Heat & Changes of State

Changes of State Solid to liquid Solid to liquid Liquid to solid Liquid to solid Liquid to gas Liquid to gas Gas to liquid Gas to liquid

Heat & Changes of State When a change of state occurs by a gain or loss of heat, the temp remains constant until the change of state is complete! When a change of state occurs by a gain or loss of heat, the temp remains constant until the change of state is complete!

Molar Heat of Fusion (∆H fus ) Fusion - when a solid absorbs heat at its freezing point Fusion - when a solid absorbs heat at its freezing point Molar heat of fusion – heat absorbed by 1 mole of a solid as it melts into a liquid (at constant temp) Molar heat of fusion – heat absorbed by 1 mole of a solid as it melts into a liquid (at constant temp) ∆H fus for water: 1 mole = 6.01kJ ∆H fus for water: 1 mole = 6.01kJ

Molar Heat of Solidification (∆H solid ) Solidification - when a liquid releases heat as it turns into a solid Solidification - when a liquid releases heat as it turns into a solid Molar heat of solidification – heat released by 1 mole of a liquid as it becomes a solid (at constant temp) Molar heat of solidification – heat released by 1 mole of a liquid as it becomes a solid (at constant temp) ∆H solid for water: 1 mole = kJ ∆H solid for water: 1 mole = kJ

∆H fus = - ∆H solid The quantity of heat absorbed by a melting solid is the same as the heat released when a liquid solidifies The quantity of heat absorbed by a melting solid is the same as the heat released when a liquid solidifies H 2 O (s) kJ → H 2 O (l) H 2 O (l) kJ → H 2 O (s)

Examples How many grams of ice at 0 °C will melt if 2.25 kJ of heat are added? How many grams of ice at 0 °C will melt if 2.25 kJ of heat are added? How many kilojoules of heat are required to melt a 10g popcicle at 0 °C? use molar mass of water How many kilojoules of heat are required to melt a 10g popcicle at 0 °C? use molar mass of water

Molar Heat of Vaporization (∆H vap ) Vaporization - when a liquid absorbs heat at its boiling point to become a vapor Vaporization - when a liquid absorbs heat at its boiling point to become a vapor Molar heat of vaporization – heat absorbed by 1 mole of a liquid as it vaporizes into a gas Molar heat of vaporization – heat absorbed by 1 mole of a liquid as it vaporizes into a gas ∆H vap for water: 1 mole = 40.7kJ ∆H vap for water: 1 mole = 40.7kJ

Molar Heat of Condensation (∆H cond ) Condensation- when a vapor releases heat as it turns into a liquid Condensation- when a vapor releases heat as it turns into a liquid Molar heat of condensation – heat released by 1 mole of a vapor as it becomes a liquid Molar heat of condensation – heat released by 1 mole of a vapor as it becomes a liquid ∆H cond for water: 1 mole = 40.7 kJ ∆H cond for water: 1 mole = 40.7 kJ

∆H vap = - ∆H cond The quantity of heat absorbed by a vaporizing liquid is the same as the heat released when a vapor condenses The quantity of heat absorbed by a vaporizing liquid is the same as the heat released when a vapor condenses H 2 O (l) kJ → H 2 O (g) H 2 O (g) – 40.7kJ → H 2 O (l)

Examples How much heat (in kJ) is absorbed when 24.8g H 2 O (l) at 100 °C & 101.3kPa is converted to steam at 100 °C ? How much heat (in kJ) is absorbed when 24.8g H 2 O (l) at 100 °C & 101.3kPa is converted to steam at 100 °C ? How many kJ of heat are absorbed when 0.46g of chloroethane (C 2 H 5 Cl, bp 12.3°C) vaporizes at it normal boiling point? Molar heat of vaporization of chloroethane is 26.4 kJ/mol.