http://www.ck12.org Chapter 17. Thermochemistry
17.2 Thermochemical Equations
Lesson Objectives
- Define enthalpy, and know the conditions under which the enthalpy change in a reaction is equal to the heat
absorbed or released. - Describe the principles behind calorimetry, and be able to calculate the heat absorbed or released during a
process that occurs in a calorimeter. - Write and solve problems with thermochemical equations.
Lesson Vocabulary
- calorimeter
- calorimetry
- enthalpy
- heat of reaction
- thermochemical equation
Check Your Understanding
Recalling Prior Knowledge
- How is specific heat used in calculating the heat absorbed or released in a process?
- What relationships are shown in a balanced chemical equation?
Heat is either absorbed or released during chemical reactions. In this lesson, you will learn about how heat changes
are measured in the lab and how quantities of heat are represented in a chemical equation.
Enthalpy
Heat changes in chemical reactions are most often measured in the laboratory under conditions in which the reacting
system is open to the atmosphere. In that case, the system is at a constant pressure. Enthalpy (H)is the heat
content of a system at constant pressure. Chemists routinely measure changes in the enthalpy of a chemical system
as reactants are converted into products. The heat that is absorbed or released by a reaction at constant pressure is
the same as the enthalpy change, which is given the symbol∆H. Unless otherwise specified, all reactions in this text
are assumed to take place at constant pressure. Therefore, heat and enthalpy change will be used interchangeably,
and q =∆H.