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Factors Affecting Equilibrium Questions.

  1. May 3, 2014 #1
    1. The problem statement, all variables and given/known data

    1) Equilibrium is achieved in a closed system where metallic Mg can react with HCl(aq) according to the following net chemical equation:

    Mg(s) + 2 H+(aq) <--> Mg+2(aq)+ H2(aq)

    A sodium hydroxide, NaOH(s), pellet is added to this system. What happens to the
    concentration of each of the species in the net equation?

    I don't know if I am correct, but does NaOH(s) act as a catalyst in this reaction?

    2) Moles of argon gas are added to the following equilibrium system, predict the direction of the equilibrium shift.

    2 NO2 (g) ↔ N2O4 (g)

    The addition of argon gas would increase pressure, thus the products would be favored.
     
  2. jcsd
  3. May 4, 2014 #2
    #2...you are correct.
    #1...please explain why you think NaOH (a strong base) catalyzes the reaction?
    Pure water has the following equilibrium reaction HOH ⇌ H⁺ +OH⁻
    What will adding NaOH do to that equilibrium?
     
  4. May 4, 2014 #3

    Borek

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    Staff: Mentor

    No, but concentration of one of the ions involved will substantially change. Have you heard about water ion product?

    Equilibrium doesn't depend on total pressure, it depends on partial pressures of the gases involved in the reaction. Have they changed?
     
  5. May 4, 2014 #4
    1. No, I didn't know about the water ion product; but after reading the link, I see that there is a dissociation.
    H2O <--> H+ + OH-

    The addition of NaOH:

    H2O + NaOH (s) <--> H+ + OH- + Na+ + OH-


    I don't know where to go from here

    2. The partial pressures have not changed, therefore it does not affect the equilibrium.
     
  6. May 4, 2014 #5

    Borek

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    Staff: Mentor

    What happens to the concentration of H+ when you add OH-?

    And that's the correct answer.
     
  7. May 4, 2014 #6
    The concentration decreases because the OH- will be used up with the H+ to maintain equilibrium.
     
  8. May 4, 2014 #7
    Mg(s) + 2 H+(aq) <--> Mg+2(aq)+ H2 (aq)

    H2O <--> H+ + OH-

    NaOH (s) <--> Na+(aq) + OH-(aq)

    Should I write the net equation to find the overall effect on each of the species?
     
  9. May 4, 2014 #8

    Borek

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    Staff: Mentor

    What happens to the equilibrium when the concentration of H+ goes down?
     
  10. May 4, 2014 #9
    Mg(s) + 2 H+(aq) <--> Mg+2(aq)+ H2 (aq)

    Equilibrium shifts left.

    Mg concentration remains the same; H+ concentration goes up; Mg+2 concentration decreases, H2 concentration decreases.
     
  11. May 4, 2014 #10

    Borek

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    Staff: Mentor

    Yes.

    That's the only part I disagree with - we already established that it goes down, which is a driving force of the equilibrium shift.

    I understand where you come from with this statement, it is just off in the context.
     
  12. May 4, 2014 #11
    I see, so would it be wrong to say it's increasing? From what I understand, we did establish is was decreasing, but at the same time, with the left shift, wouldn't it increase again?

    Thank you for your help.
     
  13. May 4, 2014 #12

    Borek

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    Staff: Mentor

    IMHO correct way to describe the situation is to say that H+ goes down after NaOH addition, shifting equilibrium to the left, and lowering concentrations of Mg2+ and partial pressure of the H2. Don't mention H+ again.
     
  14. May 4, 2014 #13
    Ok, I see what you mean. However, the question specifically asks for the concentration of each species.
    I think it would be best to write it as: initially the concentration decreases but later increases due to the shift.
     
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