CHM 3410 – Problem Set 10

Due date: Monday, November 14th. NOTE: The third hour exam is Friday, November 18th. It will cover Chapter 5, section 5.13; Chapter 6, sections 6.5 to 6.9; Chapter 20, sections 20.1 to 20.4; Chapter 21, all; plus handouts.

Do all of the following problems. Show your work.

“I knew chemistry would be worse, because I’d seen a big chart of the ninety-odd elements hung up in the chemistry lab, and all the perfectly good words like gold and silver and cobalt and aluminum were shortened to ugly abbreviations with different decimal numbers after them. If I had to strain my brain with any more of that stuff I would go mad.” – Sylvia Plath

1) In a recent paper (M. Baasandorj, A. R. Ravishankara, J. B. Burkholder, J. Phys. Chem. A 115, 10539 (2011)) the rate constant for the reaction

OH + (Z)-CF3CH=CHCF3 ® “products” (1.1)

was measured at several temperatures between 212 – 374 K. The rate constant for the reaction was fit to the expression

k = (5.73 x 10-19 cm3/molecule.s) T2 exp(678/T) (1.2)

Find the value of k and Ea for the above reaction at T = 250.0 K and T = 300.0 K. Recall that the general expression for the activation energy of a chemical reaction is

Ea = - R [d ln(k)/d(1/T)] = RT2 [ d ln(k)/dT ] (1.3)

2) For each of the following mechanisms predict the corresponding rate law

a) Formation of phosgene (COCl2)

stoic. CO + Cl2 ® COCl2

step 1 Cl2 D 2 Cl fast, reversible (k1, k-1)

step 2 Cl + CO D COCl fast, reversible (k2, k-2)

step 3 COCl + Cl ® COCl2 slow (k3)

b) Decomposition of NO2Cl

stoic 2 NO2Cl ® 2 NO2 + Cl2

step 1 NO2Cl D NO2 + Cl fast, reversible (k1, k-1)

step 2 NO2Cl + Cl ® NO2 + Cl2` slow (k2)

3) In a recombintion reaction two smaller molecules (often radicals) combine to form a larger molecule.

stoic. A + B ® AB

One mechanism for such reactions (which is similar to the mechanism for unimolecular decomposition reactions) is the following

step 1 A + B D AB* k1, k-1

step 2 AB* + M ® AB + M k2

where AB* represents activated AB molecules, and M represents inert gas molecules.

a) Using the steady state approximation for AB*, find a general expression for d[AB]/dt, the rate of formation of the recombination product.

b) Using your result in a, find the rate law corresponding to the above mechanism in the high pressure limit ( [M] ® ¥ ).

c) Usin your result in a, find the rate law corresponding to the above mechanism in the low pressure limit ( [M] ® 0).

Also do the following from Atkins:

Exercises.

21.15b The rate of a chemical reaction is found to double when the temperature is raised from 25.0 °C to 35.0 °C. Determine the activation energy for the reaction.