Solving Equilibrium Problems: NH4HS & CCl4

  • Thread starter synergix
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In summary: But don't forget that the amount of solid is also important. It is the amount of solid that will determine the partial pressure of gases. That's why you need to know the minimum mass of solid that will give an appreciable amount of gases at equilibrium.Okay so since I know the volume which is 5.0L and the K value which is 1.6x10^-4. I can use PV=nRT to find the number of moles of NH3/H2S that will be at equilibrium. Then I can use a ICE table to find the concentration of NH3 and H2S. Then I can use the molar mass of NH4HS to find the mass of NH4HS. I'm not sure
  • #1
synergix
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Homework Statement



(1) For the given system at equilibrium, NH4HS(s) <-----> NH3(g) + H2S(g), the K value is 1.6x10^-4 at 24oC, when pressures are in atmospheres.
What is the minimum mass of solid NH4HS(s) that can be introduced into a 5.0 L evacuated container so that equilibrium will result?

(2) Carbon Tetrachloride (CCl4) was once commonly used as a fire extinguisher. However, at high temperatures, CCl4 reacts with oxygen to form two deadly gases, phosgene and Cl2, according to the following chemical equilibrium.

2CCl4(g) +O2(g) <----> 2COCl2(g) + Cl2(g)

at 500oC, 0.562 mol of O2 and an unknown amount of CCL4 was introduced into a 1.00 L reaction vessel. Once the system had reached equilibrium, there was 0.263 moles of Cl2 present. Calculate the initial amount of CCL4 that was admitted into the vessel. K for this reaction at 500oC is 2.00 mol/l

The Attempt at a Solution



1) My textbook says as long as some solid is present the amount is a constant and it will not effect the concentration of ions in solution so I believe the answer would be any amount greater then zero. It is a six mark question though so I believe that is not the correct answer.

2) not to sure on this one

K=2.00Mole/L=([COCl2]^2 [Cl2]^2)/([CCl4]^2[O2])

sqrt{2.00[0.562]/[0.263]^2}= [COCl2]/[CCl4]

so this would give me the ratio of the concentrations of COCl2 over CCl4 but they should be equal to one because of the stoichiometry shouldn't they?
 
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  • #2
1) I took another shot at this one

K=1.6x10^-4= [NH3][H2S]= X^2

sqrt(1.6x10^-4)=X/[NH3]/[H2S]

[NH3]/[H2S]= 0.012649 moles/L

mass NH4HS= 0.012649moles NH3/H2S /L x 5.0L x 1mole NH4HS/ 1mole NH3/H2S x 51.11249 g/ mole NH4HS = 3.2g
 
  • #3
synergix said:
1) My textbook says as long as some solid is present the amount is a constant

Amount? Or activity?
 
  • #4
synergix said:
K=1.6x10^-4= [NH3][H2S]= X^2

sqrt(1.6x10^-4)=X/[NH3]/[H2S]

What is X?

Seems to me that you are doing tricks that have nothing to do with correct algebra :grumpy:
 
  • #5
Borek said:
What is X?

Seems to me that you are doing tricks that have nothing to do with correct algebra :grumpy:

I used x because I thought that because of the stoichiometry NH3=H2S so it just makes the calculation simpler.
 
Last edited:
  • #6
it says the concentration is a constant. " So, as long as at least some solid is present, how much solid is present is not relevant to the concentration of ions in solution."
 
  • #7
synergix said:
I used x because I thought that because of the stoichiometry NH3=H2S so it just makes the calculation simpler.

You have still not answered my question, I suppose you assume I will guess that x=NH3=H2S. What if I am too dumb?

K=1.6x10^-4= [NH3][H2S]= X^2

sqrt(1.6x10^-4)=X/[NH3]/[H2S]

This is wrong. You have taken square root only of some parts of the equation. Besides, you have already named [NH3] and [H2S] x, why do you mix now x with [NH3] and [H2S]?

"So, as long as at least some solid is present, how much solid is present is not relevant to the concentration of ions in solution."

It doesn't mean amount is constant, it means amount doesn't matter. That's not the same.
 
  • #8
Borek said:
You have still not answered my question, I suppose you assume I will guess that x=NH3=H2S. What if I am too dumb?

Yes I was assuming that. I should of been more clear

Borek said:
This is wrong. You have taken square root only of some parts of the equation. Besides, you have already named [NH3] and [H2S] x, why do you mix now x with [NH3] and [H2S]?

the square root of K = [NH3] = [H2S] = x . I have used NH3/H2S/x to indicate NH3 or H2S or x again I apologize I should have been more clear.

Borek said:
It doesn't mean amount is constant, it means amount doesn't matter. That's not the same.

I misunderstood apparently I need to be a bit more careful in my reading and writing
 
  • #9
Now, that it is all cleared, we can try to solve the question :wink:

Hint: for the equilibrium to exist you need a solid. If the amount of solid is too small, it will all decompose.There will be no solid left, there will be no equilibrium between solid and gases.
 
  • #10
So there must be an appreciable amount of solid.

Now that I have really started to think about this one I realize that this has nothing to do with concentration of ions in solution. this is about a solid decomposing into gas. So the equilibrium constant K is not concentration but pressure right? " the K value is 1.6x10^-4 at 24oC, when pressures are in atmospheres." That statement indicates that If the unit of pressure were to change the K value would also change. And since it is a equilibrium constant which equals [NH3][H2S] the K value must be the product of the the partial pressures of [NH3][H2S]. am I on the right track?
 
  • #11
Seems OK so far.
 

1. What is the definition of equilibrium?

Equilibrium is a state in which there is no net change or movement in a system. It occurs when the forward and reverse reactions are happening at equal rates, resulting in a constant concentration of reactants and products.

2. How do you determine the equilibrium constant for a reaction?

The equilibrium constant, denoted as K, is calculated by taking the ratio of the concentrations of products to reactants, each raised to the power of their respective coefficients in the balanced chemical equation.

3. What factors affect the position of equilibrium?

The position of equilibrium can be influenced by changes in temperature, pressure, and concentration of reactants and products. According to Le Chatelier's principle, when a system is disturbed, it will shift in a direction that minimizes the disturbance.

4. How do you solve equilibrium problems involving NH4HS and CCl4?

To solve equilibrium problems involving NH4HS and CCl4, you first need to write the balanced chemical equation for the reaction. Then, set up an ICE table to track the changes in concentration of each species. Finally, use the equilibrium constant expression to solve for the unknown concentration.

5. What is the significance of the reaction quotient in equilibrium problems?

The reaction quotient, Q, is a way to determine the direction in which a reaction will proceed to reach equilibrium. If Q is less than the equilibrium constant, the reaction will shift to the right to produce more products. If Q is greater than the equilibrium constant, the reaction will shift to the left to produce more reactants. When Q is equal to K, the system is at equilibrium.

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