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Vapour Pressure and Heat. What happens to a solid substance when it is heated? The compound can simply get hotter or a phase change can occur. - PowerPoint PPT Presentation
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Vapour Pressure and Heat
Phase changes can be expressed as enthalpy changes at constant
temperatures (Claussius-Clapeyron equation).
What happens to a solid substance when it is heated?
The compound can simply get hotter or a phase change can occur.
The transition from the solid phase to the liquid phase is an example of a phase change, which is often called melting. Boiling or vapourisation is an example of a phase change from the liquid to the gas phase.
TTR
H
PP
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vapo
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Phase diagrams
Show regions of P and T at which various phases are thermodynamically stable
Triple point: three phases in equilibrium
Vp curve for solid
Vp curve for liquid
Melting point line
Critical point
Supercritical fluidsHeating liquid in a closed vessel does not produce boiling. The vp (density) of the vapour rises with increasing T, as the liquid density decreases, until both are equal and a single phase exists (neither liq nor vap.).
Typical phase diagramsWater Carbon dioxide
Triple pt: 6.11mbar, 273.16K
Critical pt: 215bar, 647.3K
Triple pt: 5.11bar, 218.8K
Critical pt: 72bar, 304.2K
SolutionsA homogeneous mixture in which all of the particles have the sizes of atoms.
Driving forces for solution formation
(i) Spontaneous tendency for increasing disorder (entropy!)
(ii) Intermolecular forces
Sugar or alcohol in waterGlucose has -O-H groups along the carbon skeleton. These -O-H are polar centers.
Glucose dissolves in water because polar water molecules attach to the glucose molecules by dipole-dipole (H-bond) forces. When the attractive forces of the water molecules for the glucose exceeds the attractive forces between the glucose and its neighbouring glucose molecules the water can rip the sugar molecule out of the crystal. The glucose is "solvated" when it surrounded solvent molecules. The solvent has "dissolved" the molecule.
Water and ethyl alcohol are completely "miscible". Both water and ethanol are polar molecules with hydrogen bonding. The similarity of the two molecules results in solutions where the water and alcohol molecules are interchangeable.
Heats of SolutionThe enthalpy change between system and surroundings when
1 mole of a solute dissolves in solvent at constant pressure
solid solvent
Vapourised particles and solvent
solution
-lattice energy solvation energy
solnH
Heat of solution is zero for an ideal solution
Ideal dilute solutionsPB = xBKB Henry’s Law
Colligative Properties of Solutions
Physical properties that depend only upon the populations of particles in a mixture
Effect of solutes on the vapour pressure of solutions
Psoln = xsolventP*solvent Raoult’s Law
Molecular interpretation of Raoult’s Law
Volatile solutes and Raoult’s Law
Each component contributes its own partial pressure to the
solution vapour pressure (Dalton’s Law)
Real mixturesDeviations because of
intermolecular attractions
Boiling point elevation Freezing point depression
Entropy effect: when a solute is added to a pure liquid, the entropy is increased relative to the vapour phase. Therefore there is a weaker tendency to form a vapour (boiling point elevation). A similar molecular interpretation explains freezing point depression.
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mKT
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Osmotic pressureOsmotic membrane: semi-permeable membrane that allows passage of only solvent molecules
Dialysis membrane: membrane that allows passage of solvent and small solutes.
MRTVan’t Hoff equation
Colligative properties of solutions of electrolytes
1.00 m NaCl: F.P= -3.37C (not –1.86C as expected)!
Colligative properties depend on the concentration of particles
Remember: NaCl Na+ + Cl-
We have 2.00m of particles and should get F.P: -(2x1.86C) = -3.72C
Effect of interionic attractions account for discrepancy between actual and calculated F.P. for ionic species.
Van’t Hoff Factor compares degrees of dissociation of electrolytes
calculated
measured
T
Ti