Is condensation positive or negative enthalpy

Molar heat content of zinc above 298.15 K and at 1 atm pressure, showing discontinuities at the melting and boiling points. The enthalpy of melting (ΔH°m) of zinc is 7323 J/mol, and the enthalpy of vaporization (ΔH°v) is 115 330 J/mol.

The enthalpy of vaporization, (symbol Δ v H {\displaystyle \Delta {}_{v}H}

The enthalpy of condensation (or heat of condensation) is numerically exactly equal to the enthalpy of vaporization, but has the opposite sign: enthalpy changes of vaporization are always positive (heat is absorbed by the substance), whereas enthalpy changes of condensation are always negative (heat is released by the substance).

The enthalpy of vaporization can be viewed as the energy required to overcome the intermolecular interactions in the liquid (or solid, in the case of sublimation). Hence helium has a particularly low enthalpy of vaporization, 0.0845 kJ/mol, as the van der Waals forces between helium atoms are particularly weak. On the other hand, the molecules in liquid water are held together by relatively strong hydrogen bonds, and its enthalpy of vaporization, 40.8 kJ/mol, is more than five times the energy required to heat the same quantity of water from 0 °C to 100 °C (cp = 75.3 J K−1 mol−1). Care must be taken, however, when using enthalpies of vaporization to measure the strength of intermolecular forces, as these forces may persist to an extent in the gas phase (as is the case with hydrogen fluoride), and so the calculated value of the bond strength will be too low. This is particularly true of metals, which often form covalently bonded molecules in the gas phase: in these cases, the enthalpy of atomization must be used to obtain a true value of the bond energy.

An alternative description is to view the enthalpy of condensation as the heat which must be released to the surroundings to compensate for the drop in entropy when a gas condenses to a liquid. As the liquid and gas are in equilibrium at the boiling point (Tb), ΔvG = 0, which leads to:

Δ v S = S g a s − S l i q u i d = Δ v H / T b {\displaystyle \Delta \,_{v}S=S_{gas}-S_{liquid}=\Delta \,_{v}H/T_{b}}

As neither entropy nor enthalpy vary greatly with temperature, it is normal to use the tabulated standard values without any correction for the difference in temperature from 298 K. A correction must be made if the pressure is different from 100 kPa, as the entropy of a gas is proportional to its pressure (or, more precisely, to its fugacity): the entropies of liquids vary little with pressure, as the compressibility of a liquid is small.

These two definitions are equivalent: the boiling point is the temperature at which the increased entropy of the gas phase overcomes the intermolecular forces. As a given quantity of matter always has a higher entropy in the gas phase than in a condensed phase ( Δ v S {\displaystyle \Delta \,_{v}S}

Δ G = Δ H − T Δ S {\displaystyle \Delta \,G=\Delta \,H-T\Delta \,S}

the Gibbs free energy change falls with increasing temperature: gases are favored at higher temperatures, as is observed in practice.

Selected values

Other common substances

Common substances sorted by heat of vaporization:

See also

References

Sears, Zemansky et al., University Physics, Addison-Wessley Publishing Company, Sixth ed., 1982, ISBN 0-201-07199-1

af:Verdampingswarmte ar:حرارة تبخر ast:Entalpía de vaporización ca:Calor de vaporització cs:Měrné skupenské teplo varu da:Fordampningsvarme de:Verdampfungswärme gl:Entalpía de vaporización ko:기화열 id:Panas penguapan it:Entalpia di vaporizzazione lt:Garavimo šiluma jbo:manri febybi'o nejni hu:Párolgáshő nl:Verdampingswarmte uz:Solishtirma qaynash issiqligi sl:Izparilna toplota sr:Топлота испаравања sh:Toplota isparavanja fi:Höyrystymislämpö sv:Ångbildningsvärme th:ความร้อนแฝงของการกลายเป็นไอ uk:Питома теплота випаровування

Condensation is a change of state from the gas to the liquid phase. Most frequently refers to the conversion of water vapor to liquid water, which happens for example on cold surfaces or in nuclei in the atmosphere.

Condensation is also an example of a phase transition. The reverse process (transformation of liquid into gas phase) is called vaporization.

Endothermic phase transitions take energy, in the form of heat, from the environment. Exothermic phase transitions release heat to the environment.

Condensation releases heat to the environment, so it is exothermic. During condensation, the molecules lose kinetic and potential energy, which is released as heat (thermal energy).

Evaporation is the opposite of condensation. Thus, evaporation is an endothermic process. When molecules are in the liquid state, they are close together and attracted to each other. Energy, in the form of heat, must be given to separate them, so evaporation is an endothermic process.

What is the Endothermic and Exothermic Process?

Endothermic means that the transition takes heat (thermal energy) from the environment. Exothermic phase transitions release heat (thermal energy) into the environment.

Examples of endothermic transitions are the boiling of water and the melting of ice, which take heat from the environment.

Exothermic transitions are the condensation of water vapor and the freezing of liquid water, that gives heat to the environment.

What is Condensation?

In condensation, the molecules, initially separated in the gas phase, get closer and form a liquid (denser phase).

Condensation is commonly observed on cold solid surfaces, for example, refrigerated metal cans and inside glass windows in winter.

Also, the condensation of water vapor on the small particles of dust in the atmosphere produces clouds. Under freezing temperature conditions, atmospheric water vapor condenses to snow (the solid phase) and the process is called deposition.

Does Condensation Absorb or Release heat?

Condensation releases heat because the molecules lose kinetic and potential energies that are released as heat to the environment.

An example is the condensation of water vapor.

Why is Condensation Exothermic?

The molecules in the gas are separated by great distances (in relation to their size). They move in straight lines until they collide with another molecule or the wall of the container. So, the gas molecules have kinetic energy.

The molecules in the liquid phase are closer together and move slower than those in the gas phase.

Thus, in condensation, the gas molecules lose kinetic energy when going into the liquid state. The lost kinetic energy is released to the environment in the form of heat.

Also, the molecules in the liquid are close, there is a force of attraction between them. This means that the liquid has lower potential energy than the gas. This potential energy is lost by the gas molecules and is also transferred to the environment as heat.

The loss of kinetic and potential energy, released in the form of heat, makes condensation exothermic.

When vapor comes in contact with the surface of an object kept at a temperature lower than the saturation temperature, it condenses into a liquid with the release of heat, called latent heat. This process finds use for heating.

Thus, the reaction from left to right (Condensation) is exothermic, and from right to left (Evaporation) is endothermic.

The Phase Changes of Water. Which are Exothermic, and Which are Endothermic?

Variations in pressure or temperature produce phase changes. The phase changes can be seen in a phase diagram. A phase diagram for water is shown below, with the corresponding changes.

Figure 1. The phase diagram for water

Thus, as seen in the diagram, the transition from gas to liquid is called condensation, and the inverse vaporization. From solid to liquid is called melting, and the inverse freezing.

The direct transformation from solid to gas is called sublimation, and inverse deposition. The deposition is a type of condensation. Frost and snow are examples of deposition.

In Figure 2 we see the liquid and gas section of Figure 1.

Figure 2. A section of the phase diagram for water

At points A and C, liquid and gas are at equilibrium. At point B, only the gas (vapor) is present, and condensation is not possible.

To have condensation at constant pressure P, the temperature may be lowered from B to A; and for doing condensation at constant temperature TB, pressure has to be increased from B to C.

The Heat of Condensation

The heat of condensation is defined as “the heat evolved when a vapor changes to a liquid”. More specifically, “the quantity of heat that is evolved when the unit mass of a vapor is changed at a specified temperature to a liquid and that equals the heat of vaporization” (Merriam-Webster).

The heat added or removed during a phase change is called “latent heat”.

The heat of condensation is equal to the heat of vaporization with the opposite sign. The heat of vaporization is always positive and the heat of condensation is always negative.

The heat of condensation is equal to the change of a thermodynamic property called enthalpy, represented by H:

ΔH = Hliquid – Hgas

The gas and liquid are at the same conditions of pressure and temperature.

In general, If heat is absorbed during the process, ΔH is positive; if heat is released, then ΔH is negative.

Enthalpy of Condensation

The enthalpy of condensation is equal to the heat of condensation, which is always negative because heat is released into the environment.

The enthalpy of evaporation is always positive because energy (in the form of heat) is needed to separate the molecules. Enthalpy of evaporation and enthalpy of condensation have the same absolute value, but different signs.

The stronger the attraction between the molecules, the higher the energy needed for separating them, and the higher the enthalpy of evaporation. The enthalpy of evaporation is dependent upon pressure.

Enthalpies are measured in units of energy per mole, or Joule/mol. Table 2 shows enthalpies of condensation at pressure = 1 atm for a series of liquids:

Molar Heat of Condensation

The molar heat of condensation is the heat evolved when one mole of vapor is condensed. One mole is equal to 6.02214076×1023 molecules, or a mass in grams equal to the molecular weight in daltons.

In the case of water, whose molecular weight is 18.015 daltons, the mass of a mole is 18.015 grams.

The value of the molar heat of condensation is equal to the enthalpy (Table 2) with the changed sign. So, the molar heat of condensation for water at P = 1 atm and T= 100 °C is 40660 J/mol.

What Conditions Cause Condensation?

Condensation occurs when warm, moist air collides with cold surfaces. When this moisture-packed warm air comes into contact with a chilly surface, it cools down quickly and releases the water, which turns into liquid droplets on the cold surface.

Examples are water condensing on a cold beer can, glass windows in winter, walls, mirrors, and eyeglasses.

Clouds form in the upper atmosphere when the invisible water vapor rises from the earth’s surface and temperature drops.

Water then condenses in the always present small particles, like dust. The droplets may then coalesce to produce rain drops. The condensation at low altitudes is similar but it is called fog.

FAQs

Is Condensation a physical change or a chemical change?

Condensation of molecules is a physical change. Chemical changes involve dissociation and/or the formation of chemical bonds.

For example, no chemical bonds are formed or broken in the condensation of water vapor. Chemical changes occur in chemical reactions, where new molecules are formed.

Is Sublimation endothermic or exothermic?

Sublimation is endothermic because the environment gives energy (in the form of heat) needed for separating the molecules in the solid and increasing their kinetic energy.

An example is the direct transformation of frozen water to vapor.

Is Evaporation endothermic or exothermic?

Evaporation is endothermic because energy (in the form of heat) is needed for overcoming the forces of attraction between the molecules and for increasing their kinetic energy.

What processes use evaporation and condensation?

• Solar desalination.

In solar desalination, water is heated by the sun in a specially designed still, and the vapor is condensed on a cold surface. This method does not need another source of energy, like distillation followed by condensation.

• Refrigeration and air-conditioning.

A cycle including evaporation and condensation of a special fluid (generally tetrafluoroethane) is used for refrigeration and air-conditioning.

The evaporation part, being endothermic, takes heat from the environment thus lowering the temperature.

• Heat pumps

Heat pumps also use evaporation and condensation cycles to transfer heat from the outside to houses and buildings using evaporation-condensation cycles.

They consume less energy than other heating systems. Heating pumps can also be used for cooling.

Condensation is exothermic (it gives away energy in the form of heat). The heat comes from the kinetic and potential energy of the molecules.

As a result, the change of a property called enthalpy is negative – the enthalpy of the condensed mass decreases in the process. The enthalpy change is measured in units of energy/mass, frequently J/kg or J/mol.

An important example of condensation is the condensation of water vapor or moisture.