Demonstration of Molecular Enthalpy

Purpose:

To describe the forms of energy a substance has.
To explain how the forms of energy change as molecules absorb and release energy.
To demonstrate changes in molecular enthalpy.

Materials:

Sodium thiosulfate crystals (Reagent grade)
Test tubes (13 * 100 mm)
Bunsen burner, or heating element.
Test tube tongs
Solid rubber stopper

Procedure:

1) Fill a clean and dry test tube 1/2 full with sodium thiosulfate crystals. Note the shape and colour of the solid.
2) Heat the test tube on a hot plate or with a bunsen burner, until the crystals melt. A clear liquid is preferable

Important:

Do not let the sodium thiosulfate boil.

3) Cool the test tube under a water tap until the surface of the test tube is cool enough to hold.

Important:

1) Do not allow any of the water to enter test tube during cooling
2) Be careful to cool the test tube but not allow the sodium thiosulfate to solidify.
3) Cork the test tube with a solid rubber stopper.
4) Shake the test tube an record your observations in a table of your own making..

Questions:

To answer these questions refer back to the discussion of molecular enthalpy.

1) What form of kinetic and potential energy does the sodium thiosulfate have in it's solid state?
2) When you heated the test tube which forms of energy or energies increased in the molecule?
3) When you cooled the molecule which forms of energy decreased? Which new form of energy still remained as long as the thiosulfate was in a liquid state (that was not present in the solid state)?
4) Shaking the liquid allows the molecules to line up in a solid. What energy change as the liquid becomes a solid?
5) Where then did the heat come from when the test tube was shaken?