Thermochemistry

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Chapter 5

• Thermochemistry

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Terms I

• Thermochemistry
• Study of
• Kinetic Energy
• Energy of
• Potential Energy
• Energy of

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Units of E

• JOULE
• Calorie vs. calorie
• 1 Cal
• 1 cal is the amount of heat necessary to raise
  the temperature of 1 g of water 1oC
• 1 cal J

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Terms II

• Force
• A
• Work
• Energy used to move an object
• W
• Energy
• Capacity to do

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First Law of Thermodynamics

• Energy
• ? E

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Equation II

• ? E
• q heat in or heat out of system
• w work done on or by the system

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Practice

• Calculate the change in the internal energy of
  the system for a process in which the system
  absorbs 140 J of heat from the surroundings and
  does 85 J of work on the surroundings.

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Endothermic Reaction

• Process when a system
• Gets
• ?H gt

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Exothermic Reaction

• Process when a system
• Gets
• ?H lt

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State Function

• Describes the condition of a system
• Does not matter how the system got there
• For example

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ENTHALPY

• Heat
• Represented by
• Is a

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Enthalpy Equation

• ?H Hfinal Hinitial qp
• P indicates constant pressure

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Enthalpy of Reaction

• ?Hrxn H(products) H(reactants)
• 2H2O (g) ? 2H2 (g) O2 (g) ?H
• 2H2 (g) O2(g) ? 2H2O(g) ?H

Return
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Rules for Enthalpy and RXNS

1. Enthalpy is an extensive property
2. Enthalpy for a reaction
3. Enthalpy change depends on state of reactants and
   products

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Practice

• 2Mg(s) O2(g) ? 2MgO(s) ?H-1204 kJ
• Is this
• How much heat is transferred when 2.40 g of Mg
  are reacted?
• How many grams of MgO are produced during an
  enthalpy change of 96.0 kJ?

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Continued

• 2Mg(s) O2(g) ? 2MgO(s) ?H-1204 kJ
• How many kJ of heat are absorbed when 7.50 g of
  MgO(s) are decomposed into magnesium and oxygen
  at constant pressure?

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Calorimetry

• The study of
• Heat capacity (C)
• Temperature change of an object as it
• Amount of heat required to
• Greater heat capacity means

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Calorimetry

• Molar heat capacity (Cmolar)
• Specific heat capacity (Cp)
• Equation

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Calorimetry

• What is the heat required to raise 400.00 g of
  water by 34.50oC?
• What is the heat lost when 200.00 g of iron
  changes from 115.50 oC to 22.00 oC?

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Calorimetry

• Large beds of rocks are used in some solar heated
  homes to store heat. Assume that the specific
  heat of rock is 0.082 J/g-K. Calculate the
  quantity of heat absorbed by 50.0 kg of rocks if
  their temperature increases by 12.0 oC

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Flashback

• Law of conservation of energy (First law of
  thermodynamics)

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Calorimetry

• You have heated a 55.00 g piece of iron, Cp
  0.385 J/g-K, to 200.00 oC. You then put the iron
  into water in a calorimeter. There are 300.00 g
  of water at 22.00 oC. What is the final
  temperature of the mixture?

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Calorimetry

• In the calorimetry lab you will be mixing an acid
  and a base and studying the temperature changes.
• You mix 35.00 mL each of 1 M HCl and 1 M NaOH in
  a calorimeter. The temperature increases from
  21.0 to 27.5 oC. What is the enthalpy change for
  the reaction in kJ/mol HCl?

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Hesss Law
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Enthalpy of Formation

• Energy change for the formation
• Symbolized by ?Hf
• Tables are for standard conditions
• 1 atm
• 25oC
• Appendix C

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Enthalpy of Formation

• Units for ?Hof are in
• Magnitude depends on state
• 2H2(g) O2(g) ? 2 H2O(l) ?Hof -285.8
• 2H2(g) O2(g) ? 2 H2O(g) ?Hof -241.8

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Enthalpy of Formation

• By definition the standard enthalpy of
  formation of the most stable form of ANY element
  is 0 kJ/mol
• ?Hof C(s) 0
• ?Hof H2(g) 0

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Enthalpy of RXN

• ?Horxn Sn ?Hof (products) - Sm ?Hof (reactants)

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