For the reaction at

CHEMISTRY · Multi-part · AP · Chemistry · Kinetics · Kinetics

Problem

For the reaction

2\,\text{NO}(g) + \text{Cl}_2(g) \to 2\,\text{NOCl}(g)

25\,^{\circ}\text{C}

, a proposed mechanism yields the rate expression

\text{rate} = k_{\text{obs}}[\text{NO}]^2,

because

\text{Cl}_2

is present in large excess so that its concentration remains effectively constant during the initial rate measurement. A student collects the following data at

25\,^{\circ}\text{C}

Trial	$[\text{NO}]$ (M)	Initial rate ( $\text{M}\cdot\text{s}^{-1}$ )
1	$0.010$	$2.0 \times 10^{-5}$
2	$0.020$	$7.8 \times 10^{-5}$
3	$0.050$	$1.9 \times 10^{-4}$

(a) Calculate the numerical value and units of

k_{\text{obs}}

for each trial. (b) Evaluate whether the constancy (or lack thereof) of

k_{\text{obs}}

across the three trials supports the proposed rate law. Use your calculated values to justify your answer. (c) The student repeats Trial 2 but doubles the initial concentration of

\text{Cl}_2

while keeping

[\text{NO}] = 0.020\,\text{M}

. Predict the effect, if any, on the initial rate according to the proposed rate law, and explain how the outcome would provide additional evidence for or against the mechanism.

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