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Latent Heats, Vapor Pressure and Sublimation Critical Point

Latent heat, Vapour pressure, Boiling point, Melting and freezing point, Vapour pressure of the mixture of liquids, Sublimation critical point.

Latent heat

Substances changing state absorb or release heat as a result of their change of state, also called transformational heat. Taking 1g of ice at 0C and converting it to 1g of water at 0C (the latent heat of fusion), 336 joules of energy are required. The release of this latent energy occurs when matter changes from a high to a low kinetic energy state (e.g. from water to a stream). There are practical applications of latent energy. Melting ice, for example, removes a substantial amount of energy from the skin, whereas the solidifying paraffin wax gives off a significant amount of heat, which is therefore warming to the skin.

Vapour pressure

When a closed system is at a specified temperature, the vapor pressure represents the pressure exerted by the vapor at equilibrium with the non-vaporized phases (liquid or solid). Equilibrium vapor pressure is a measure of how quickly a liquid evaporates. When particles escape a liquid (or solid), the liquid tends to lose its properties. A substance's vapor pressure is a measure of how strongly it evaporates at a given temperature. Vapour pressure can be described as the pressure that results from vapor present above liquid surfaces.

The molecules in liquids unevenly acquire kinetic energy, so some of them move faster than others due to greater energy acquisition. In the vapor phase (gas phase), molecules with enough energy to overcome intermolecular attractions can escape from the surface. It is a process known as evaporation.

Molecular molecules in vapor have a larger average kinetic energy than molecules in the liquid state, so the temperature of the liquid drops upon evaporation. Various factors determine how fast a liquid evaporates, such as temperature, the area of the surface, the pressure above the liquid, and the forces that pull the liquid to the surface.

Another form of condensation occurs when the molecules of liquid that are in the vapor phase collide, transferring energy to other molecules and becoming liquid again. As the concentration of molecules in the vapor phase increases, so does the rate of condensation in the vapor phase. Evaporation and condensation are equal in their rates at dynamic equilibrium conditions. According to the following equation, the vapour pressure of a liquid is proportional to its absolute temperature

Calculating Clausius-Clapeyron equation:

log p2/p1 = ‘H (T2 – T1 )/ 2.303 RT1T2

Where,
The vaporization heat of a molecule is H;

Vapour pressure p1 and p2 at absolute temperatures T1 and T2 can be expressed as a number.

Liquid molecules gain kinetic energy as the temperature of the liquid increases. Vapour pressure increases as the number of molecular transitions increase with the kinetic energy of the molecules. Further raising the temperature increases the density of the vapor while decreasing that of the liquid. After a while, the densities of both phases are equal, and they cannot be distinguished from each other. At this point, the liquid phase is unavailable and the critical temperature is reached. A liquid phase is not present above this temperature.

Boiling point

At the boiling point, the vapor pressure equals the atmospheric pressure of the liquid. This temperature causes a liquid to change from a liquid state to a vapor state. A liquid's normal boiling point can be determined by calculating the pressure at which it will boil (also known as atmospheric pressure boiling point or atmospheric boiling point at 1 atm, or atmospheric pressure at sea level). When a liquid reaches its boiling point, all of the heat absorbed is transformed into vapor, so the temperature does not increase until the vapor has completely replaced the liquid. Latent heat of vaporization is what causes that heat to be released.

Melting and freezing point

Throughout history, melting points have been used to define the temperature at which solids become liquids. All energy released during melting is converted to the latent heat of fusion, which maintains a constant temperature. When the melting point is reached, solid and liquid phases are coexistent in equilibrium. It is identified as the freezing or crystallizing point of a liquid when it is at the point of change from a liquid state to a solid-state.

Vapour pressure of the mixture of liquids

As each component's molar concentration in the mixture solution affects the partial pressure exerted by each component, miscible liquids (mixtures of liquid and liquid) exhibit a proportional partial vapor pressure. P is the total vapor pressure given by the following equation:

P = PA + PB = PoAXA + PoBXB

The mole fractions XA and XB represent the components XA and XB, respectively. Vapour pressure A is directed outward by pure component A, whereas vapor pressure B is directed outward by pure component B.

PB stands for the partial vapor pressure exerted by A and PA stands for the partial vapor pressure exerted by B in the liquid mixture. Several liquids can coexist without being combined, but each liquid exerts its vapor pressure without any support from the others. Therefore, P is the vapor pressure total:

P = PoA + PoB

Sublimation critical point

When a solid becomes a vapor, it transitions from the solid to the liquid state without passing through the liquid state. Sublimation occurs when a substance's triple point on its phase diagram is below its endothermic phase transition temperature and pressure. Triple points (in thermodynamics) are pressure or temperature limits at which three phases of a substance can coexist at the same time. Phase transition occurring when a solid becomes a gas without passing through a liquid first. A winter's worth of dry ice is a gaseous form of solid, frozen carbon dioxide, which sublimates at a cold -78.5 degree C (-109.3 degree F). The fog you see is caused when dry ice melts and sublimates, becoming purified. Cold gases like carbon dioxide and cold, moist air are the main ingredients in fog.

Sublimation has many advantages and some of them includes:
  1. Among the most important advantages of sublimation is its ability to purify matter.
  2. Only the smallest amount of the product is lost.
  3. It doesn't require solvents.
  4. Solvents are almost eliminated from compounds.
  5. The best purification method is sublimation for substances weighing less than 100 mg.
A molecule is sublimated into vapor by absorbing heat, which provides sufficient energy to overcome the attraction of its neighbor molecules. Endothermy refers to the process of changing state through the release of energy. When the enthalpy of fusion and vaporization are added together, the sublimation enthalpy (or heat of sublimation) of the material can be calculated.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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