# Introduction To Energy Changes In Chemical Reactions

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### Introduction

Energy is something that can move matter and do work.

All substances (chemicals) have energy stored in the form called chemical energy (chemical potential energy). In this topic, we are interested in the energy changes that take place during chemical reactions.

During a chemical reaction, the chemical energy can be changed into other forms of energy, like:

• Heat energy – E.g. Lighting a fire cracker. Chemical energy is changed into heat energy, light energy and sound energy.
• Light energy
• Sound energy
• Electrical energy – E.g. Within a simple electric cell, chemical energy is changed into electrical energy.

Every substances (chemicals) possesses a characteristic internal energy which is due to its structure and physical state. This is known as the heat content or enthalpy.

• The unit for enthalpy is H.

Most energy changes during a chemical reaction involve heat energy. The amount of heat energy given out or taken in (absorbed) during a chemical reaction is called the enthalpy change or the heat of reaction.

• Unit for enthalpy change is $\Delta \text{H}$.
• $\Delta \text{H}$ is usually measured in kilojoules (kJ).
• $\Delta \text{H} = \text{Energy content of the products }-\text{Energy content of the reactants}$

A reaction that gives out heat to the surroundings is called an exothermic reaction.

A reaction that takes in heat from the surroundings is called an endothermic reaction.

We will explore more about exothermic and endothermic reaction in the later sub-topics.