Energy Basics

Why does sodium metal react with water to make hydrogen gas, but sodium chloride simply dissolves? This can be explained by studying the relationship between chemistry and energy, called ‘thermochemistry’.

All forms of matter are associated with energy, which can be measured as heat or work. 

The two main types of energy are potential and kinetic energy.

Potential energy describes the positional forces acting on the object. A rock at the top of a hill has more potential energy than a rock at the bottom because the rock at the top is further from the Earth’s center of mass.

Kinetic energy is the energy of a moving object. If a rock is pushed off the hill, it loses potential energy but it gains kinetic energy as it moves.

This also applies to the atoms or molecules that make up an object. An object at rest still has kinetic energy because its component atoms can vibrate.

This form of kinetic energy is called thermal energy. An object has higher kinetic energy and consequently higher thermal energy when the atoms move more vigorously. This object is perceived as hot.

Molecules also have potential energy called chemical energy, which is associated with the relative position of electrons and nuclei. 

During a reaction, chemical energy is converted into other forms of energy, such as light, or energy is released, such as in the form of heat.

The amount of energy transformed depends on the molecular structure of reactants.

In a gas lamp, acetylene burns into carbon dioxide and water, releasing a large amount of energy as light.

On the other hand, adding sodium hydroxide to hydrochloric acid, releases a smaller amount of energy, as heat.

To study energy changes, it helps to divide the universe into a ‘system’, which contains the process being observed, and the ‘surroundings’, which is everything else.

The system can be open, meaning that both energy and mass can be exchanged between the system and surroundings. The system can be closed, where only energy is exchanged, or the system can be isolated, where neither mass nor energy is exchanged.