0.12 Measuring energy changes in chemical reactions

Introduction

Energy is a central theme of the study of Chemistry. The most common chemical reactions are carried out entirely for their production of energy. The most common sources of energy for use by humans are all chemical reactions. And the source of energy in the human body is entirely from chemical reactions. The industries of production, transportation, storage, and conversion of energy sources are overwhelmingly chemically based.

To this point in our studies, we have discussed energy extensively but only to help us understand the stability of atoms, the electronic structure of atoms, the stability of chemical bonds, the geometry of molecules, the bonding in metals and salts, and so forth. We have not yet studied the energy changes which accompany chemical reactions.

This study and the next mark a significant transition in our studies. Rather than focus entirely on the structure of atoms and molecules, we will now consider observations of chemical properties on the macroscopic level. One of the major goals of developing chemical models is to relate the structures of individual atoms and molecules to the properties we observe for substances and reactions in bulk amounts. This might seem an insurmountable task. Since a bulk sample of a substance may contain literally trillions of trillions of particles, relating the properties of those particles to the properties of the bulk seems to require an incomprehensible amount of information. In this study and those that follow, we begin that process. First, we make detailed observations about the amounts of energy which are released or absorbed during chemical reactions and develop a method for measuring reaction energies for bulk reactions. Then, in the next study, we will relate the energies of chemical bonds to the energies of chemical reactions. But first, we must relate the energies of chemical reactions to things we can measure directly and easily.

Introduction

To make any progress with energy in Chemistry, we must assume some basic principles about energy from Physics. Energy is the capacity to do work, where work is the application of a force over a distance. We can therefore tell whether an object possesses energy by determining whether it has the capacity to accelerate another object. Keep in mind that this is capacity to do work. It is not necessary for an object to actually do work to have energy. We often speak two broad types of energy, kinetic and potential. Kinetic energy is the energy associated with motion. An object in motion has the capacity to do work on another object by either pulling it, pushing it, or crashing into it, for examples. Potential energy is the energy associated with position. If by changing position an object can do work on another object, then it has potential energy. For example, a book on a table has potential energy because, if it were to fall to the floor, it could accelerate an object tied to it during the fall. During this study, we will relate chemical energy to these forms of energy.