Vapor Pressure and Vapor Pressure and BoilingBoiling

Vapor PressureVapor Pressure – the pressure exerted by a – the pressure exerted by a vaporvapor in equilibrium with its in equilibrium with its liquidliquid state. state.

Liquid molecules at the surface Liquid molecules at the surface escapeescape into into the the gasgas phase. phase.

These gas particles create These gas particles create pressurepressure above above the the liquidliquid in a closed container.in a closed container.

Vapor Pressure Vapor Pressure increasesincreases with with increasing temperature.increasing temperature.

20oC 80oC

•As temperature increases, the amount of vapor generated by a liquid in a closed container increases. 

•This occurs because as the liquid gains kinetic energy, the molecules can overcome the intermolecular forces of attraction that are prevalent in the liquid phase.

Evaporation vs BoilingEvaporation vs Boiling EvaporationEvaporation - when a liquid - when a liquid gas at any gas at any

temperaturetemperature VaporizationVaporization – When a liquid – When a liquid gas when gas when heat heat

is applied or at the is applied or at the boilingboiling temperature temperature BoilingBoiling – occurs when the vapor pressure above – occurs when the vapor pressure above

the liquid the liquid equalsequals the atmospheric pressure. the atmospheric pressure.

Normal Boiling PointNormal Boiling Point - - --boiling temperature when pressure boiling temperature when pressure

is at is at 1atm1atm or or 760torr.760torr.

Boiling Point is a physical constant. Boiling Point is a physical constant. It depends on the It depends on the strengthstrength of the of the attraction between molecules.attraction between molecules.

Strong attraction between molecules Strong attraction between molecules = = higherhigher boiling point.boiling point.

Weaker attraction between Weaker attraction between molecules = molecules = lowerlower boiling point.boiling point.

Vapor Pressure CurvesVapor Pressure Curves

Graph shows how boiling points change with change in vapor pressure.

Boiling Points Change with Boiling Points Change with pressure changes.pressure changes.

Less pressureLess pressure = = lower boiling lower boiling pointpoint (example = water boils at (example = water boils at lowerlower temperatures at temperatures at highhigh elevations- water boils at elevations- water boils at 7676ooCC on Mt. on Mt. Everest).Everest).

Higher PressureHigher Pressure = = higherhigher boiling boiling point (example = pressure cooker – point (example = pressure cooker – cooks cooks fasterfaster because you cook at a because you cook at a higherhigher temperature.)temperature.)