5 Steps to a 5 AP Chemistry 2019

Kinetics ❮ 203

The experimenter must then determine the concentration of one or more substances later,

or record some measurable change in the solution. Unless there will be an attempt to

measure the activation energy, the temperature should be kept constant. A thermometer is

needed to confirm this.

“Clock” experiments are common kinetics experiments. They do not require a separate

experiment to determine the concentration of a substance in the reaction mixture. In clock

experiments, after a certain amount of time, the solution suddenly changes color. This

occurs when one of the reactants has disappeared, and another reaction involving a color

change can begin.

In other kinetics experiments, the volume or pressure of a gaseous product is moni-

tored. Again, it is not necessary to analyze the reaction mixture. Color changes in a solu-

tion may be monitored with a spectrophotometer. Finally, as a last resort, a sample of the

reaction mixture may be removed at intervals and analyzed.

The initial measurement and one or more later measurements are required. (Remember,

you measure times; you calculate changes in time [Dt]). Glassware, for mixing and dilut-

ing solutions, and a thermometer are the equipment needed for a clock experiment. Other

kinetics experiments will use additional equipment to measure volume, temperature, etc.

Do not forget: In all cases, you measure a property and then calculate a change. You never

measure a change.

Common Mistakes to Avoid

1. When working mathematical problems, be sure your units cancel to give you the desired
   unit in your answer.


2. Be sure to round your answer off to the correct number of significant figures.


3. In working rate law problems, be sure to use molarity for your concentration unit.


4. In writing integrated rate laws, be sure to include the negative sign with the change in
   reactant concentration, since it will be decreasing with time.


5. Remember that the rate law for an overall reaction must be derived from experimental
   data.


6. In mathematically determining the rate law, be sure to set up the ratio of two experi-
   ments such that the concentration of only one reactant has changed.


7. Remember that in most of these calculations the base e logarithm (ln) is used and not
   the base 10 logarithm (log).

TIP