Endothermic and Exothermic Reactions

Name Date Period

Lab Partners:

Endothermic and Exothermic Reactions

Some chemical reactions absorb energy and are called endothermic reactions. Many chemical reactions release or give off energy. Chemical reactions that release energy are called exothermic reactions. You will study one endothermic reaction and one exothermic reaction in this experiment.

The same idea holds true for phase changes, which we know are physical changes rather than chemical changes/reactions. Those phase changes that absorb energy are considered endothermic and those that release energy are exothermic.

In Part I of this experiment, you will study the chemical reaction between citric acid solution and baking soda. An equation for the reaction is

H3C6H5O7(aq) + 3 NaHCO3(s) 3 CO2(g) + 3 H2O(l) + Na3C6H5O7(aq)

In Part II, you will study the chemical reaction between magnesium metal and hydrochloric acid. An equation for this reaction is

Mg(s) + 2 HCl(aq) H2(g) + MgCl2(aq)

OBJECTIVES

In this experiment, you will

•  observe two chemical reactions

•  hopefully use a computer to measure temperature changes

•  determine the change in temperature, Dt, for each of the reactions

•  identify endothermic and exothermic reactions

•  identify endothermic and exothermic phase changes

MATERIALS

400mL beaker
Vernier computer interface / Electronic balance
Vernier Temperature Probe / citric acid, H3C6H5O7, solution
50-mL graduated cylinder / baking soda, NaHCO3
Styrofoam or paper cup / hydrochloric acid, HCl, solution (0.3 M)
Plastic stirring rods / magnesium, Mg, ribbon

Figure 1

PROCEDURE

Part I Citric Acid Plus Baking Soda

1. Place a paper/styrofoam cup into a 400-mL beaker as shown in Figure 1. Measure out 30.0 mL of citric acid solution into the Styrofoam cup. Use a pipette as needed and remember not to contaminate the stock solution! If necessary, put any extra/waste into to the beaker labeled “citric acid waste.” Place the temperature probe into the citric acid solution so that it has time to equilibrate.

2. Prepare the computer interface as directed by your teacher. Do not start data collection yet.

3. Weigh out 10.0 g of solid baking soda on a piece of weighing paper. (Remember to press “zero”, put weigh paper on balance, press “zero” and then measure out the 10.0g.)

4. Make sure that the temperature probe has been in the citric acid solution for at least 45 seconds before this next step:

•  Click the green arrow to begin data collection on your computer interface.

•  After a few temperature readings (about 30 seconds or so) have been made, add the baking soda to the citric acid solution.

•  Stir the solution with to ensure good mixing using a blue plastic pipette.

•  Collect data until a minimum temperature appears to have been reached and the temperature readings just begin to increase again, then click STOP.

•  Record your qualitative observations of this reaction in Table #2 below. Be sure to note any indicators/clues of chemical change and also whether the temperature increased or decreased upon adding the baking soda.

5. Click the graph button, then “analyze,”, then statistics, and then click “temperature.” It should then show you the minimum and maximum temperatures. Check that these make sense relative to the data you observed and then record these in Table 1.

6. Dispose of the reaction products, cup, and used stirrer, as directed by your teacher. Rinse off your temperature probe.

Part II Hydrochloric Acid Plus Magnesium

7. Measure out 30.0 mL of HCl solution into a clean paper cup. CAUTION: Handle this acid with care. It can cause painful burns if it comes in contact with your skin or gets into your eyes. Use a pipette as needed and remember not to contaminate the stock solution! If necessary, put any extra/waste into to the beaker labeled “HCl Waste.” Place the temperature probe into the HCl solution so that it has time to equilibrate.

8. Get a piece of magnesium metal from the teacher/cart.

9. The temperature probe must be in the HCl solution for at least 45 seconds before this step.

•  Click the green arrow to begin data collection.

•  After a few temperature readings (about 30 seconds) have been made, add the Mg ribbon to the HCl solution.

•  Stir the solution with a fresh blue plastic pipette to ensure good mixing. CAUTION: Do not breathe the vapors!

•  Collect data until a maximum temperature has been reached and the temperature readings just begin to decrease, then click STOP..

•  Record your qualitative observations of this reaction in Table #2 below. Be sure to note any indicators/clues of chemical change and also whether the temperature increased or decreased upon adding the Mg.

10. Click the graph button, then “analyze,” then statistics, and then click “temperature.” It should then show you the minimum and maximum temperatures. Check that these make sense relative to the data you observed and then record these in Table 1.

11. Dispose of the reaction products in the back sink and flush with plenty of water. Please pick out the solid Mg ribbon from the sink and put in the garbage instead. Be SURE to have your goggles on. Clean and dry all materials and put back in proper locations/cabinets. Throw out the plastic stirring rods.

12. Rinse your temperature probe and remove from the computer interface and put back in the bag. Go see Mrs. Jimenez with your probe and computer interface and cover. Sign your name and # in the book to record equipment usage and then give back to Mrs. Jimenez.

RESULTS

Table #1 – Temperature Ranges of Chemical Reactions

Part I RXN / PART II RXN
Minimum Temperature
Maximum Temperature

Table #2 – Qualitative OBSERVATIONS

PART i RXN / Part II RXN
Qualitative Observations

Results analysis

1. Calculate the temperature change, DT, for each reaction by subtracting the minimum temperature from the maximum temperature (DT = Tmax – T min). Use the 3 step method of writing formula, substituting in with units, and solving for ∆T with units in answer.

Part I Calculation ∆T

Part II Calculation ∆T

2. Tell which reaction is endothermic. Tell which reaction is exothermic. Explain why for each very briefly. Recall that if a reaction is endothermic, it will absorb heat from its environment/surroundings. If it is exothermic, it will release heat to its environment/surroundings.

3. For each of the two reactions, list all the specific clues that a chemical reaction was happening. Table #2 observations could be helpful with this.

Reaction I:

Reaction II:

4. Now apply what you have seen here today regarding chemical reactions to phase changes which are physical changes.

Which of the phase changes are endothermic? (List 3)

Which of the phase changes are exothermic? (List 3)

MORE ON RESULTS ANALYSIS

Attach to this “report,” a handmade bar graph on graph paper. It should have two bars for each reaction – one for minimum temperature and one for maximum temperature. Use different colors so that both minimum temperatures are one color and both maximum temperatures are another color and then use a key instead of labeling them as such.

Physical Science with Computers 5 - XXX