Physical chemistry

Physical chemistry is the study of how chemical systems behave using the principles of physics. It examines phenomena at both the macroscopic level, which involves large-scale observations, and the microscopic level, which looks at tiny particles. Scientists in this field use concepts like motion, energy, force, and time to explain chemical behavior.

By applying physics, researchers can understand how atoms and molecules interact to form complex substances. This science is often considered a supra-molecular science because many of its foundations relate to the bulk properties of matter rather than just individual atoms. This includes studying things like chemical equilibrium and colloids.

One major goal of physical chemistry is to predict the properties of chemical compounds. To do this, scientists must describe how atoms are bonded together. This requires precise knowledge of where the nuclei of atoms are located. They must also understand how electrons are distributed around those nuclei.

By mastering these details, researchers can explain why materials have specific physical traits. They can study intermolecular forces to understand why a material has certain tensile strength or surface tension. They can also study how ions affect the electrical conductivity of different materials.

Chemical thermodynamics is a vital branch that studies how heat and work interact. It looks at the relationship between a chemical system and its surroundings during a phase change or a chemical reaction. This field, often called thermochemistry, helps determine if a reaction will happen spontaneously. It can also set limits on how much energy an internal combustion engine can convert into work.

Thermodynamics provides links between different properties, such as the rate of change of entropy and the thermal expansion coefficient of a gas. This helps engineers decide if a specific reactor or engine design is actually feasible.

While thermodynamics looks at energy limits, chemical kinetics focuses on the speed of reactions. This branch investigates which reactions occur and how fast they proceed. A key concept in kinetics is the transition state. This is a state where chemical species have higher energy than the original reactants or the final products. This state acts as a barrier to the reaction. Generally, a higher energy barrier results in a slower reaction rate.

Kinetics also studies how temperature, reactant concentration, and catalysts can be used to optimize these reaction rates.

Physical chemistry also uses statistical mechanics to bridge the gap between tiny particles and large systems. In a mixture, there might be a massive number of particles, perhaps around the Avogadro constant of 6 × 10^23. It is impossible to track every single particle's position and speed. Instead, statistical mechanics allows scientists to describe these huge groups using just a few variables like pressure, temperature, and concentration. This method helps predict everyday properties from molecular characteristics without needing to rely only on experimental correlations. It provides a mathematical way to understand how microscopic movements create macroscopic reality.

The history of this field began with Mikhail Lomonosov, who coined the term in 1752. He presented a lecture course at Petersburg University about explaining chemical operations through physical experiments. The modern era of the science emerged between the 1860s and 1880s. A major milestone occurred in 1876 when Josiah Willard Gibbs published his work on the equilibrium of heterogeneous substances. This paper introduced essential concepts like Gibbs energy, chemical potentials, and the phase rule. Later, in 1887, Wilhelm Ostwald and Jacobus Henricus van 't Hoff founded the first specialized journal for the field. Ostwald, van 't Hoff, and Svante August Arrhenius were all leading figures who won Nobel Prizes in Chemistry between 1901 and 1909.

Today, the field includes advanced sub-disciplines like quantum chemistry and spectroscopy. Quantum chemistry applies quantum mechanics to determine the shape and strength of chemical bonds. It also explains how nuclei move and how light is absorbed or emitted by compounds. Spectroscopy is a closely related field that studies how electromagnetic radiation interacts with matter. Modern developments also include astrochemistry and nuclear chemistry. Scientists can now use calculation algorithms to predict more than 20 different physicochemical properties, such as boiling points or vapor pressure, from a chemical structure alone. This allows them to understand molecules even before they are physically synthesized in a lab.