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Published byRoland Allison Modified over 9 years ago
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Heat & Changes of State
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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
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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!
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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
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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 = - 6.01kJ ∆H solid for water: 1 mole = - 6.01kJ
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∆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) + 6.01kJ → H 2 O (l) H 2 O (l) - 6.01kJ → H 2 O (s)
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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
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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
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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
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∆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) + 40.7kJ → H 2 O (g) H 2 O (g) – 40.7kJ → H 2 O (l)
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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.
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