Let us continue our thermochemistry review as we proceed into chemical thermodynamics with the familiar examples of melting and vaporization. When heat is added to a solid below its melting point, the temperature begins to rise. Rising temperature means that the average kinetic energy of the particles is increasing (although with greater range of vibration along lines of intermolecular force, the electrostatic potential energy component is also increasing). So picture the molecules vibrating along lines of intermolecular force.
Molar heat content of zinc above 298.15 K and at 1 atm pressure, showing discontinuities at the melting and boiling points.
If the solid is heated at its melting point, at the melting point, the temperature remains constant until the solid has melted. The melting process requires the heat of fusion to flow into the matter to allow the particles to escape from the rigid intermolecular binding of the solid state into the less tight liquid arrangement.
When mutually attracting charged particles (the polarities which lead to intermolecular force) are moved apart from one another, the electrostatic potential energy of the system is increased. The heat flow into the matter at the melting point increases the electrostatic potential energy of the particles along lines of intermolecular force.
After all of the solid has melted, heating the liquid can now raise its temperature until the boiling point is reached. The heat of vaporization then causes the liquid to boil at constant temperature. Similar to the heat of fusion, the internal energy increase brought about by the heat of vaporization is not increasing the average kinetic energy of the particles (the temperature is constant during vaporization) but the electrostatic potential energy increases as the particles are separated each from each other. The heat flow is not only increasing the internal energy. There is also a large change in the volume of the system, so the heat that flows in must also perform pressure-volume work. After all the liquid has been vaporized, the addition of more heat then raises the temperature of the gas. After vaporization, in which the particles escape from their mutual well of electrostatic binding energy, the internal energy increase caused by heat flow leads to increased particle kinetic energy (and higher temperature).
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