define phase transitions and phase transition temperatures explain the relation between phase change temperatures and also intermolecular attractive forces

We witness and also utilize changes of physics state, or phase transitions, in a good number the ways. Together one example of worldwide significance, consider the evaporation, condensation, freezing, and melting the water. These transforms of state space essential aspects of ours earth’s water cycle and also many other herbal phenomena and technical processes of central importance to our lives. In this module, the essential aspects of step transitions room explored.

You are watching: The vapor pressure of ammonia at several different temperatures is shown below.

Vaporization and also Condensation

When a liquid vaporizes in a close up door container, gas molecules cannot escape. As these gas step molecules move randomly about, lock will occasionally collide with the surface of the condensed phase, and also in some cases, this collisions will result in the molecules re-entering the condensed phase. The readjust from the gas phase to the fluid is dubbed condensation. When the rate of condensation becomes equal to the rate of vaporization, neither the amount of the liquid nor the quantity of the vapor in the container changes. The vapor in the container is then stated to it is in in equilibrium with the liquid. Save in mind that this is no a revolution situation, together molecules space continually exchanged in between the condensed and gaseous phases. Such is an instance of a dynamic equilibrium, the status of a mechanism in i m sorry reciprocal procedures (for example, vaporization and also condensation) take place at same rates. The press exerted by the vapor in equilibrium v a liquid in a closeup of the door container in ~ a provided temperature is referred to as the liquid’s vapor press (or equilibrium vapor pressure). The area that the surface of the fluid in contact with a vapor and also the dimension of the vessel have no result on the vapor pressure, although castle do affect the time forced for the equilibrium to it is in reached. We deserve to measure the vapor pressure of a liquid by put a sample in a closeup of the door container, favor that illustrated in figure \(\PageIndex1\), and also using a manometer to measure up the rise in push that is because of the vapor in equilibrium v the condensed phase.


Figure \(\PageIndex1\): In a close up door container, dynamic equilibrium is reached once (a) the price of molecule escaping native the fluid to come to be the gas (b) increases and eventually (c) equates to the rate of gas molecule entering the liquid. As soon as this equilibrium is reached, the vapor pressure of the gas is constant, back the vaporization and also condensation processes continue.

The thedesigningfairy.comical identities that the molecules in a liquid identify the types (and strengths) the intermolecular attractions possible; consequently, different substances will certainly exhibit various equilibrium vapor pressures. Reasonably strong intermolecular attractive pressures will offer to impede vaporization and also favoring “recapture” that gas-phase molecules as soon as they collide with the fluid surface, resulting in a reasonably low vapor pressure. Weak intermolecular attractions present less that a barrier to vaporization, and a decreased likelihood that gas recapture, yielding reasonably high vapor pressures. The following example illustrates this dependence of vapor pressure on intermolecular attractive forces.

Example \(\PageIndex1\): Explaining Vapor press in terms of IMFs

Given the shown structural formulas because that these 4 compounds, define their family member vapor pressures in terms of varieties and extents the IMFs:

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Figure \(\PageIndex3\): The boiling clues of liquids are the temperature at which your equilibrium vapor pressures equal the push of the surrounding atmosphere. Regular boiling points room those matching to a press of 1 atm (101.3 kPa.)

Example \(\PageIndex2\): A Boiling point at lessened Pressure

A typical atmospheric pressure in Leadville, Colorado (elevation 10,200 feet) is 68 kPa. Usage the graph in figure \(\PageIndex3\) to identify the boiling allude of water in ~ this elevation.


The graph that the vapor press of water matches temperature in figure \(\PageIndex3\) indicates that the vapor press of water is 68 kPa at about 90 °C. Thus, at around 90 °C, the vapor press of water will certainly equal the atmospheric pressure in Leadville, and also water will certainly boil.

The quantitative relation in between a substance’s vapor pressure and also its temperature is defined by the Clausius-Clapeyron equation:



\(ΔH_vap\) is the enthalpy of vaporization because that the liquid, \(R\) is the gas constant, and \(\ln A\) is a constant whose value counts on the thedesigningfairy.comical identification of the substance.

Equation \(\ref10.4.1\) is frequently rearranged into logarithmic kind to productivity the direct equation:

\<\ln P=−\dfracΔH_\cevapRT+\ln A \label10.4.2\>

This linear equation might be express in a two-point format that is convenient for use in various computations, together demonstrated in the examples and also exercises that follow. If in ~ temperature \(T_1\), the vapor press is \( P_1\), and at temperature \(T_2\), the vapor pressure is \(T_2\), the matching linear equations are:

Exercise \(\PageIndex3\)

At 20.0 °C, the vapor push of ethanol is 5.95 kPa, and at 63.5 °C, the vapor pressure is 53.3 kPa. Use this info to calculation the enthalpy of vaporization for ethanol.


47,782 J/mol = 47.8 kJ/mol

Example \(\PageIndex4\): Estimating Temperature (or Vapor Pressure)

For benzene (C6H6), the regular boiling suggest is 80.1 °C and also the enthalpy that vaporization is 30.8 kJ/mol. What is the boiling suggest of benzene in Denver, whereby atmospheric push = 83.4 kPa?


If the temperature and vapor press are recognized at one point, in addition to the enthalpy that vaporization, ΔHvap, climate the temperature that corresponds to a various vapor press (or the vapor pressure that corresponds to a different temperature) have the right to be figured out by making use of the Clausius-Clapeyron equation (Equation \(\ref10.4.1\)) :

Enthalpy of Vaporization

Vaporization is one endothermic process. The cooling effect can be apparent when you leaving a swimming swimming pool or a shower. Once the water on her skin evaporates, it removes heat from her skin and causes you to feel cold. The energy adjust associated v the vaporization procedure is the enthalpy that vaporization, \(ΔH_vap\). Because that example, the vaporization that water at typical temperature is stood for by:

\<\ceH2O(l)⟶\ceH2O(g)\hspace20pxΔH_\cevap=\mathrm44.01\: kJ/mol\>

Example \(\PageIndex5\): using Enthalpy of Vaporization

One method our human body is cooled is through evaporation that the water in sweat (Figure \(\PageIndex4\)). In very hot climates, we deserve to lose as lot as 1.5 l of sweat per day. Although sweat is not pure water, we can acquire an approximate worth of the lot of heat removed through evaporation by assuming that it is. How much warmth is forced to evaporate 1.5 l of water (1.5 kg) at T = 37 °C (normal body temperature); \(ΔH_vap = 43.46\, kJ/mol\) at 37 °C.

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Figure \(\PageIndex5\): (a) This manufacturer of ice has a temperature of −12.0 °C. (b) after ~ 10 minutes the ice has absorbed enough warm from the air to warm to 0 °C. A small amount has melted. (c) Thirty minute later, the ice has actually absorbed an ext heat, however its temperature is still 0 °C. The ice cream melts without transforming its temperature. (d) only after all the ice has melted walk the heat soaked up cause the temperature to boost to 22.2 °C. (credit: modification of job-related by mark Ott).

If we stop heating throughout melting and place the mixture that solid and also liquid in a perfect insulated container for this reason no warm can get in or escape, the solid and also liquid phases stay in equilibrium. This is practically the situation with a mixture that ice and also water in a very great thermos bottle; almost no warmth gets in or out, and also the mixture of heavy ice and also liquid water continues to be for hours. In a mixture the solid and also liquid at equilibrium, the reciprocal processes of melting and also freezing take place at same rates, and the quantities of solid and liquid because of this remain constant. The temperature in ~ which the solid and also liquid phases that a offered substance room in equilibrium is dubbed the melting point of the solid or the freezing suggest of the liquid. Use of one term or the other is normally dictated by the direction of the phase transition being considered, because that example, hard to fluid (melting) or fluid to hard (freezing).

The enthalpy of combination and the melting allude of a crystalline solid count on the toughness of the attractive forces between the units present in the crystal. Molecules through weak attractive forces kind crystals v low melt points. Crystals consists of particles with stronger attractive pressures melt at greater temperatures.

The amount of heat required to adjust one mole the a problem from the hard state come the liquid state is the enthalpy of fusion, ΔHfus of the substance. The enthalpy of blend of ice is 6.0 kJ/mol at 0 °C. Blend (melting) is one endothermic process:

\<\ceH2O_(s) \rightarrow \ceH2O_(l) \;\; ΔH_\cefus=\mathrm6.01\; kJ/mol \label10.4.9\>

The reciprocal process, freezing, is an exothermic procedure whose enthalpy change is −6.0 kJ/mol in ~ 0 °C:

\<\ceH_2O_(l) \rightarrow \ceH_2O_(s)\;\; ΔH_\cefrz=−ΔH_\cefus=−6.01\;\mathrmkJ/mol \label10.4.10\>

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Figure \(\PageIndex7\): for a offered substance, the sum of that is enthalpy of combination and enthalpy that vaporization is around equal come its enthalpy of sublimation.


Video \(\PageIndex3\): review of phase changes with regards come kinetics.

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Phase transitions are processes that convert matter indigenous one physical state into another. Over there are six phase transitions in between the 3 phases of matter. Melting, vaporization, and also sublimation room all endothermic processes, request an intake of heat to get rid of intermolecular attractions. The reciprocal transitions of freezing, condensation, and deposition are all exothermic processes, involving warmth as intermolecular attractive pressures are developed or strengthened. The temperatures at which phase transitions happen are established by the family member strengths the intermolecular attractions and also are, therefore, dependent on the thedesigningfairy.comical identity of the substance.