Chapter 6: Thermochemistry
After a consideration of energy and definitions of some basic terms, the relationship between heat, work, and
internal energy—the first law—is described. The first law is applied to heats of reactions, after these heats first
are determined with calorimetry, leading to Hess’s law. The determination of heats of reaction is simplified
with the use of standard enthalpy changes of formation. Consideration of the energy changes involved in
combustion and respiration concludes the chapter.
Summary/objective of Sections
Potential and kinetic energies are both defined, along with units of energy: J, kJ, cal, and kcal.
Define kinetic and potential energy.
6.2 Thermochemistry: Some Basic Terms
Within this section are defined a number of terms: system (open, closed, and isolated); surroundings; internal energy;
thermal energy; chemical energy; heat; thermal equilibrium; exothermic; endothermic; work; and pressure-volume work.
Define the following: system and surroundings, heat, and work.
6.3 Internal Energy (
), State Functions, and the First Law of Thermodynamics
A statement of the law of conservation of energy leads to the summary equation:
Positive heat and work
transfer energy into the system, while negative values transfer energy out of the system. Although absolute internal
energy cannot be determined, changes in it can be determined, and these changes depend only on starting and ending
conditions, because internal energy is a state function. Heat and work, on the other hand, are path functions, a distinction
that is enhanced with a mountain-climbing analogy.
State the first law of thermodynamics and know how to apply it.
Describe what is meant by a state
6.4 Heats of Reaction and Enthalpy Change,
Heat of reaction is the quantity of heat absorbed during a chemical reaction: exothermic if heat is given off (q< 0)
endothermic if heat is absorbed (q > 0). The work involved in most chemical reactions is pressure-volume work,
and so the heat absorbed at constant pressure is the change in a state function, termed the
Enthalpy change is related to chemical reactions by assuming that each chemical formula in the reaction stands for the
amount in moles of that reagent.
Thus, multiplying the reaction multiplies the value of
, and reversing the reaction
changes the sign of
Define enthalpy and calculate enthalpy changes.
6.5 Calorimetry: Measuring Quantities of Heat
After heat capacity, molar heat capacity, and specific heat are defined, instructions are given on measuring them, along
with sample calculations.
This leads to measuring heats of reaction in solution.
Bomb calorimeters measure the heats of
reactions involving gases.