CHM 112 Section 1

Homework Assignment #4
Due February 1, 2006 by 8:05 a.m.

Practice (not to be turned in): Problems: 13.38, 13.40, 13.41

Consider the following questions:

0) Even in the gas phase, where kinetic-molecular theory works well, theoretical calculation of rate constants from first principals generally does not succeed. Explain this statement in terms of collision theory.

1) A modest temperature rise, say 10 oC, increases the collision frequency by only a few per cent. However, this same temperature rise can increase the rate constant by a factor of two or more. Explain.

2) The activation energy for an endothermic reaction is always equal to or greater than the enthalpy change. Explain.

3) The relationship between the magnitudes of the activation energy and the enthalpy for exothermic reactions is random. Explain.

4) A mixture of hydrogen gas and oxygen gas in a vessel is stable at room temperature. However, if this mixture is exposed to a spark there is an immediate explosion that consumes all of the limiting reagent. Explain in terms of collision theory.

5) In the reaction between iodine atoms to form iodine molecules the orientation term in collision theory does not contribute. Explain.

6) Using the appropriate experimental techniques, gas phase molecules can be emitted from a nozzle into a vacuum such that all of the molecules have about the same direction and about the same speed. This molecular beam is said to be "cold" and is, therefore, unreactive. Explain why there is so little reactivity in these molecules that contain substantial kinetic energy.

7) A mixture of gas phase fluorine atoms will react much faster to form fluorine molecules than a mixture of iodine atoms to form iodine atoms at the same temperature. Yet, the bond energies of F2 and I2 are about the same. Explain in terms of collision theory.

8) In the context of collision theory, the rate constant k is a probability factor, not a true probability. Explain the difference between a probability and a probability factor.

9) Many reactions are known that occur in the solid state, i.e., molecules in solids that react with themselves. Often, the orientation term is the most important factor for determining the rate constant. Explain why.

Turn in the following on a clean, full sheet of paper (not torn out of a notebook):

Name
Student ID Number
The answer to the question corresponding to the last digit of your Student ID Number.