(©2002, François G. Amar, All rights reserved)

Summary for Week 7

Reading:

For this week: Chapter 5.5-5.8

A look ahead: Chapter 6

Homework problems:

Beginning of Chapter 5: 2, 5, 8, 12, 14, 15,16, 18, 19, 22, 28, 31

End of Chapter 5: 33, 37, 39, 42, 46, 47, 50, 53, 56, 58, 67, 70

Key concepts and equations:

• Standard enthalpy or heat of formation: DH°_f (kJ/mol) is tabulated for many compounds.

  Use tabulated values to predict the heat of reaction

  • formation reaction is: (elements in standard state) --> compound

    Some examples (1 and 2 are formation reactions, 3 is a combustion reaction):

    1)     H2 (g) ---> H2 (g)        D H°f = D Hrxn = Hfinal - Hinitial = 0
          
          2)     H2 (g)             +            1/2 O2 (g)           --->            H2O (l)
             (1 mol)x(0 kJ/mol)       +       (1/2 mol)x(0 kJ/mol)  --->           (1 mol)x(-285.83 kJ/mol) 
          
                               reactants (initial)                   --->       products (final) 
          
                 D H°rxn = Sproducts(molesxDH°f) - Sreactants(molesxDH°f)  
           
           D H°rxn = -285.83 kJ for reaction (2) as written
          
    3)     C2H6 (g)             +            7/2 O2 (g)       --->        2 CO2 (g)          +       3 H2O (l)
            (1 mol)x(-84.68 kJ/mol)   +      (7/2 mol)x(0 kJ/mol)  --->     (2 mol)x(-393.5 kJ/mol) + (3 mol)x(-285.83 kJ/mol) 
          
                               reactants (initial)                   --->       products (final) 
          
                 D H°rxn = Sproducts(molesxDH°f) - Sreactants(molesxDH°f)  
            
           D H°rxn = -1559.81 kJ for reaction (3) as written

• Calorimetry: measuring heat flows

  • at constant P (q_P=DH_system)

  or
  • at constant V (q_V=DE_system)
• Specific heat, c = heat required to raise temperature of 1 gram of substance by 1 K (J/g.K)
  • c_H2O = 1 calorie/g.K = 4.186 J/g.K

    we can often approximate the specific heat of dilute aqueous solutions by

    the value for pure water.

• Heat capacity, C= c x mass
• Constant pressure calorimetry:
  • q_solution = C_solution x D T = c_solution x mass x D T

  but

  • D H_rxn = q_rxn = - q_solution

    Example: if a reaction performed in a coffee cup calorimeter is exothermic then the

    heat absorbed by the solution, q_solution is positive (temperature of solution rises)

    But this is an exothermic reaction (DH_rxn<0) in which no work is done so q_rxn = - q_solution.

• Constant volume calorimetry:
  • q_calorimeter = C_calorimeter x D T

  but

  • D E_rxn = q_rxn = - q_calorimeter= - C_calorimeter x D T

    Example: if a reaction performed in a bomb calorimeter is exothermic then the

    heat absorbed by the calorimeter assembly, q_calorimeter is positive (temperature of calorimeter rises)

    But this is an exothermic reaction (DE_rxn<0) in which no work is done so q_rxn = - q_calorimeter.

• Hess's Law: reaction enthalpy of a process carried out in steps is equal to the sum of the reaction enthalpies for the individual steps.
• Fuel values: expressed in kJ/g. Multiply mass times fuel value to get heat generated by given quantity of fuel or food. Use % composition to get total values for mixtures of proteins, carbohydrates, etc.