Phase change 1 Flashcards
Intramolecular forces
(chemical bonding forces) hold atoms together ina molecule. Influence chemical properties.
Intermolecular forces
the (nonbonding) electrostatic forces exist betweenthe molecules (particles). Responsible for physical propertiesof the phase and phase changes.
Phases of Matter
Each phase is a physically distinct homogeneouspart of a system
Enthalpy change
Each phase change is accompanied by an Enthalpy change, ∆H, given in units of kilojoules per mole (measured at 1 atmand the temperature of the change)
Phase change differences
*Solid/liquid are still close together no major change in intermolecular forces. *Liquid/gas have to separate the molecules and overcome intermolecular forces. There is a large potential energy change at liquid/gas phase transition
Quantitative Aspects of Phase Changes: Within a phase
Within a phase, heat flow is accompanied by a change in temperature, since the average Ek of particles changes.
Quantitative Aspects of Phase Changes: during a phase change
Heat flow occurs at a constant temperature, as the average distance between particles changes
Liquid –Gas Equilibria
In a closed system, phase changes are usually reversible. The system reaches a state of dynamic equilibrium, where molecules are leaving and entering the liquid at the same rate.
Vapor Pressure
Vapor pressure is the pressure extracted by the vapour present above the liquid and gas phases.
DYNAMIC equilibrium
Some fast-moving molecules escape into the gas phase
*These gas molecules exert pressure on the liquid surface
*The pressure as more molecules enter the gas phase
*Some gas molecules collide with the surface and stick to it, re-entering the liquid phase
*Eventually, the rate of molecules evaporating equals the rate of molecules condensing
Temperature and Intermolecular interactions affect vapor Pressure
As temperature increases, the fraction of molecules with enough energy to enter the vapour phase increases, and the vapour pressure increases. The weaker the intermolecular forces, the more easily particles enter the vapor phase, and the higher the vapor pressure. Intermolecular Forces (IF) P
Clausius-Clapeyron Equation
P in Torr, R = 8.314 J/(mol.K)
Solid-Liquid Equilibria
Solid, particles vibrate about fixed positions
*Increase T, vibrations increase until enough Ek for particles to break free i.e. melting begins
*Some molecules in the liquid phase collide with the solid and become fixed in position again.
*At the melting point (the freezing point) A dynamic equilibrium occurs where the melting rate = the freezing rateSolid-Liquid EquilibriaMelting / Freezing point; T=constantLiquids and Solids are nearly incompressible. P hardly affects the melting point. P vs T is a straight nearly vertical line
Solid-Gas Equilibria
Solids with high enough vapour pressures include dry ice (CO2), iodine and solid room deodorizersIodine sublimes, come into contact with ice and the gas is deposited. Substances sublime because the intermolecular attractions and atmospheric pressure are too low to keep particles near to one another after they leave the solid state
H2O phase diagram
The higher the P, the lower the T at which water freezes!At atm P, solid water melts and then vaporizes.
The solid-liquid line slants to the left for H2O, because the solid is less dense than the liquid. Water expands on freezing. An increase in pressure favours the liquid phase
