A student monitors the reversible reaction at

CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)

Problem

A student monitors the reversible reaction

2\,\text{A(g)} \rightleftharpoons \text{B(g)}

400\,\text{K}

. Using a highly sensitive flow probe, the student records the rates of the forward and reverse processes at four different times after mixing pure A(g). The data are summarized in the table below.

Time (s)	Rate of $2\,\text{A} \rightarrow \text{B}$ ( $\text{mol}\cdot\text{L}^{-1}\cdot\text{s}^{-1}$ )	Rate of $\text{B} \rightarrow 2\,\text{A}$ ( $\text{mol}\cdot\text{L}^{-1}\cdot\text{s}^{-1}$ )
$0$	$4.0 \times 10^{-3}$	$0.00$
$50$	$2.7 \times 10^{-3}$	$5.5 \times 10^{-4}$
$100$	$1.9 \times 10^{-3}$	$1.9 \times 10^{-3}$
$150$	$1.4 \times 10^{-3}$	$2.0 \times 10^{-3}$

Use the information in the table to answer the following questions. (a) At which recorded time, if any, is the reaction mixture at equilibrium? Justify your answer using the numerical data. (b) For the time interval between

0\,\text{s}

and

50\,\text{s}

, describe the net direction of conversion and support your description with appropriate reference to the data. (c) Between

100\,\text{s}

and

150\,\text{s}

, the reverse rate becomes larger than the forward rate. Explain what this indicates about the change taking place in the composition of the mixture during that interval.

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