The general form of the rate law for this reaction is \[ \text { Rate }=k[\mathrm{NO}]^{x}\left[\mathrm{Cl}_{2}\right]^{y} \] We can determine the values of \( x \) and \( y \) by comparing the rates from the various experiments. To determine the value of \( y \), we use the results from experiments 1 and 2 , in which only \( \left[\mathrm{Cl}_{2}\right] \) changes. In these experiments, \( [\mathrm{NO}] \) is held constant and \( \left[\mathrm{Cl}_{2}\right] \) is doubled. The rate also doubled. Thus the reaction is first order with respect to \( \left[\mathrm{Cl}_{2}\right] \). Or mathematically, \[ \begin{array}{l} \frac{0.36}{0.18}=\frac{k(0.10)^{x}(0.20)^{y}}{k(0.10)^{x}(0.10)^{y}}=\frac{(0.20)^{y}}{(0.10)^{y}} \\ 2.0=(2.0)^{y} \\ y=1 \end{array} \] To determine the value of \( x \), we use the results from experiments 2 and 3 , in which only \( [\mathrm{NO}] \) changes. Here, as the \( \mathrm{NO} \) concentration doubles \( \left(\mathrm{Cl}_{2}\right. \) concentration is constant), the rate increases by a factor of four. Thus the reaction is second order with respect to NO. Or mathematically: \[ \begin{array}{l} \frac{1.44}{0.36}=\frac{k(0.20)^{x}(0.20)^{y}}{k(0.10)^{x}(0.20)^{y}}=\frac{k(0.20)^{x}}{k(0.10)^{x}} \\ 4.0=(2.0)^{x} \\ x=2 \end{array} \] The rate law can now be written: \[ \text { Rate }=k[\mathrm{NO}]^{2}\left[\mathrm{Cl}_{2}\right] \] b What is the value of the rate constant? Rate constant \( = \) \( \square \) \[ \frac{\mathrm{L}^{2}}{\mathrm{~mol}^{2} \cdot \min } \]
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For the reaction $X_{2}+Y+Z \longrightarrow X Y+X Z$, it is found that doubling the concentration of $X_{2}$ doubles the reaction rate, tripling the concentration of $Y$ triples the rate, and doubling the concentration of $Z$ has no effect. (a) What is the rate law for this reaction? (b) Why is it that the change in the concentration of Z has no effect on the rate? (c) Suggest a mechanism for the reaction that is consistent with the rate law.
Order and rate law of a reaction The overall order of an elementary step directly corresponds to its molecularity. Both steps in this example are second order because they are each bimolecular. Furthermore, the rate law can be determined directly from the number of each type of molecule in an elementary step. For example, the rate law for step 1 is rate=k[NO2]^2. The exponent "2" is used because the reaction involves two NO2 molecules. The rate law for step 2 is rate=k[NO3]^1[CO]^1=k[NO3][CO] because the reaction involves only one molecule of each reactant, the exponents are omitted. Analyzing a new reaction Consider the following elementary steps that make up the mechanism of a certain reaction: 2A → B + C B + D → E + C Part A: What is the overall reaction? Express your answer as a chemical equation. Part B: Which species is a reaction intermediate? Part C: What is the rate law for step 1 of this reaction? Express your answer in standard MasteringChemistry notation. For example, if the rate law is k[A][C]^3, type k*[A]*[C]^3. Part D: What is the rate law for step 2 of this reaction? Express your answer in standard MasteringChemistry notation. For example, if the rate law is k[A][C]^3, type k*[A]*[C]^3.
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