When steam changes to water the kinetic energy and temperature remain the same, however bonds are created between the molecules. Since bonds are being created, why is this not an endothermic process where energy is absorbed and turned into bond energy?

  • $\begingroup$ You have it backwards. Hydrogen bonds (not covalent bonds) are broken endothermically upon vaporization and re-formed exothermically upon condensation. Unlike ice, liquid water forms very unstable and transient molecular clusters. In a hydrogen bond, the potential energy between a hydrogen and the oxygen of a different molecule is negative. $\endgroup$ Jul 8 '19 at 12:05
  • $\begingroup$ @Bert Barrois hydrogen bonds are not broken upon vaporization. The water molecules remain intact and simply move farther apart. There are no chemical changes, just physical changes. $\endgroup$
    – Bob D
    Jul 9 '19 at 0:14
  • $\begingroup$ @ Bob D -- A hydrogen bond is not a covalent bond. It is a weak (3 to 6 kcal/mol) electrostatic attraction between H and O of two different molecules. $\endgroup$ Jul 9 '19 at 11:20

Bonds are not created when steam condenses. Water molecules in the vapor phase are far apart from one another and come closer together during condensation. That lowers the potential energy of the water molecules.

Since the total internal energy of the water is the sum of its potential and kinetic energies, and the kinetic energy doesn’t change (temperature being constant during a phase change) the overall internal energy decreases. The loss is heat transfer out of the steam.

Why does the molecules being closer together decrease the potential energy?

Consider first a phase change from liquid water to water vapor. There are intermolecular attraction forces between the molecules of water. It takes energy (in the form of heat) to pull them apart in order for a phase change to occur from liquid to steam. Separating them increases their potential energy. An analogy (not exact) is it takes energy (in the form of work) to separate an object from the surface of the earth which increases its gravitational potential energy.

Now consider a phase change from water vapor back to liquid water. The opposite of the above occurs. The molecules come closer together and the potential energy decreases. That requires the release of an equal amount of energy in the form of heat. The gravity analogy is an object loses potential energy (converts it to kinetic energy) as it falls back to (decreases its separation from) the earth.

Hope this helps.

  • $\begingroup$ Why does the molecules being closer together decrease the potential energy? $\endgroup$
    – Melkor
    Jul 8 '19 at 11:49
  • $\begingroup$ See my answer to your follow up question in the revision to my answer. Hope it helps clarify things for you. $\endgroup$
    – Bob D
    Jul 8 '19 at 12:32

The process of bind two things together releases energy. You’d need to add energy to separate the bound items again.

Examples that might help:

  • The Moon is bound to the earth. It would take energy (lots of energy!) to lift it far away
    • An electron is bound electrically to a proton in a hydrogen atom. You have to add the ionization energy to pull it away, and it liberated energy (a photon) when it was captured.

So the weak binding that holds the water molecules together as a liquid liberates energy. That’s the latent heat.

It’s a common misconception that chemical bonds “contain” (positive) energy. But think about what happens if you burn hydrogen and oxygen: you make water, which contains stronger bonds than the individual H and O molecules, and the net result is a liberation of heat: the molecules have less energy than they started with.

  • $\begingroup$ I understand up to the chemical bonds part, wouldn't the heating add energy to the system which as you say would create stronger bonds? $\endgroup$
    – Melkor
    Jul 8 '19 at 12:09
  • $\begingroup$ Why do you think heating makes stronger bonds? $\endgroup$ Jul 8 '19 at 12:10
  • $\begingroup$ The heating adds energy to the system, so in your example shouldn't the bonds holding the water together be weaker than before? $\endgroup$
    – Melkor
    Jul 8 '19 at 13:03
  • $\begingroup$ To get steam to condense to liquid, you reduce the energy. Each bond is what it is, with a certain amount of negative energy (energy released). As condensation happens, that released energy become heat energy which you remove to cause more condensation. $\endgroup$ Jul 8 '19 at 13:15

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