Chemistry Outcomes Review: Chapter 7

Chemical Energetics: Enthalpy

Everything you do, including thinking, requires energy. In this chapter we examined various forms of energy and transfers of energy between chemical and physical systems. We learned that the first law of thermodynamics is the principle of conservation of energy. Changes in the internal energy of a system, ∆E, are combinations of thermal and mechanical energy (heat and work) transfers to or from the system. We found that the same change in internal energy could be brought about by different heat and work transfers. A change in internal energy is a function only of the initial and final states of the system, but heat and work transfers depend upon the path from beginning to end.

We introduced a new state function called enthalpy, H, that accounts for the internal energy change plus any pressure-volume work involved in the change. Calorimetric measurements at constant pressure provide a measure of ∆H for reactions. We can use these enthalpy data (among others) to get bond enthalpies and enthalpies of formation for compounds. Using these values, we can calculate the enthalpy change for essentially any reaction. We cannot, however, use energy to predict the direction of changes; we will pursue that goal in the next chapter.

Check your understanding of the ideas in the chapter by reviewing these expected outcomes of your study.

You should be able to:

  • Identify the forms of energy transferred in physical and chemical changes and show how energy is conserved in the changes [Sections 7.1, 7.2, 7.3, 7.4, 7.10, and 7.11].
  • Draw molecular level representations of thermal energy (undirected kinetic energy) and mechanical energy (directed kinetic energy) transfers [Sections 7.2, 7.3, and 7.10].
  • Identify whether a thermal energy transfer occurs by radiation, conduction, and/or convection [Section 7.3].
  • Define and identify the variables that are functions of state and those that are functions of the path for a given change [Sections 7.4, 7.10, and 7.11].
  • Identify the system and the relevant surroundings for a given change [Section 7.5]
  • Define and identify open, closed, and isolated systems [Section 7.5].
  • Use the data from calorimetric measurements to calculate a calorimeter constant and the thermal energy transferred to or from a reacting system [Section 7.6].
  • Use the data from constant pressure calorimetric measurements to calculate the enthalpy change for the reacting system [Section 7.6].
  • Define and give examples of homolytic bond cleavage reactions [Section 7.7].
  • Write equations for homolytic bond cleavage and homolytic bond formation for compounds, use bond enthalpies to calculate the enthalpy changes associated with these processes, and obtain the enthalpy change for gas phase reactions [Section 7.7].
  • Draw enthalpy level diagrams that show how bond enthalpies combine to give the enthalpy change for a reaction [Section 7.7].
  • Use bond enthalpies to predict whether, for given atoms, reactions will favor singly-bonded or multiply-bonded products [Section 7.7].
  • Define standard states for elements and compounds and write the equations whose enthalpy changes are the standard enthalpies of formation of the compounds [Section 7.8].
  • Use standard enthalpies of formation to calculate the standard enthalpy change for a reaction [Section 7.8].
  • Draw enthalpy level diagrams that show how standard enthalpies of formation combine to give the standard enthalpy change for a reaction [Section 7.8].
  • Define coupled reactions and identify examples based on the definition [Section 7.9].
  • Show whether coupling between two reactions would be an energetically favorable combination [Section 7.9].
  • State the first law of thermodynamics in terms of internal energy, heat, and work and use it to analyze a change that occurs by different pathways [Sections 7.10 and 7.11].
  • Explain the difference between ∆E and qV and between ∆H and qP for a change [Section 7.11].
  • Use pressure and volume data or the ideal gas equation to calculate the pressure-volume work done in a reaction that involves gases [Sections 7.10 and 7.11].
  • Calculate ∆E, given ∆H for a reaction, and vice versa [Section 7.11].
  • Determine whether a process is consistent with (allowed) by the first law of thermodynamics [Section 7.12].