresponds to a negative concentration of NO2, which is physically impossible.
 
  • #3
Chestermiller said:
Those are both correct solutions to your equation, but x=0.65 corresponds to a negative concentration of NO2, which is physically impossible.
Hello! Yes, that's essentially what I was asking. Sorry if I made it all unclear! So to know the right answer, we are plugging 0.65 and 0.39 into (1-2x). What I don't understand is why don't we plug it into K=x/(1-2x)^2 instead? I mean, we were given 2 moles of NO2, wouldn't it be reasonable then to calculate for 1-2x squared?
 
  • #4
doridoridori said:
Hello! Yes, that's essentially what I was asking. Sorry if I made it all unclear! So to know the right answer, we are plugging 0.65 and 0.39 into (1-2x). What I don't understand is why don't we plug it into K=x/(1-2x)^2 instead? I mean, we were given 2 moles of NO2, wouldn't it be reasonable then to calculate for 1-2x squared?
That is what you do plug into. Try those values and see what you get.
 

FAQ: Chemical Equilibrium: find concentration from 2 solutions (ICE table)

What is chemical equilibrium?

Chemical equilibrium is a state in a chemical reaction where the forward and reverse reactions occur at the same rate, resulting in no overall change in the concentrations of reactants and products.

How is equilibrium constant (K) calculated?

The equilibrium constant (K) is calculated by dividing the concentration of the products by the concentration of the reactants, each raised to the power of their respective coefficients in the balanced chemical equation.

What is an ICE table and how is it used?

An ICE table is a tool used to organize the initial, change, and equilibrium concentrations of reactants and products in a chemical reaction. It is used to determine the unknown concentrations at equilibrium by applying the principles of stoichiometry.

Can the concentration of a reactant or product be calculated at any point during a reaction?

Yes, the concentration of a reactant or product can be calculated at any point during a reaction as long as the initial concentrations and equilibrium constant are known.

How does changing the temperature affect the equilibrium constant?

Changing the temperature can affect the equilibrium constant by shifting the equilibrium in either the forward or reverse direction. An increase in temperature favors the endothermic reaction, while a decrease in temperature favors the exothermic reaction.

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