U7 Phase Changes & Heating Curves Notes

Phase Changes & Heating Curves Changes of State ●​ A change of state occurs whenever matter changes from one state to another ○​ Matter may change back and forth between any 2 states ○​ These are physical changes and are reversible Processes that Cause Changes of State ●​ ●​ ●​ ●​ ●​ ●​ Melting: Solid to liquid Freezing: Liquid to solid Sublimation: Solid to gas Deposition: Gas to solid Condensation: Gas to liquid Evaporation: Liquid to gas Role of Energy in Changes of State ●​ Energy of particles in matter determines the matter’s state ○​ Particles of gases have more energy than liquids ○​ Particles of liquids have more energy than solids ●​ Particles of matter either absorb or lose energy to change from one state to another Melting ●​ Particles of most solids are packed tightly together in an orderly arrangement ○​ Motion of individual particles is restricted to vibrational motion about a fixed point ○​ Solids are almost completely incompressible and have the highest densities ●​ As a solid is heated, particles vibrate more rapidly and eventually the structure begins to break down Melting Point ●​ Melting point is the temperature at which a solid changes into a liquid ○​ The disruptive vibrations overcome the attractive forces holding the solid together ○​ Depended on the strength of the attractive forces ■​ Ionic bonds are strongest and have the highest melting points ■​ Intermolecular forces also play a role ●​ Hydrogen bonds are stronger than other IMF Sublimation ●​ Solids have vapor pressure, through less than liquids ○​ Ex: Snow banks disappear even if the temp stays below freezing ●​ Sublimation is the change of state from a solid to a gas without passing through the liquid state ○​ Ex: dry ice (Solid carbon dioxide) ●​ Opposite is deposition (gas to solid) Evaporation ●​ Vaporization is the process in which liquid is converted into a gas ●​ Evaporation is the conversion of of a liquid into vapor below the boiling temperature of the liquid ●​ Condensation - some water molecules become vapor, and equal number of water vapor molecules condense back into the liquid state ●​ In order for a liquid molecule to escape into the gas state, the molecule must have enough kinetic energy to overcome the intermolecular forces in the liquid ○​ The remaining molecules in the liquid now have lower average kinetic energy so as evaporation occurs, the temperature decreases Boiling ●​ As liquid is heated, the average kinetic energy of its particles increases ○​ Rate of evaporation increases and now more escape to vapor phase until all have enough kinetic energy to vaporize ○​ Boiling point is the temperature at which the vapor pressure of a liquid is equal to the external pressure Vapor Pressure ●​ In a sealed container, the rate of evaporation is equal to the rate of condensation ●​ ●​ ●​ ●​ ●​ ●​ - dynamic equilibrium The vapor molecules can’t escape so they exert pressure on the walls of the container Vapor pressure is a measure of the pressure (force per unit area) exerted by a gas above liquid in a sealed container Strong intermolecular forces produce a lower rate of evaporation and lower vapor pressure Weak intermolecular forces produce a higher rate of evaporation and higher vapor pressure As temp increases, the vapor pressure increases Vapor pressure is dependent on the temperature ●​ Temp dependence is not linear Vapor pressure curves ●​ Vapor pressure curves illustrate the building points of various liquids ○​ Shows the graph of vapor pressure as a function of temperature ○​ A line is drawn indicating standard pressure ○​ Where the line intersects the vapor pressure curve is the building point of that liquid Vapor pressure and IMF ●​ Boiling points of liquids also correlate to the strength of the intermolecular forces ○​ Liquids held together by weak dispersion forces have a high vapor pressure so it has a lower boiling point ○​ Water is held together by hydrogen bonds which are stronger and have a lower boiling vapor pressure so they have a higher boiling point ●​ Boiling points are also affected at higher altitudes because of the reduced surface pressure ○​ Higher altitudes = lower boiling points