Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

Monthly Update sign up
Mailing List signup

Unit 7: The Energy in Chemical Reactions—Thermodynamics and Enthalpy

Section 1: Introduction

In science, energy represents the ability to do work or transfer heat. All living things require energy and have evolved ways to harness it from their environment. Plants absorb the energy of the sun directly, and animals acquire energy from food. Early humans acquired energy from food like other animals, but then developed new ways to capture it, starting with fire, a chemical reaction. (Figure 7-1)


Figure 7-1. Prometheus

© Wikimedia Commons, Public Domain.

View More


Figure 7-1. Prometheus

In Greek mythology, Prometheus is credited with bringing fire to humankind from the gods. As the earliest example of humans harnessing chemical energy, fire’s importance to the development of human culture cannot be overstated. The light from fire allowed humans to remain active after dark, and it provided protection from predators and insects. Cooking also makes food more digestible, allowing the body to extract more energy from it.

Today, we use energy not just to power our bodies, but also to power vehicles, lights, computers—all of the machines and technology associated with modern life. Most of that energy comes from burning fossil fuels—which, as we will see, is also a chemical reaction.

Because energy is vital to so many aspects of our existence, a special branch of chemistry developed to study the energy of chemical reactions: thermochemistry. Energy takes many different forms: Objects in motion possess a certain kind of energy, and objects lifted against the force of gravity possess another. The most important types of energy for chemists are:

  • Chemical energy—the energy stored in chemical bonds, and
  • Thermal energy—the energy associated with temperature.

Thermochemistry focuses on energy changes that occur within a particular system, the place where the reaction is taking place, such as a laboratory container. A system can exchange energy with its surroundings through heat and work.

Energy is central to chemical reactions: Some release it, while others require energy to be added before they can take place. Understanding the role of energy in chemistry provides insight into all kinds of questions about daily life. For example, how does sweating cool us off on hot days? And why do vegans often eat larger meals than omnivores?

At a larger scale, thermochemistry is a critical part of many industrial and technological processes. It explains why societies typically shift from bio-based fuels such as wood to fossil fuels, which contain more energy per unit of fuel as they industrialize. (See Unit 12 to learn about nuclear chemistry as a way of obtaining even more energy from matter.)



The study of energy in chemical reactions.


© Annenberg Foundation 2017. All rights reserved. Legal Policy