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Homework Help: Physical Chemistry- Rates and Kinetics

  1. Dec 4, 2013 #1
    1. The problem statement, all variables and given/known data

    The Reaction; 2NO(g) + Cl2 (g) → 2NOCl (g)

    Is second order in NO and first order in Cl2. Five moles of nitric oxide and two moles of Cl2 were brought together in a volume of 2dm3 and initial rate was 2.4x10-3 moldm-3sec-1. What will be the rate when one half of the chlorine has reacted?

    2. Relevant equations

    Rate = k [NO]2 [Cl2]

    3. The attempt at a solution

    Initial Rate= 2.4x10-3 moldm-3sec-1

    2.4x10-3 = k [NO]2 [Cl2]

    5mols of NO in 2dm3
    = 5mols in 2L
    ∴ 1L contains 2.5mols
    [NO] = 2.5M

    2mols of Cl2 in 2dm3
    = 2mols in 2L
    ∴ 1L contains 1mol
    [Cl2] = 1M

    2.4x10-3 = k (2.5)2 (1)
    k = 2.4x10-3/6.25
    = 0.000384

    When half of the chorine is used (1mol), molarity = 0.5M

    ∴ Rate = (0.000384) (2.5)2 (0.5)
    Rate = 0.0012moldm-3sec-1

    Am I going about this the right way or this completely wrong?
  2. jcsd
  3. Dec 4, 2013 #2


    User Avatar

    Staff: Mentor

    Close, but no cigar yet.

    How much NO was consumed together with the chlorine?
  4. Dec 4, 2013 #3
    Wait, sorry I read the question wrong. Don't mind me, listen to Borek.
  5. Dec 4, 2013 #4
    So does that mean that since half the Cl2 (0.5mols) is used then half the NO is used (2.5mols)?

    [NO] 2.5mols in 2dm3
    2.5mols in 2L
    ∴ 1L = 1.25mols
    [NO] = 1.25M

    Rate = (0.000384) (1.25)2 (0.5)
    = 0.0003moldm-3sec-1
  6. Dec 5, 2013 #5


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    Staff: Mentor

    No, it is not about "half". It is a trivial stoichiometry.
  7. Dec 5, 2013 #6
    From the balanced equation, a 2: 1 mol ratio is noted between NO and Cl2

    So if one mol of Cl2 is used up then 2 mols NO would be used up

    [NO] = 1M
    [Cl2] = 0.5M

    Rate= (0.000384) (1)2 (0.5)
    = 0.000192moldm-3s-1
  8. Dec 5, 2013 #7


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    Staff: Mentor


    Please pay attention to what you are doing, you have made a second careless mistake. You were right up to 2 moles of NO being consumed.
  9. Dec 5, 2013 #8
    I thought the volume was given so that the concentration can calculated an utilized in the rate expression. Am I not supposed to calculate the concentration based on the mols being consumed? :confused:
  10. Dec 5, 2013 #9


    User Avatar

    Staff: Mentor

    Using your logic once all chlorine is consumed, and there is no more chlorine present, its concentration is 1 M, as 2 moles were consumed. So all was consumed, but the concentration has the maximum value. Something doesn't sound right.
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