(©2007, François G. Amar, All rights reserved)

Summary for Week 8

Reading:

For this week: Chapter 5.5-5.8

Homework problems:

End of Chapter 5: 46, 47, 50, 53, 56, 58, 67, 71, 75

OWL Assignment for Week 8:

Chapter 5 assignment (called Ch 6 in OWL) thermodynamics: due October 28, 2007

Key concepts and equations:

• Standard enthalpy or heat of formation: H°_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)        H°f = 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) 
          
                 H°rxn = Sproducts(molesxH°f) - Sreactants(molesxH°f)  
           
           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) 
          
                 H°rxn = Sproducts(molesxH°f) - Sreactants(molesxH°f)  
            
           H°rxn = -1559.81 kJ for reaction (3) as written

• Calorimetry: measuring heat flows

  • at constant P (q_P=H_system)

  or
  • at constant V (q_V=E_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
• Heat transfer in a calorimeter
• Constant pressure calorimetry:

  q_solution = C_solution x T = c_solution x mass x T

  but

  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 (H_rxn<0) in which no work is done so q_rxn = - q_solution.

• Constant volume calorimetry:

  q_calorimeter = C_calorimeter x T

  but

  E_rxn = q_rxn = - q_calorimeter= - C_calorimeter x 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 (E_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, and other fuels.