Change of stateChange of state

Change of state and energy Change of state and energy consumptionconsumption

FusionFusion

Heat of fusion is energy required to convert Heat of fusion is energy required to convert the solid into the liquid at the same the solid into the liquid at the same temperaturetemperature

Originally called latent heat because no Originally called latent heat because no temperature rise occurstemperature rise occurs

Freezing releases same quantity of heatFreezing releases same quantity of heat

Vapour pressureVapour pressure

There is always a There is always a certain pressure of a certain pressure of a gas in equilibrium with gas in equilibrium with the liquid phase, even the liquid phase, even at temperatures well at temperatures well below the boiling pointbelow the boiling point

Consequence of range Consequence of range of energies of of energies of molecular motionmolecular motion

Vapour pressure and boilingVapour pressure and boiling

Molecules do not all have Molecules do not all have the same energythe same energy

High energy molecules High energy molecules escape the liquid – vapour escape the liquid – vapour pressurepressure

When vapour pressure = When vapour pressure = atmospheric pressure atmospheric pressure boiling occursboiling occurs

Sublimation is direct Sublimation is direct transition of solid to gas transition of solid to gas (dry ice)(dry ice)

Molecular energy and change of stateMolecular energy and change of state

Combining of molecules generates heat - Combining of molecules generates heat - condensationcondensation

Input of energy breaks molecular bonds - Input of energy breaks molecular bonds - vaporizationvaporization

SublimationSublimation

Solid converts directly Solid converts directly to a gasto a gas

““Dry ice” – solid carbon Dry ice” – solid carbon dioxide sublimes to the dioxide sublimes to the gas at room gas at room temperaturetemperature

BoilingBoiling

Heat of vaporization is Heat of vaporization is energy required to energy required to convert liquid into convert liquid into vapor at the same vapor at the same temperaturetemperature

Condensation releases Condensation releases the same quantity of the same quantity of energyenergy

Heat, temperature and change of stateHeat, temperature and change of state

Evaporation requires greater heat input than Evaporation requires greater heat input than melting because of the greater number of bonds melting because of the greater number of bonds to be brokento be broken We lick our fingers to extinguish a burning candleWe lick our fingers to extinguish a burning candle

At phase change, no change in T

At phase change, no change in T

Heat input in boiling much

greater

Energy calculationsEnergy calculations

Calculate heat required to convert 15.0 g water at Calculate heat required to convert 15.0 g water at 25.0ºC to steam at 100ºC.25.0ºC to steam at 100ºC.

Calculation is sum of two parts:Calculation is sum of two parts:1.1. Heating the liquidHeating the liquid

1.1. Mass waterMass water

2.2. Temperature change Temperature change

3.3. SHSH waterwater

2.2. Boiling the liquidBoiling the liquid1.1. Heat vaporization waterHeat vaporization water

2.2. Mass waterMass water

State change calculationState change calculation

Calculate heat required to convert 15.0 g water at Calculate heat required to convert 15.0 g water at 25.0ºC to steam at 100ºC.25.0ºC to steam at 100ºC.

SH water = 4.18 J/gSH water = 4.18 J/gºC, ºC, ΔΔHHvapvap = 2,259 J/g = 2,259 J/g