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.
To relate the heat of solution involved to the two-step process of dissolving.
Apparatus and Materials:
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.
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.
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
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%
% error ammonium nitrate = %