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