Ms. Keever's Science Classes

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Investigating Changes of State

Equilibrium Vapor Pressures

BEFORE: Read the following three paragraphs.

Everyone knows that if you leave an open beaker of water out in the room for a period of time, the water will evaporate. However, if you place a tight cover on the beaker, the water will still be there several weeks later. Is there nothing happening in the covered beaker? Actually, some of the water will evaporate and some will condense. After putting the lid on the system, the system will reach equilibrium between the number of water molecules that are in the liquid state, and the number of water molecules that are in the vapor state. Chemists would express what is occurring like this:

water (l)    <—>    water (g)

The double arrow indicates that the reaction is going forward and backward at the same rate. It does not indicate that there is the same number of molecules in the liquid state as there are in the vapor state. This is called equilibrium and it is referred to as dynamic equilibrium because the molecules are constantly changing places even though there is no net change in the number of molecules in each phase.

Because some of the molecules are in the gas phase, they exert a vapor pressure. This is called the equilibrium vapor pressure and it is temperature dependent. That is to say that, if we increase the temperature, more of the molecules will go in to the vapor phase and a new equilibrium will be established at a new temperature. All liquids (and some solids) have a vapor pressure. During this activity, you will use Excel to graph the equilibrium vapor pressures of some substances other than water and then use your graph to answer questions about the liquids.

Purpose: In this activity you will graph equilibrium vapor pressure versus temperature and interpret the graph.

Materials: Computer with Excel and a printer connection.

Procedure:

1. Open Excel.

2. Using Excel, enter the following data into the spreadsheet. You can cut and paste from the chart below.

3. Highlight all the data. Do NOT highlight the labels at the top of each column (temperature, etc.)

4. Click on the Chart Wizard at the top of Excel.

5. Select XY (Scatter) and then select the second type, data points connected by smooth lines.

6. Click NEXT. You should see three graphs, each in a different color. Click on the SERIES tab at the top of the pop-up window.

7. Click on each series in turn in the box at the left. Name each series: Series 1 = Chloroform, Series 2 = Ethanol, Series 3 = Acetic Acid.

8. Click NEXT and make sure the TITLES tab at the top is selected.

9. The chart title should be Vapor Pressures. The x-axis should be labeled Temperature, °C. The y-axis should be labeled Pressure, torr.

10. Go to the GRIDLINES tab. Click on (select) major gridlines and minor gridlines for both axes.

11. Click NEXT and make sure As Object In is selected. Click FINISHED.

12. Your graph should show up on your spread sheet. Drag it underneath your data, and adjust its size (enlarge) so you can read the details of the individual graphs.

13. Now right-click on the temperature numbers at the bottom of the graph, the x-axis. You should get a pop-up that says FORMAT AXIS. Click on that. A new pop-up opens. Click on the SCALE tab at the top. For the MAJOR UNIT enter 10. For the MINOR UNIT enter 5.

14. Do the same for the pressure numbers on the left side of the graph, the y-axis. For the major unit enter 100. For the minor unit enter 20. Resize the graph to be able to clearly read it if necessary.

AFTER:

15. In the cells just below your graph, answer the following questions. You may need your textbook for the two definitions.

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