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Latent Heat

Key Concepts

  • Latent Heat (enthalpy) is the "hidden" heat when a substance absorbs or releases heat without producing a change in the temperature of the substance, eg, during a change of state.

  • Latent Heat (enthalpy) of Fusion is the heat absorbed per mole when a substance changes state from solid to liquid at constant temperature (melting point).

  • Latent Heat (enthalpy) of Vaporization (vaporisation) is the heat absorbed per mole when a substance changes state from liquid to gas at constant temperature (boiling point).

  • Latent Heat (enthalpy) of Sublimation is the heat absorbed per mole when a substance changes state from solid to gas, without going through the liquid phase, at constant temperature.

Changes of State

When a substance melts or boils (vaporizes) it absorbs energy, an endothermic reaction.

When a substance condenses to a liquid or freezes to a solid, it releases energy, an exothermic reaction.

Change of State Name Energy is ΔH = ΔH sign
solid → liquid melting or fusion absorbed Latent Heat of
Fusion
+
liquid → solid solidification or freezing released -Latent Heat of
Fusion
-
liquid → gas vaporization or boiling or evaporation absorbed Latent Heat of
Vaporization
+
gas → liquid condensation or liquefaction released -Latent Heat of
Vaporization
-
solid → gas sublimation absorbed Latent Heat of
Sublimation
+
gas → solid condensation released -Latent Heat of
Sublimation
-

Heating and Cooling Curves

Heating Curves

A substance is heated at a uniform rate:

  • Temperature of the solid rises uniformly until the melting point is reached.

  • At the melting point heat is absorbed and used to melt the solid without any temperature change (latent heat of fusion), all the energy is going into weakening the intermolecular forces between the particles in the solid.

  • When all the solid has melted to a liquid, the temperature starts to increase uniformly again until the boiling point is reached.

  • At the boiling point heat is absorbed without any change in temperature (latent heat of vaporization), all the energy absorbed is being used to overcome the intermolecular forces between the particles in the liquid.

  • When all the liquid has been vaporized to gas the temperature will once again increase.

Cooling Curves

A substance is cooled:
  • Temperature of the gas falls until the boiling point is reached.

  • At the boiling point the gas condenses to a liquid, releasing the latent heat of vaporization, causing the temperature to remain constant.

  • When all the gas has liquefied, the temperature falls again until the freezing point is reached.

  • At the freezing point the liquid solidifies to a solid, releasing the latent heat of fusion, causing the temperature to remain constant.

  • When all the liquid has solidified, the temperature once again decreases.

Latent Heats for Some Common Substances

Enthalpies (heats) of fusion and evaporation at the transition temperature for each change of state.
Melting Melting Point
(oC)
Latent Heat
of Fusion ( kJ mol-1)
Boiling Boiling Point
(oC)
Latent Heat
of Vaporization (kJ mol-1)
H2(s) → H2(l) -259 0.12 H2(l) → H2(g) -253 0.92
O2(s) → O2(l) -219 0.44 O2(l) → O2(g) -183 6.82
N2(s) → N2(l) -210 0.36 N2(l) → N2(g) -196 2.8
H2O(s) → H2O(l) 0 6.01 H2O(l) → H2O(g) 100 40.66
NH3(s) → NH3(l) -78 5.65 NH3(l) → NH3(g) -33 23.35
CH4(s) → CH4(l) -182 0.94 CH4(l) → CH4(g) -161 8.18
CH3OH(s) → CH3OH(l) -98 3.16 CH3OH(l) → CH3OH(g) 64 35.27

The stronger the intermolecular forces acting between the molecules in the substance are, the more energy is required to weaken those forces to cause the solid to melt.

Water, ammonia and methanol all have hydrogen bonds acting between the molecules, while the other substances are non-polar and have only weak intermolecular forces (Van der Waals, Dispersion, or London Forces) acting between the molecules.

The energy required to melt water, ammonia and methanol is greater than the energy required to melt the other tabulated substances.

Some substances, such as iodine and carbon dioxide, do not undergo the transition:

solid → liquid → gas

Instead they sublime:

solid → gas


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