Change of state

Change of state and energy consumption

Fusion  Heat of fusion is energy required to convert the solid into the liquid at the same temperature  Originally called latent heat because no temperature rise occurs  Freezing releases same quantity of heat

Vapour pressure  There is always a certain pressure of a gas in equilibrium with the liquid phase, even at temperatures well below the boiling point  Consequence of range of energies of molecular motion

Vapour pressure and boiling  Molecules do not all have the same energy  High energy molecules escape the liquid – vapour pressure  When vapour pressure = atmospheric pressure boiling occurs  Sublimation is direct transition of solid to gas (dry ice)

Molecular energy and change of state  Combining of molecules generates heat - condensation  Input of energy breaks molecular bonds - vaporization

Sublimation  Solid converts directly to a gas  “Dry ice” – solid carbon dioxide sublimes to the gas at room temperature

Boiling  Heat of vaporization is energy required to convert liquid into vapor at the same temperature  Condensation releases the same quantity of energy

Heat, temperature and change of state  Evaporation requires greater heat input than melting because of the greater number of bonds to be broken  We lick our fingers to extinguish a burning candle At phase change, no change in T Heat input in boiling much greater

Energy calculations  Calculate heat required to convert 15.0 g water at 25.0ºC to steam at 100ºC.  Calculation is sum of two parts: 1.Heating the liquid 1.Mass water 2.Temperature change 3.SH water 2.Boiling the liquid 1.Heat vaporization water 2.Mass water

State change calculation  Calculate heat required to convert 15.0 g water at 25.0ºC to steam at 100ºC.  SH water = 4.18 J/gºC, ΔH vap = 2,259 J/g