MODULE
#13: Thermodynamics
EXPERIMENT
13.1
Determining the ΔH of a Chemical Reaction*
Supplies : · Two Styrofoam coffee cups · Thermometer · Vinegar · Mass scale · Measuring tablespoon and ½ teaspoon · Lye (This is commonly sold in supermarkets with the drain cleaners. A popular brand is Red Devil® Lye. If you cannot find lye, any powdered drain cleaner ought to work.) · Safety goggles 1. In order to perform this experiment, you must be able to measure out a relatively small mass of lye. The problem is, your mass scale is not very good at measuring small masses. Thus, what we need to do is figure out how many grams are in a teaspoon of lye. Then we can use measuring spoons as a quick way of measuring out a small mass of lye. To do this, measure the mass of 10 tablespoons of lye. 2. Remembering that each tablespoon represents 3 teaspoons, realize that the mass you just measured is the mass of 30 teaspoons of lye. Now take that mass and divide it by 30. The result is the mass of 1 teaspoon of lye. You should recognize this process as another example of calibration. You have just calibrated your measuring spoons to tell you the mass of lye. 3. In a moment, you will add ½ teaspoon of lye to some vinegar. Calculate what mass that is by taking the mass of 1 teaspoon of lye and dividing by 2. This will be the mass of lye that you will use in the experiment. 4. Now we can begin the real experiment. Nest the two coffee cups as you did in Experiment 2.2 to make a calorimeter. 5. Pour 100.0 mL (½ cup) of vinegar into your calorimeter. 6. Place your thermometer in the vinegar and let it sit for three minutes. After that time, record the temperature of the vinegar. This will be the initial temperature in the experiment. 7. Now add ½ teaspoon of lye to the vinegar, and begin stirring the mixture with your thermometer. 8. As time goes on, the following reaction will take place:
1. This reaction is exothermic; thus, you should see the temperature begin to rise. Read the temperature every 30 seconds. Once the temperature falls or stays constant for two consecutive readings, you can end the experiment. 2. Now you are ready to do your calculations. Remember, calorimetry experiments use the change in temperature to determine the heat transferred. The equation we use is: q = mcΔT
3.
The specific heat (c) of vinegar is 4.1 4. The mass of the vinegar in the calorimeter will be its volume (100.0 mL) times its density (which equals 0.99 g/mL). In addition to this mass, however, you must add the mass of ½ teaspoon of lye, because that was also in the calorimeter. In the end, then, the mass of the calorimeter contents is the mass of the vinegar plus the mass of the lye added. 5. Subtract the initial temperature in the experiment from the final temperature to get ΔT. 6. Now you can calculate the heat absorbed by the contents of the calorimeter. Take the specific heat given in step #11 times the mass you calculated in step #12, and then multiply it by the ΔT you calculated in step #13. This gives you the heat absorbed by the calorimeter's contents. 7. Since the heat absorbed by the calorimeter's contents came from the chemical reaction, you have just calculated the ΔH of the reaction! 8. We aren't quite finished, however. The ΔH we calculated was the amount of energy released when the mass of lye that we used in the experiment reacted with vinegar. It would be more useful if we could determine the ΔH per mole of NaOH that reacts. To do that, all we have to do is take the ΔH we just calculated and divide it by the number of moles of NaOH that we used. 9. Take the mass of lye and divide it by the molar mass of NaOH. That will give you the number of moles of NaOH that you used in the reaction. 10. Divide the ΔH that you calculated in step #14 by the number of moles you calculated in step #17. The result is the ΔH of the reaction in J/mole. 11. Clean up your mess.
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