Chemical equilibrium concentrations

In summary, the conversation discusses a problem involving finding equilibrium concentrations given the equilibrium constant and chemical equation. There is confusion about whether the given values and equation are correct and a discussion about solving the problem using an ICE table and the equilibrium constant expression. The final conclusion is that the initial equation and value given for k may not be consistent with chemical reality and should be adjusted for more reasonable concentrations.
  • #1
cj2222
14
0
I have a problem in which It asks to find the equilibrium concentrations but it only give the value of the equilibrium constant and the chemical equation. Is this even possible to solve or does it not have enough information?

Here is the problem

Determine the equilibrium concentrations for the reaction below. K = 2.6 x 10-6.

Mg2+ + 2OH- <====> Mg(OH-) (solid)
 
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  • #2
Mg(OH-) or Mg(OH)2?

Write expression for the equilibrium constant. Is there any dependence between concentrations of the ions produced when solid dissolves?
 
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  • #3
Can you use 1 as the equilibrium concentration of Mg(OH)2 and use this on the ICE table to find the other equilibrium concentrations or would this give you the wrong answer?

because k = 1/[Mg2+][OH-]

or would the answer be

1/(x)(2x)^2 = 2.6 x 10^-6

where x = the equilibrium concentration of Mg2+
 
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  • #4
1 is OK, but not as a concentration - magnesium hydroxide is a solid, so its activity is assumed to be 1. There is no need for ICE table - as long as there is solid present, its activity is 1. Just 1.

Your k is wrong - please write balanced reaction of magnesium dissolution first, then use it to construct reaction quotient.
 
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  • #5
Mg2+ + 2OH- <===> Mg(OH)2k = 1/[Mg2+][OH-]2

The concentrations of OH- is going to be 2 times the concentration of Mg2+

so 2.6 x 10-6 = 1/(x)(2x)2so x = 45.81

the concentration of Mg2+ is 45.81

the concentration of OH- is 91.62

Is this correct or not because I can't think of any other way to do it?
 
  • #6
You are on the right track, but numbers you get are absurd (BTW, you should list units in your answer).

IMHO that's because your initial reaction equation and value given for k are not consistent with chemical reality, which is probably not your fault.
 
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  • #7
yeah, the reaction is supposed to be reversed, k is correct.

The reaction is supposed to be


Mg(OH)2 <===> Mg2+ + 2OH-


which would give much smaller and more reasonable concentrations.
 
  • #8
My sources give pKsp for Mg(OH)2 as something between 10 and 11, which means k should be around 10-10 or 10-11. 10-6 looks way too large.
 

FAQ: Chemical equilibrium concentrations

1. What is chemical equilibrium?

Chemical equilibrium is a state in a chemical reaction where the concentrations of the reactants and products remain constant over time. This means that the forward and reverse reactions are occurring at equal rates, resulting in no overall change in the concentrations of the substances involved.

2. How is the equilibrium concentration of a substance determined?

The equilibrium concentration of a substance is determined by the equilibrium constant, which is a ratio of the products to the reactants at equilibrium. This constant can be calculated using the concentrations of the substances at equilibrium.

3. Can the equilibrium concentration of a substance be changed?

Yes, the equilibrium concentration of a substance can be changed by altering the conditions of the reaction, such as temperature or pressure. This can shift the equilibrium in favor of either the reactants or the products, resulting in a change in the equilibrium concentration of a substance.

4. How does Le Chatelier's principle relate to equilibrium concentrations?

Le Chatelier's principle states that when a stress is applied to a system at equilibrium, the system will shift in the direction that relieves the stress. In terms of equilibrium concentrations, this means that if the concentration of a substance is increased, the equilibrium will shift to decrease that concentration, and vice versa.

5. How does the equilibrium concentration of a substance affect the rate of a reaction?

The equilibrium concentration of a substance does not directly affect the rate of a reaction, as the rate is determined by the rate of the forward and reverse reactions. However, changes in the equilibrium concentrations of substances can affect the rate by altering the equilibrium constant and shifting the equilibrium in favor of one reaction over the other.

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