Thermal energy is the total kinetic energy associated with the random movement of a substance’s constituent particles, including atoms and molecules. The introduction of energy, often referred to as heat, increases this random motion. Temperature is the quantitative measure of the average kinetic energy of these moving particles. Therefore, an increase in thermal energy leads to a corresponding increase in temperature, reflecting that the particles are, on average, moving faster.
Increased Molecular Kinetic Energy and Motion
Adding thermal energy directly increases the kinetic energy of a substance’s molecules, causing them to move more vigorously. This motion is distributed across three primary types of molecular movement. The simplest is translation, which is the physical movement of the entire molecule from one location to another.
For molecules composed of two or more atoms, the added energy also fuels internal motions. Rotation involves the molecule spinning around its axis. Vibration causes the atoms within a molecule to oscillate, stretching and bending their bonds.
In a gas, translational motion is the most significant form of kinetic energy that determines temperature. In liquids and solids, however, molecules are held closely together, and energy is partitioned among translation, rotation, and especially vibration. The molecules in a solid cannot move freely but instead vibrate more intensely in their fixed positions as thermal energy is absorbed.
Driving Changes in States of Matter
The increased molecular motion resulting from higher thermal energy drives physical state changes, known as phase transitions. Attractive forces, known as intermolecular forces, hold the molecules together, defining the solid or liquid state. The kinetic energy of the molecules constantly works against these attractive forces.
When a solid is heated, the molecules vibrate until they gain enough kinetic energy to partially overcome the forces locking them into a rigid structure. This threshold is the melting point, where the solid transitions into a liquid as the molecules gain the freedom to move past one another. Continued addition of thermal energy to the liquid increases molecular speed until the energy is sufficient to completely overcome the remaining intermolecular forces.
At the boiling point, the liquid turns into a gas, with molecules escaping the attractive forces and moving independently. During these transitions, the temperature remains constant even as energy is continuously added. This occurs because the energy is being used to break the attractive bonds rather than increasing the speed of the molecules, and this absorbed energy is referred to as latent heat.
Influence on Chemical Reaction Rates
An increase in thermal energy significantly influences the speed at which chemical reactions occur. Chemical reactions require molecules to collide with the correct orientation and a minimum amount of energy, called the activation energy, to successfully rearrange their bonds. Thermal energy boosts the reaction rate through two distinct effects.
First, the faster movement of molecules at higher temperatures means they collide more frequently. Second, the increased kinetic energy ensures that a greater proportion of those collisions meet or exceed the activation energy threshold. The distribution of molecular energies shifts, making high-energy collisions much more common, even though the activation energy itself does not change.
For many reactions, a temperature increase of just \(10text{°C}\) can nearly double or triple the reaction rate. This exponential increase occurs because the enhanced molecular speed not only increases the number of collisions but also dramatically raises the probability that any given collision will have the necessary force to break existing bonds and form new products.