POWERPOINT DONE BY: AMANDA HOFSTAEDTER, KEVIN ROGERS & KAYLA SCULLION Phase Changes

“Energy Changes Accompanying Phase Changes” Every phase change is accompanied by a change in energy of the system Solid – Liquid  Molecules/ions are ‘fixed’ in position as a solid  Increasingly vibrate as temperature increases When melted, units are free to move Heat of fusion H fus : enthalpy change for melting a solid

“Energy Changes Accompanying Phase Changes” Liquid - Gas  As temperature increases, so does molecular movement  Measured by increase of gas-phase molecules over liquid Molecules exert vapor pressure Vapor pressure increases until equals atmospheric pressure Liquid then boils and molecules move into gaseous state Heat of vaporization H vap Solid - Gas  Molecules of a solid can go directly into gaseous state  Heat of sublimation: H sub (sum of Heat of Fusion & Vaporization)

“In a solid, the particles are packed in a rigid configuration, giving the substance a definite shape and size. In a liquid, the particles are close together but may move with respect to one another, giving the substance a definite volume but a fluid shape. In a gas, the particles may occupy the entire volume of the container, so that their shape and volume are both defined by the container.” Internal structure of the three physical states.

“Energy Changes Accompanying Phase Changes” First Law of Thermodynamics  Heat absorbed by liquid in vaporizing must be evolved when reverse process occurs (condensation)  Heat of Condensation is equal in magnitude and opposite in sign from Heat of Vaporization  Heat of Deposition exothermic = Heat of Sublimation endothermic  Heat of Freezing exothermic = Heat of Fusion endothermic

Diagram of Phase Transitions Table to Show Signs of Enthalpy Changes Phase TransitionChange of Enthalpy Fusion (Melting) (solid to liquid) ΔH > 0; enthalpy increases (endothermic process) Vaporization (liquid to gas) ΔH > 0; enthalpy increases Sublimation (solid to gas) ΔH > 0; enthalpy increases Freezing (liquid to solid) ΔH < 0; enthalpy decreases (exothermic process) Condensation (gas to liquid) ΔH < 0; enthalpy decreases Deposition (gas to solid) ΔH < 0; enthalpy decreases Process of Phase Transitions

Generic Heating/Cooling Curve

“Heating Curves” Graph of the temperature of a system versus the amount of heat added  Diagonal lines show heating of one phase from lower to higher temperature  Horizontal lines show conversion of one phase to another at a constant temperature  Constant because temperature increase is used to overcome attractive forces Cooling has opposite effect  Supercooling: remove heat so rapidly, temporarily cool substance below its freezing point without forming a solid  Highly unstable – see videovideo

“Critical Temperature and Pressure” A gas will liquefy when pressure is applied to it  Critical Temperature: highest temperature at which a distinct liquid phase can form  Critical Pressure: pressure required to bring liquefaction at the critical temperature Gases become more difficult to liquefy as temperature increases because of inc. kinetic energy  Nonpolar & low molecular weight substances have lower measurements than polar & higher molecular weight substances Due to intermolecular forces

Notice that at or above 374 (the critical temperature for water) only water vapor exists in the tube. No matter how much pressure is applied, the vapor cannot be liquefied.

“Critical Temperature and Pressure” “For every gaseous substance, a temperature can be reached at which the motional energies of the molecules are sufficient to overcome the attractive forces that lead to the liquid state, regardless of how closely the molecules are forced to approach by increasing the pressure.”  Useless to try to liquefy a gas by applying pressure if the gas is above the critical temperature

Section Questions Two pans of water are on different burners on a stove. One is burning vigorously while the other is boiling gently. What can be said about the temperature of the water in the two pans? The temperature is the same. A large container of water and a small container of water are at the same temperature. What can be said about the relative vapor pressures of the water in the two containers? They are the same because they are at the same temperature. Volume or surface area does not matter.

Section Questions On this phase diagram why does the line that separates the gas and liquid phases end rather than go to infinite pressure and temperature? The liquid-gas line of a phase diagram ends at the critical point, the temperature and pressure beyond which the gas and liquid phases are indistinguishable.

TEXTBOOK: BROWN, LEMAY, AND BURSTEN. CHEMISTRY THE CENTRAL SCIENCE. UPPER SADDLE RIVER: PEARSON EDUCATION, WEBSITES: RIALS/THERMOCHEM/PHASE.HTM E/PHASE.HTML Works Cited