thermochemistry energy in state changes. copyright © pearson education, inc., or its affiliates....

Thermochemistry

Energy in State Changes

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Heats of Fusion and Solidification

•All solids absorb heat as they melt to become liquids.

• The gain of heat causes a change of state instead of a change in temperature.

• The temperature of the substance undergoing the change remains constant.


Heats of Fusion and Solidification• The heat absorbed by one mole of a solid

substance as it melts to a liquid at constant temperature is the molar heat of fusion (ΔHfus).

• The molar heat of solidification (ΔHsolid) is the heat lost when one mole of a liquid substance solidifies at a constant temperature.


Heats of Fusion and Solidification

•The quantity of heat absorbed by a melting solid is exactly the same as the quantity of heat released when the liquid solidifies.

ΔHfus = –ΔHsolid


Heats of Fusion and Solidification• The melting of 1 mol of ice at 0°C to 1 mol

of liquid water at 0°C requires the absorption of 6.01 kJ of heat.

• The conversion of 1 mol of liquid water at 0°C to 1 mol of ice at 0°C releases 6.01 kJ of heat.

ΔHfus = 6.01 kJ/mol

ΔHsolid = –6.01 kJ/mol

H2O(s) → H2O(l)

H2O(l) → H2O(s)


•How many grams of ice at 0°C will melt if 2.25 kJ of heat are added?

Using the Heat of Fusion in Phase-Change Calculations


.

KNOWNS UNKNOWN

Initial and final temperature are 0°CΔHfus = 6.01 kJ/mol

ΔH = 2.25 kJ

mice = ? g

• Find the number of moles of ice that can be melted by the addition of 2.25 kJ of heat.

• Convert moles of ice to grams of ice.


•Calculate the amount of heat absorbed to liquefy 15.6 g of methanol (CH4O) at its melting point. The molar heat of fusion for methanol is 3.16 kJ/mol.


•Calculate the amount of heat absorbed to liquefy 15.6 g of methanol (CH4O) at its melting point. The molar heat of fusion for methanol is 3.16 kJ/mol.

ΔH = 15.6 g CH4O

= 1.54 kJ32.05 g CH4O

1 mol 3.16 kJ

1 mol


Heats of Vaporization and Condensation

•A liquid that absorbs heat at its boiling point becomes a vapor.

• The amount of heat required to vaporize one mole of a given liquid at a constant temperature is called its molar heat of vaporization (ΔHvap).


•This table lists the molar heats of vaporization for several substances at their normal boiling point.

Heats of Physical Change

Substance ΔHfus (kJ/mol) ΔHvap (kJ/mol)

Ammonia (NH3) 5.66 23.3

Ethanol (C2H6O) 4.93 38.6

Hydrogen (H2) 0.12 0.90

Methanol (CH4O) 3.22 35.2

Oxygen (O2) 0.44 6.82

Water (H2O) 6.01 40.7


Heats of Vaporization and Condensation

•Condensation is the exact opposite of vaporization.

• When a vapor condenses, heat is released.

• The molar heat of condensation (ΔHcond) is the amount of heat released when one mole of vapor condenses at its normal boiling point.


Heats of Vaporization and Condensation•The quantity of heat absorbed by a vaporizing liquid is exactly the same as the quantity of heat released when the vapor condenses.

ΔHvap = –ΔHcond


•A heating curve graphically describes the enthalpy changes that take place during phase changes.

Remember: The temperature of a substance remains constant during a change of state.


•How much heat (in kJ) is absorbed when 24.8 g H2O(l) at 100°C and 101.3 kPa is converted to H2O(g) at 100°C?

Using the Heat of Vaporization in Phase-Change Calculations


Multiply the mass of water in grams by the conversion factors.

ΔH = 24.8 g H2O(l)

= 64.21 kJ

18.0 g H2O(l)

1 mol H2O(l)

1 mol H2O(l)

40.7 kJ


Heat of Solution

During the formation of a solution, heat is either released or absorbed.


Heat of Solution

• The enthalpy change caused by the dissolution of one mole of substance is the molar heat of solution (ΔHsoln).

During the formation of a solution, heat is either released or absorbed.


CaCl2(s) → Ca2+(aq) + 2Cl–(aq)

ΔHsoln = –82.8 kJ/mol

Heat of Solution

•A practical application of an exothermic dissolution process is a hot pack.• In a hot pack, calcium chloride, CaCl2(s),

mixes with water, producing heat.


Heat of Solution•The dissolution of ammonium nitrate, NH4NO3(s), is an example of an endothermic process.

• The cold pack shown here contains solid ammonium nitrate crystals and water.

• Once the solute dissolves, the pack becomes cold.

• The solution process absorbs energy from the surroundings.

NH4NO3(s) → NH4+(aq) + NO3

–(aq)

ΔHsoln = 25.7 kJ/mol


•How much heat (in kJ) is released when 2.50 mol NaOH(s) is dissolved in water?

Calculating the Enthalpy Change in Solution Formation


Multiply the number of moles by the conversion factor.

ΔH = 2.50 mol NaOH(s)

= –111 kJ 1 mol NaOH(s)

–44.5 kJ


How much heat (in kJ) is absorbed when 50.0 g of NH4NO3(s) are dissolved in water if Hsoln = 25.7 kJ/mol?


How much heat (in kJ) is absorbed when 50.0 g of NH4NO3(s) are dissolved in water if Hsoln = 25.7 kJ/mol?

ΔH = 50.0 g NH4NO3

= 16.1 kJ80.04 g NH4NO3

1 mol 25.7 kJ

1 mol


Key Concepts

•The quantity of heat absorbed by a melting solid is exactly the same as the quantity of heat released when the liquid solidifies; that is, ΔHfus = –ΔHsolid.

•The quantity of heat absorbed by a vaporizing liquid is exactly the same as the quantity of heat released when the vapor condenses; that is, ΔHvap = –ΔHcond.

•During the formation of a solution, heat is either released or absorbed.


Glossary Terms

• molar heat of fusion (ΔHfus): the amount of heat absorbed by one mole of a solid substance as it melts to a liquid at a constant temperature

• molar heat of solidification (ΔHsolid): the amount of heat lost by one mole of a liquid as it solidifies at a constant temperature

• molar heat of vaporization (ΔHvap): the amount of heat absorbed by one mole of a liquid as it vaporizes at a constant temperature


Glossary Terms

• molar heat of condensation (ΔHcond): the amount of heat released by one mole of a vapor as it condenses to a liquid at a constant temperature

• molar heat of solution (ΔHsoln): the enthalpy change caused by the dissolution of one mole of a substance

thermochemistry energy in state changes. copyright © pearson education, inc., or its affiliates....

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