Endothermic and Exothermic Reactions

 Some chemical reactions absorb energy and are called endothermic reactions. Many chemical reactions 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.

 You will study the 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)

 You will study the 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

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

•  identify endothermic and exothermic reactions

Figure 1

 

 PROCEDURE

1.   Obtain and wear goggles.

 Citric Acid Plus Baking Soda

 2.   Mass your calorimeter. Measure out 30.0 mL of citric acid solution in the graduated cylinder and place it in the calorimeter. Mass the calorimeter with acid solution and record so that the mass of the acid can be calculated. Place the thermometer into the citric acid solution and record the temperature after 15 seconds with no change in the reading.

 3.   Weigh out 10.0 g of solid baking soda on a piece of weighing paper.

 4.   Add the baking soda to the citric acid solution.  Stir the solution to ensure a good mixing.    
   Continue to watch the temperature carefully.  Record the lowest or highest temperature that is
    reached.

 5.   Dispose of the reaction products into the plastic waste cups.

 Hydrochloric Acid Plus Magnesium

 6.   Measure out 30.0 mL of HCl solution into the clean graduated cylinder and transfer into a cleaned calorimeter. CAUTION: Handle this acid with care. It can cause painful burns if it comes in contact with your skin or gets into your eyes.   Mass and record to find the mass of the acid solution. Place the thermometer into the HCl solution and record the temperature after 15 seconds with no change in the reading.

 7.   Get approximately 0.1 grams of magnesium.  Record actual mass of magnesium used.

8.   Add the Mg to the HCl solution.    

•  Stir the solution to ensure good mixing. CAUTION: Do not breathe the vapors!

•  Collect data until a minimum or maximum temperature has been reached.

9.         Dispose of the reaction products into the plastic waste cups.

  

DATA

1.      Mass of calorimeter:                                           ______ g

2.      Mass of citric acid and calorimeter:                     ______ g

3.      original temperature of citric acid solution:           ______ °C

4.      final temperature of citric acid/baking soda:         ______ °C

5.      mass of hydrochloric acid and calorimeter:          ______ g

6.      original temperature of hydrochloric acid:            ______ °C

7.      final temperature of HCl/Mg mixture:                  ______ °C

5.   actual mass of Mg used:                                     ______  g

 OBSERVATIONS

   PROCESSING THE DATA

 1.   Calculate the temperature change, Dt, for each reaction by subtracting the minimum temperature from the maximum temperature (Dt = t_max – t _min).

   2.   Tell which reaction is endothermic. Tell which reaction is exothermic. Explain.

 3.   For each reaction, describe three ways you could tell a chemical reaction was happening.

 EXTENSIONS

  1.      Determine the energy effect, in joules per gram of  NaHCO₃.

  2.      Determine the energy effect, in joules per gram of  Mg.