A concrete demonstration of a rate process using colored water and a buret. It can be used to illustrate anything from the idea of a flow rate in mL/sec to the mathematical analysis of first order kinetics.

Buret Kinetics Apparatus (see diagram)
Plastic dish pan to catch water
Colored water in squirt bottle
Splits Timer (digital watch will do)

The apparatus is a permanent setup as shown in the figure below. Place the plastic dish pan in position to catch water as it squirts from the nozzle. Fill the squirt bottle with distilled water and add a few drops of food coloring to make it clearly visible to your audience.

One point that should be made clear is that since you are measuring the amount of water in the buret, the zero point at the top represents 50 mL (assuming a 50 mL buret), and the 50 mL point at the bottom represents an empty buret (0 mL).

This demonstration should accompany a discussion of kinetics and rates of chemical reactions. It is a concrete example of a physical process changing with an easily determinable rate law. The apparatus and demonstration can be used to illustrate a variety of aspects of rates and kinetic processes.

Repeat the measurement when the water level has dropped another 10 or 15 mL in the buret. The calculated rate of flow will be smaller than the first one because the height of the water column, and hence the pressure forcing the water through the exit tube, is less. Use this observation to convince your class that the rate of flow from the buret is not constant, but changes as the amount of water remaining changes. This is also true of many processes, including chemical reactions where the amounts or concentrations of reactants are changing as the reaction preceeds.

Graphing Data. This demonstration apparatus provides an excellent opportunity to make a series of measurements that can be subsequently graphed and discussed. Fill the buret above the top line with the colored water. Start the water flow at a convenient rate (experiment with this...I find that a flow that draws about 45 mL in 2.5 to 3 minutes works best). Tell your students that you will announce the elapsed time every time another 5 mL of water has drained from the buret and request one or two students to record the announced times when the water level in the buret is 50, 45, 40, 35, 30, 25, 20, 15, 10, and 5 mL. When the water level reaches the 50 mL point (the 0 mL line at the top of the buret), start your stopwatch and announce "zero seconds!" When the water crosses the 45 mL line (the 5 mL mark on the buret), click your split timer and read the time to the class. Restart your split timer after each reading! Continue reading until the water reaches the 5 mL mark; this may take a long time if the flow rate is too slow in which case you may wish to stop at 10 mL. Have your students graph the volume versus time data that you have obtained. The result will be a good looking concave downward curve (exponential decay).

Data Analysis. If you have discussed the "integrated forms" of rate laws with your students, you can work with the experimental data to find the actual rate law and rate constant k governing the rate of flow of water from the buret. The rate will be found to be first order in the volume of water (actually, the height of the water column), and graphing the log of the volume versus the measured time will give a straight downward sloping line, the slope of which will be equal to -k, the negative of the rate constant.

Graphing Data. Depending on the sophistication of your class, the demonstration can stop with just the data collection and graphing. For a more advanced group the resulting graph can be used to estimate the volume of water in the buret at any given time, and the slope of the curve can be related to the flow rate at any time. The first graph below shows typical data for buret volume vs. time.

The second graph shows these data plotted as lnV vs. time. All points except possibly the last fall on a pretty good straight line, demonstrating (to the knowledgeable) a first order rate relationship.

The third graph below shows the same data plotted as 1/V vs. time. This is not a good fit for second order.

Data Analysis. Mathematically, the relationship between the height of the water column h (proportional to the volume because the buret is a cylinder of constant diameter) in the buret and its rate of change with respect to time t is

where k is a constant of proportionality, known in chemical kinetics as the rate constant. When this equation is integrated, the relationship between h and time t is

It is hard to imagine any hazards with this experiment. Do not divide by zero in any of your calculations and try not to drop the apparatus on your foot. Do not stab yourself with the sharp point of the buret. Do not drink the colored water. The colored water can be poured down the drain.

The following article contains a good discussion of this experiment as well as information on other configurations of the apparatus (more than one buret, different shape burets, etc.

D.A.Davenport, J. Chem. Educ., 52, 379 (1975).