Effect of pressure on melting point

To understand how such a nucleation barrier comes about, it is necessary to understand how curvature of a particle affects its melting temperature, which in turn requires a review of the effect of pressure on the melting point. We start by writing the differential of G as 

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Although the effect of pressure on melting point is very small, its direction is still important To decide whether the melting point will be increased or decreased by compression, a simple principle is applied. An increase in pressure favors the formation of the more dense phase. 

Effect of Pressure on Melting Point (Normal) Substances... 

The Le Chatelier principle itself can also be derived from the Second Law of Thermodynamics (Frame 14) is of much wider applicability than just to chemical reactions. It applies to all systems which are in equilibrium. Other examples might be solid - liquid (see Frame 26 where conclusions regarding the effect of pressure on melting point are in agreement with the application of the Le Chatelier principle), solid - solid equilibria or equilibria in solution (see Section 49.7 below). 

Effect of Pressure on Melting Point, 7m and Boiling Point,7b Case (i) (Normal) Substances for Which l/n > 4 > K... 

Figure 12.3 Effect of pressure on melting and boiling points. Solid line indicates high pressure dashed line low pressure.

Effect of pressure on the melting point of a solid, (a) When ihe solid is die more dense phase, an increase in pressure converts liquid to solid the melting point increases, (b) If the liquid is the more dense phase, an increase in pressure converts solid to liquid and the melting point decreases. 

Melting point The temperature at which the solid and liquid phases of a substance are in equilibrium with each other, 13 effects of pressure on, 235 liquids, 235 low, 235 metals, 245... 

Bunsen (1850), Hopkins (1854), Batelli (1887), and de Yissier (1892) also made experiments on the effect of pressure on the melting-point of bodies of the wax-type. The latter found for acetic acid ... 

Temperature and pressure are the two variables that affect phase equilibria in a one-component system. The phase diagram in Figure 15.1 shows the equilibria between the solid, liquid, and vapour states of water where all three phases are in equilibrium at the triple point, 0.06 N/m2 and 273.3 K. The sublimation curve indicates the vapour pressure of ice, the vaporisation curve the vapour pressure of liquid water, and the fusion curve the effect of pressure on the melting point of ice. The fusion curve for ice is unusual in that, in most one component systems, increased pressure increases the melting point, whilst the opposite occurs here. 

A sublimation process is controlled primarily by the conditions under which phase equilibria occur in a single-component system, and the phase diagram of a simple one-component system is shown in Figure 15.30 where the sublimation curve is dependent on the vapour pressure of the solid, the vaporisation curve on the vapour pressure of the liquid, and the fusion curve on the effect of pressure on the melting point. The slopes of these three curves can be expressed quantitatively by the Clapeyron equation ... 

These relationships are represented in an exaggerated manner in fig. 2. The lines AO, BC and OD represent the vapour pressures of sulphur I., sulphur II. and molten sulphur, respectively OE and CF show the effect of pressure change on the melting-points of the two crystalline forms, whilst the effect of pressure on the transition temperature is indicated by BF. 

The density of cyclohexane has been measured from its melting point to the critical point.1 13Figure 40 12 presents the data of Reamer and Sage on the effect of pressure on the density or cyclohexane.1 Data on the other three compounds arc available up to the boiling... 

Data to construct the phase diagram were taken from S. P. Clark, Jr., Effect of Pressure on the Melting Points of Eight Alkali Halides, J. Chem. Phys., 31, 1526-1531 (1959). 

This curve shows the effect of pressure on the melting point of ice or freezing point of water. As the curve OC slopes towards the pressure axis, the melting point is lowered as the pressure is increased. This fact can also be predicted by Le-Chatelier s principle, as ice melts with decrease in volume. 

The Clapeyron-Clausius equation (1.52) for solid = liquid equilibrium cannot be integrated easily since Vs cannot be ignored in comparison with Vt. Also the laws of liquid state are not so simple as those for gaseous state. However, this equation can be used for calculating the effect of pressure on the melting point of a solid. Eq. 1.52 can also be used for calculating heats of fusion from vapour pressure data obtained at different temperatures. 

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