Thermochemistry Lab #1     Heat of Solution of a Solid

Note references: Specific Heat and Heat Capacity, Enthalpy, Phase Changes and Calorimetry.

When a solid dissolves in water, the process always has a energy change associated with it. Examples exist for both endothermic and exothermic heats of solution. However, the dissolving process itself is really a two-step process. The first step, that of breaking down the solid crystal, is endothermic while the second step, that of hydrating the individual particles released into the solvent, is exothermic. For an ionic compound, ionic bonds are broken while ion-dipole forces are formed. Ion-dipole forces are formed when the slightly positive hydrogen atoms are attracted to the negative ions, and the slightly negative oxygen atoms are attracted to the positive ions. The overall heat of solution depends on the relative amounts of energy involved in the two individual steps.

Hydration of ions. Hydration involves a complex redirection of forces of attraction and repulsion. Before this solution forms, water molecules are attracted only to each other; and Na+ and Cl- ions have only each other in the crystal to be attracted to. In the solution, the ions have water molecules to take the places of their oppositely charged counterparts; and water molecules find ions more attractive then even other water molecules.

In this experiment, you will determine the heats of solution for the dissolving of ammonium nitrate and of sodium acetate in water, using a simple styrofoam cup as a calorimeter.

Objectives:
To measure experimentally the amount of heat involved in the dissolving of ammonium nitrate and of sodium acetate in water.

To relate the heat of solution involved to the two-step process of dissolving.

Apparatus and Materials:
2 styrofoam cups (bring your own, paper is not suitable)
thermometer
100 mL graduated cylinder
balance
stirring rod
sodium acetate,  NaCH3COO
ammonium nitrate, NH4NO3

Procedure:
1. Find the data table included with this lab and familiarize yourself with its layout.

2. Accurately find the mass of a sample of solid ammonium nitrate of approximately 15 grams.

3. Find the mass of the pair of dry styrofoam cups.

4. Add about 150 mL of distilled water to the cup and find the mass of the cup and water.

5. stir the water briefly with the thermometer and record the temperature to the nearest 0.2oC.

6. Dissolve the solid in the water, stirring with the thermometer, and record the maximum temperature difference from the initial reading.

7. Rinse out the cup, dry it thoroughly, and repeat the experiment using a sample of about 15 grams of solid sodium acetate in place of the ammonium nitrate.

Calculations:
1. From your data, calculate the following for each part of the experiment.
a) the temperature change of the water.
b) the mass of water.
c) the quantity of heat absorbed (or given off) by the water during the dissolving.
d) the number of moles of solid used.
e) the quantity of heat involved per mole of solid dissolved. This is called the molar heat of solution.

2. Find the accepted values for the molar heats of solution for these solids in a chemistry handbook, and calculate the percentage error of your experimental values.

Questions:
1. Write an equation for the dissolving process for each solid. Include the heat term in each equation.

2. Consider the fact that dissolving is actually a two-step process. Describe each step using both words and a diagram. Designate each step as endothermic or exothermic.

3. How does the nature of these individual steps combine to determine whether the overall process will be endothermic or exothermic?
 


Heat of Solution of a Solid

Data Tables

Ammonium nitrate 
Sodium acetate
Mass of empty styrofoam cup =  
  g
  g
Mass of cup and water =
  g
  g
Mass of NaOH used = 
  g
  g
Initial temperature of H2O =
oC oC
Final temperature of H2O = 
oC oC

Calculations


     Ammonium   nitrate  
     Sodium acetate    
Mass of H2O used = g
g
Formula of solid =          
____________
____________
Molecular mass of solid = g/mol
g/mol
Number of moles of solid =                            
mol
mol
Temperature change = oC oC
Energy released in reaction =   
kJ
kJ

                                                               

Research- What is the accepted value of the heats of solution for both of the above solids?

Ammonium Nitrate =        _____________________ kJ/mole

Sodium acetate =              _____________________ kJ/mole

Percent error calculation:

% error = accepted value - your value * 100%
                       accepted value

% error                ammonium nitrate =                                   %

                               sodium acetate =                                    %