How can we determine the stoichiometry of a reaction?

Goals: Students will determine the stoichiometry or mole ratio of a reaction by using the temperature probe to measure the amount of heat evolved.

Safety:

  1. Safety goggles must be worn at all times.
  2. Both the sodium hydroxide and acids are corrosive to eyes and can irritate the skin. If any skin comes in contact with the solutions wash with soap and water immediately.

Prelab:

In this activity, we will combine various amounts of two chemicals that react. Both chemicals have been dissolved in water at the same concentration, or molarity. The molarity indicates the number of moles of a species per liter solution.

1.  In Part B, you will combine various volumes of sulfuric acid solution and sodium hydroxide solution. This reaction is exothermic – the amount of heat evolved is proportional to the amount reacted. Look at the amounts in your data table for Part B. Is there one that will react more (i.e., get hotter) than the others, or will they all produce the same amount of heat? Why?

2.  In Part C, you will combine different amounts of acetic acid solution and sodium hydroxide solution. Look at the amounts in your data table for Part C. Is there one that will react more (i.e., get hotter) than the others, or will they all produce the same amount of heat? Why?

Procedure:

Part A: Calibrating the temperature probe (for shorter classes, this will be done for you).

  1. To calibrate the probe, go to Experiment and choose Calibrate. Click on Perform Now. Place both the temperature probe and the thermometer in the ice water bath. Wait for the thermometer to reach a steady reading and for the voltage reading on the screen to slow down. (The voltage reading will never be completely stable.) Read the temperature from the thermometer, enter it in the box on the screen, and click on Keep. Repeat in the hot water bath. Click OK when you are finished.

Part B: Sodium hydroxide and sulfuric acid.

  1. Obtain two small foam cups and place them inside a 400 mL beaker to stabilize them. This is your calorimeter. Measure 50 mL sodium hydroxide and pour it into the foam cups. Place the temperature probe in the cup (see figure). Check that your temperature sensor registers room temperature. If not, notify the MCL staff.
  2. Measure 10 mL sulfuric acid, but DO NOT add it to the sodium hydroxide in the cup.
  3. Press Collect and wait 30 seconds for the temperature probe to stabilize. Quickly, pour the sulfuric acid in, and gently stir with the probe.
  4. Allow the program to collect temperature data for the full 75 seconds. Store your latest run by choosing Data and the Store latest run. Look in the Table Window to determine the lowest temperature (before the reaction occurred) and the highest temperature (after the sulfuric acid was added). Record these values in Data Table B.
  5. Pour the used solutions from the cup into the waste beaker and rinse the cups with water.
  6. In the Table Window, double click on Temperature. Change the column color to a color of your choice (this is for the new run) and click “OK”.
  7. Repeat the experiment with the other proportions in the Data Table for Part B.

Data Table for Part B

Run / Volume Sodium Hydroxide (mL) / Volume Sulfuric Acid (mL) / Ratio of Sodium Hy-droxide: Sulfuric Acid / Lowest temp. (oC) / Highest temp. (oC) / Temp. Change (oC)
1 / 50 / 10
2 / 40 / 20
3 / 30 / 30
4 / 20 / 40
5 / 10 / 50

Part C: Sodium Hydroxide and Acetic acid

  1. Erase your data from the sulfuric acid runs by choosing Data, Clear all Data, and Yes.
  2. Rinse the sulfuric acid graduated cylinder with distilled water. You will use this cylinder to measure your acetic acid.
  3. Repeat the procedures from Part B using the acetic acid solution instead of sulfuric acid. Record your results in the table on the next page.

Data Table for Part C

Run

/ Volume Sodium Hydroxide (mL) / Volume Acetic Acid (mL) / Ratio of Sodium Hy-droxide: Acetic Acid / Lowest temp. (oC) / Highest temp. (oC) / Temp. change (oC)
1 / 50 / 10
2 / 40 / 20
3 / 30 / 30
4 / 20 / 40
5 / 10 / 50

Postlab:

1.  Look at your data and circle the runs in each table that showed the greatest temperature change.

2.  What is the ratio of sodium hydroxide to sulfuric acid in the run with the greatest temperature change?______Given this, write a balanced chemical reaction (the products are water and sodium sulfate).

3.  What is the ratio of sodium hydroxide to acetic acid in the run with the greatest temperature change?______Given this, write a balanced chemical reaction (the products are water and sodium acetate).

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Exp. 37 Page