Chemistry 102 Laboratory

Experiment 5:  Chemical Equilibrium (Kinetics measured by Pressure)

Laboratory

In this experiment you will measure the rate of decomposition of carbonic acid by measuring the rate of production of carbon dioxide in the following reaction:

H₂CO₃  <–>  CO₂(g)  +  H₂O

The equilibrium constant for this reaction, in terms of concentration, is given by the expression:

Keq  =  [CO₂]/ [H₂CO₃]

In this experiment, however, the pressure of carbon dioxide, rather than its concentration, is to be measured.  In particular, the pressure is measured as a function of time, and at two different temperatures.  Since the equilibrium and chemical rates are both functions of temperature, two different rates will be measured at the two different temperatures.  Since this decomposition reaction is known to be first order in the concentration of the carbonic acid, the rate varies as the exponent of the absolute temperature of the solution.  For every 10^o increase in temperature, the rate of the reaction will approximately increase by a factor of 2.  Equilibrium is attained when the pressure remains a constant with time.  In this experiment, you will make use of the ideal gas equation and the fact that the equilibrium constant may be stated in terms of the gas pressure of carbon dioxide as well as the concentration (mass) of carbon dioxide.

Instructions

Computer Setup

1. Obtain a Pressure Sensor and plug it into an Analog channel of the interface box.  Make sure the interface box is on, then start Science Workshop.  Use the analog plug button to select your channel and then select the Absolute pressure sensor.  Double click on the pressure sensor icon, and allow the sensor to warm up for 5 minutes.

2. Using the barometer in the Main Lab, obtain the current atmospheric pressure in mm Hg.  Convert to atm.  In the screen for senor calibration, change kPa to atm for the pressure units.  In this same screen change the low pressure known value from 300 to the atmospheric pressure you measured in “1” above.  After the current reading of the pressure stabilizes, click on read to calibrate the pressure.  You are done with this part.

3. Check to make sure the sampling rate is set for Fast at 10 Hz by looking at the Sampling Options.  Bring up a graph window by dragging its icon over that of the pressure sensor.

Getting Set Up

4. Insert the barb of a quick release connector into one end of the plastic tubing that comes with the pressure sensor.

5. Obtain a one hole rubber stopper that fits a 250 mL glass bottle or flask.

6. Place a drop of glycerin on the bottom end of the hole of the stopper.

7. Insert the glass part of an eyedropper, tip up, through the hole in the stopper; the tip must clear the top of the stopper.

8. Carefully fit the glass tip into the open end of the plastic tubing.

9. Align the quick-release connector with the connector on the PRESSURE PORT of the Pressure Sensor.

10. Push the connector onto the port and then turn the connector clockwise until it clicks.

11. DO NOT INSERT THE STOPPER IN THE BOTTLE YET!!!  The pressure sensor should still be open to the atmosphere at this point.

Recording Data

12. Obtain 100 mL of room temperature (a posted value) soda water and place it in the 250 mL flask.

13. When everything is ready, click REC to begin recording data.

14. QUICKLY insert the stopper in the bottle; make sure there is a tight seal. DO NOT shake or stir the contents of the bottle.

15. Observe the changes in pressure as the carbonic acid in the soda water decomposes in the bottle.

16. Record for 6 minutes.

17. Slowly remove the stopper from the bottle and allow the pressure readings to stabilize.  This is to determine whether or not the air pressure or calibration have changed.

18. Stop the recording.  Determine the initial rate of decomposition of carbonic acid in the same manner employed in experiment 4 (i.e. a linear fit of the data).  Also record the final pressure of the gas in the flask.

19. Obtain 100 mL of cold soda water and place it into a 250 mL flask which has been cooled on ice in an ice bath.  Leave the water and the flask in the ice bath and return to your lab station.Repeat the procedure with cold soda water.