Chapter 11, Problem 120QRT

In a time-resolved picosecond spectroscopy experiment, Sheps, Crowther, Carrier, and Crim (Journal of Physical Chemistry A, Vol. 110, 2006; pp. 3087–3092) generated chlorine atoms in the presence of pentane. The pentane was dissolved in dichloromethane, CH₂C1₂. The chlorine atoms are free radicals and are very reactive. After a nanosecond the chlorine atoms have reacted with pentane molecules, removing a hydrogen atom to form HCl and leaving behind a pentane radical with a single unpaired electron. The equation is

Cl · (dcm) + C₅H₁₂(dcm) → HCl(dcm) + C₅H₁₁ · (dcm)

where (dcm) indicates that a substance is dissolved in dichloromethane. Measurements of the concentration of chlorine atoms were made as a function of time at three different concentrations of pentane in the dichloromethane. These results are shown in the table.

1. (a) Determine the order of the reaction with respect to chlorine.
2. (b) Determine whether the reaction rate depends on the concentration of pentane in dichloromethane. If so, determine the order of the reaction with respect to pentane.
3. (c) Explain why the concentration of pentane in dichloromethane does not affect the data analysis that you performed in part (a).
4. (d) Write the rate law for the reaction and calculate the rate of reaction for a concentration of chlorine atoms equal to 1.0 μM and a pentane concentration of 0.23 M.
5. (e) Sheps, Crowther, Carrier, and Crim found that the rate of formation of HCl matched the rate of disappearance of Cl. From this they concluded that there were no intermediates and side reactions were not important. Explain the basis for this conclusion.