Heat of Solution Reactions Lab

    Chemical and physical changes are usually accompanied by the liberation or absorption of energy.  If energy is evolved, the reaction is said to be EXOTHERMIC.  If the energy is absorbed, the reaction is ENDOTHERMIC. 

   Heat is a form of energy.  The calorie or joule is the unit used to express heats of reaction.  The calorie is defined as the amount of heat required to raise the temperature of 1 gram of water one Celsius degree.  For conversions, 1 calorie = 4.185 joules.

    Energy may be transformed from one kind to another within an isolated (or closed) system but the total energy does not change.  If the change in energy of a system can be measured and if this change is due solely to a chemical reaction, then the energy change must be equal to that of the chemical reaction itself.

    In this experiment a simple calorimeter will be used and the change in energy of the system will be measured by observing the temperature of a given weight of water before and after the reaction occurs.  The specific heat capacity of water (i.e., the energy required to raise the temperature 1^o C of 1 gram of the material) is very nearly 1.00 cal/g^oC for temperatures between 0^o C and 100^o C.  Thus, if a calorimeter contained 100 grams of water at    23.0^o C initially and after the reaction took place there were still 100. grams of water and the temperature was now 30.0^o C, the energy liberated in the reaction would be

      Q  (cal)  =   mass (g)  x  Dt (^oC)  x  c_p  (cal/g^oC)

   700. cal     =   100. g      x  7.00^o C      x    1.00 cal/g^oC

    This calculation assumes that no energy was required to raise the temperature of the calorimeter itself and that no energy was lost, or gained, by the calorimeter during the experiment.

    Accurate calorimeters are very expensive and very tedious to operate.  In this experiment a simple calorimeter will be made by placing one styrofoam (polystyrene) cup inside another and covering the inner cup with a piece of cardboard to minimize heat loss or gain from the surface of the liquid.  Expanded polystyrene is a good insulator and has a very high specific heat capacity.

 Procedure:

1. Measure as accurately as possible with a graduated cylinder 75.0 mL of water and transfer the water to the inner styrofoam cup.  Record the temperature of the water.

 2. Mass accurately,  a quantity of NaOH between two and three grams.

 3. Add the NaOH to the water, continually swishing the cup gently, and with constant observation of the temperature until it remains constant for about 15-20 seconds.  Record this temperature.

 4. Repeat using NH₄NO₃.

 5. After completing all data collection for this experiment, move on to the second experiment.

Data Table:

mass of 75.0 mL of water                     ______

mass of NaOH                                     ______

initial temperature of water                    ______

final temperature of solution                   ______

 mass of 75.0 mL of water                  ______

mass of NH₄NO₃                                 ______

initial temperature of water                    ______

final temperature of solution              ______

Questions:

1. Is the dissolution of the NaOH in water an exothermic or endothermic process?  What about the ammonium nitrate?

 

2. Assuming 1 mL of water has a mass of 1 gram and the specific heat of the dilute solution is the same as water, calculate the number of calories involved in the dissolution of the different materials. (use actual mass of H₂O)

    # of cal  =   75 g  x  Dt  x  1.00 cal/g^oC                                 # of joules = 75 g  x  Dt  x  4.185 J/gºC

   NaOH      ______________ cal                                               NaOH   _______________ J

      NH₄NO₃   ______________ cal                                          NH₄NO₃  ______________ J

3. How many moles of each substance were dissolved?

    # of moles  =   grams dissolved  |  1 mole of substance

                                                     |   formula mass

   NaOH           ____________   mole

   NH₄NO₃        _____________ mole

   

4. Calculate the number of calories (and joules) that would be involved if one mole of the substance were dissolved in water.

    # of cal or J   =     # of cal or J (see question 2 above)             
        mole                     # of moles  (see question 3 above)

    NaOH         ______________ cal/mole                                  NaOH    ____________ J/mol

      NH₄NO₃      ______________ cal/mole                               NH₄NO₃ ____________ J/mol

 5.  Calculate the joules/gram absorbed or released when the sodium hydroxide and ammonium nitrate was dissolved in water.