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Enthalpy Changes

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Enthalpy Changes Phase changes involve changes in the potential energy of a system. Chemists use the term enthalpy change ( H) to refer to the potential energy ... – PowerPoint PPT presentation

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Title: Enthalpy Changes


1
Enthalpy Changes
  • Phase changes involve changes in the potential
    energy of a system.
  • Chemists use the term enthalpy change (?H) to
    refer to the potential energy change of a system
    during a chemical reaction or physical change.
  • Enthalpy change is measured in kJ/mol.

2
Enthalpy Changes in Chemical Changes
  • The enthalpy change of a chemical reaction
    represents the difference between the potential
    energy of the products and the potential energy
    of the reactants.
  • ?Hrxn Hproducts - Hreactants

3
Endothermic vs Exothermic
  • Endothermic reaction when a reaction results in
    a net absorption of energy.
  • Temperature of surroundings decreases
  • Exothermic reaction when a reaction results in a
    net release of energy.
  • Temperature of surroundings increases

4
Representing Enthalpy Changes
  • Enthalpy of reaction ?Hrxn is the enthalpy change
    of a chemical reaction.
  • Enthalpy depends on Temp. And pressure.
  • ?Hrxn is the standard enthalpy of reaction which
    occurs et SATP (25C and 100kPa)
  • Aka. Heat of Reaction

5
Representing Exothermic Reactions
  • Use a thermochemical equation, showing the
    energy released as a productH2(g) ½ O2(g) ?
    H2O(l) 285.8kJ
  • Indicate the enthalpy of reaction as a separate
    expression.H2(g) ½ O2(g) ? H2O(l) ?Hrxn
    -285.8kJ/mol
  • Use an enthalpy diagram.

6
Representing Endothermic Reactions
  • Use a thermochemical equation, showing the
    energy gained as a reactant117.3kJ MgCO3(s) ?
    MgO(s) CO2(g)
  • Indicate the enthalpy of reaction as a separate
    expression.MgCO3(s) ? MgO(s) CO2(g)
    ?Hrxn117.3kJ/mol
  • Use an enthalpy diagram.

7
Enthalpy of Reaction
  • Enthalpy of reaction is linearly dependant on the
    amount of substances that react.
  • If the amount of reactants doubles, the enthalpy
    change also doubles.

8
Standard Molar Enthalpy of Formation
  • Standard molar enthalpy of formation (?Hf) is
    the quantity ofenergy that is absorbed or
    released when one mole of a compound is
    formed directly from its elements in their
    standard states.

9
Standard Molar Enthalpy of Combustion
  • Standard molar enthalpy of combustion (?Hcomb)
    is the enthalpy associated with the combustion of
    1 mol of a given substance.

10
Practice
  • P 643 15-18

11
Calculating Enthalpy Changes
  • Use this formula to calculate the heat absorbed
    or released by a chemical reaction

12
Sample Problem Heat of Formation and Combustion
Reactions
13
Sample Problem Heat of Formation and Combustion
Reactions
14
Practice
  • P 645 19-23

15
Enthalpy Changes and Changes of State
  • Changes of state involve changes in the potential
    energy of a system only.
  • The temperature of the system undergoing the
    state change remains constant.

16
Representing a Phase Change
17
Enthalpy Changes associated with Phase Changes
  • Molar enthalpy of vaporization, ?Hvap the
    enthalpy change for the state change of one mole
    from liquid to gas.
  • Molar enthalpy of condensation, ?Hcond the
    enthalpy change for the state change of one mole
    from gas to liquid.
  • Molar enthalpy of melting, ?Hmelt the enthalpy
    change for the state change of one mole from
    solid to liquid.
  • Molar enthalpy of freezing, ?Hfre the enthalpy
    change for the state change of one mole from
    liquid to solid.
  • Molar enthalpy of solution, ?Hsoln the enthalpy
    change when mol of a solute dissolves in a
    solvent.

18
Practice
  • P 648 24-29
  • Section Review
  • At this point you should be able to answer
    questions, 1,2,3,4,5,6,7,8
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