This animation shows hydrogen molecules and chlorine molecules translating and then reacting to form hydrogen chloride. Close-ups portray the electronic energy levels of the reactants--hydrogen and chlorine--and the product--hydrogen chloride. Because the ground state energy level of hydrogen chloride is lower than the ground energy levels of the reactants, energy is released during the reaction. Put another way, the energy of the product is lower than the energy of the reactants and therefore delta E is negative.

In the first part of this animation, ice is maintained at a temperature of minus 20. The free energy of liquid water is higher than the free energy of ice at this temperature and therefore the free energy change is positive. The ice, therefore, does not melt at minus 20 degrees. In the second part of the animation the temperature is increased to plus 20 degrees. At this temperature the free energy of the product is lower than that of the reactant and, consequently, the free energy change for the melting of ice is negative. The ice therefore melts.

At absolute zero, elemental chlorine exists in the solid state, where the atoms within the molecules undergo vibration. As the temperature is increased, the molecules begin to move in their lattice positions. When the melting point of 40 degrees is reached, the molecules leave their lattice positions and move about through the container.