For the reaction 2 NO (g) + Cl2 (g) → 2 NOCl (g) If the concentration of NO is tripled, the rate of the
reaction increases by a factor of nine. If the concentration of Cl2 is cut in half, the rate of the reaction is
decreased to half the original rate. Find the order of reaction for each reactant and write the rate expression for
the reaction.

->I don't understand the part where it says "If the concentration of Cl2 is cut in half, the rate of the reaction is
decreased to half the original rate." How would you find the order of reaction for Cl2?

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  1. rate= k[Cl2][NO]^2

    rate is first order for Cl2 (it is to power 1)
    rate is first order of NO2 (it is to the second power)

  2. Are you saying that the coefficients of the balanced reaction equation define order of reaction?

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  3. Ms Nicole, you need to re-read the problem and check that the [Cl2] from Exp-1 to Exp-3 was 'doubled' NOT cut in half. The rate decreased to 1/2 the original rate only when the concentration of Cl2 is DOUBLED. If [Cl2] is cut in half and the rate also drops to 1/2 of Rate 1, the rate constant for Experiment 3 will not match the rate constants for Exp 1 & Exp 2.

    Given: 2NO + Cl2 => 2NOCl
    Empirical Rate Law = k[NO]^x[Cl^2]^y
    Determine x, y & k ...
    Exp [NO] [Cl2] Rate (R)
    Exp-1: [NO]1 [Cl2]1 R1(Refnc)

    Exp-2 [NO]2= [Cl2]2=
    3[NO]1 [Cl2]1 R2=9(R1)
    Holding [Cl2] constant:

    Exp-3 [NO]3= [Cl2]3=
    [NO]1 2[Cl2]1 R3=1/2(R1)
    Holding [NO] constant:

    In general, order of reaction indicates a rate trend as a function of concentration changes. Typical interpretations are based upon a reference rate (usually and 'Exp-1' and compared to the effect of changing one component of the reaction at a time while holding all other components constant.

    For Reference Rate = R
    0 Order => Chg Conc => ∆R = 0

    1st Order => 2[Conc] => 2R
    => 3[Conc] => 3R
    => 4[Conc] => 4R ...

    2nd Order => 2[Conc] => 4R
    => 3[Conc] => 9R
    => 4[Conc] =>16R ...
    The overall order of reaction is the sum of the exponential values determined from experiment. (Note)Rate Laws for reactions as in this problem can ONLY be determined by experimental process. In Chem Kinetics analysis, one can NOT assume the coefficients of the balanced equation of the reaction is the order of reaction. It can only be determined experimentally.

    For your problem:
    Exp 1 to 2 => 2[NO]=>9R1 =>[NO]^2 (2nd Order w/to NO.

    Exp 1 to 3=>2[Cl2]=>1/2R1=>[Cl2]^-1 (-1st Order W/to Cl2.

    Rate Law:
    Rate = k[NO]^2[Cl2]^-1
    = k([NO]^2/[Cl2])

    Solving for k=Rate[Cl2]/[NO]^2 and substitution concentration and rate values from each row gives the following k-value for each experiment;i.e., ...
    k = R[Cl2]/[NO]^2

    This would not be true if you halved Cl2 and halved rxn rate. The exp 3 k-value (rate 'constant') would not match exp 1 & exp 2 k-values. Good Luck :-)

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