Physical chemistry

Physical chemistry is a major branch of chemistry concerned with understanding chemical systems by applying the theories, principles, and methods of physics. It investigates both macroscopic and microscopic behaviour in matter, using concepts such as energy, force, motion, thermodynamics, quantum theory, and statistical mechanics to explain how chemical substances behave and interact. The discipline forms a bridge between chemistry and physics, enabling the interpretation of chemical phenomena through quantitative physical laws.
Physical chemistry differs from chemical physics in its emphasis on supramolecular and bulk properties, such as chemical equilibrium, thermodynamic relationships, and colloidal behaviour. It provides a conceptual framework for explaining how chemical reactions occur, what drives them, and how the structural and energetic properties of substances determine their observable characteristics.

Scope and Core Areas

Physical chemistry encompasses a wide array of topics aimed at explaining how chemical systems evolve in time and under varying conditions. A foundational concern is the role of intermolecular forces and how they influence the properties of materials, including plasticity, tensile strength, and surface tension in liquids. These forces help determine states of matter, solubility, vapour pressure, and many other physical characteristics.
Another pivotal area is chemical kinetics, which focuses on the rates of chemical reactions and the factors affecting them. Reaction rates depend on variables such as temperature, concentration, and the presence of catalysts. Physical chemists study transition states—high-energy configurations that reactants must pass through in order to transform into products. The height of the energy barrier associated with the transition state plays a central role in determining reaction speed.
Electrochemistry forms another core component of the discipline, exploring how chemical reactions produce or consume electric current. This includes the study of ionic species, conductivity, electrode processes, and applications such as batteries, fuel cells, and corrosion. Surface science is closely related, addressing phenomena such as adsorption, catalysis, and membrane behaviour, often important in biological and industrial systems.
Thermodynamics is integral to physical chemistry, governing how heat, work, and energy change during chemical processes. Chemical thermodynamics evaluates whether a reaction is spontaneous, how far it can proceed, and how energy can be converted into work. Thermochemistry, a related area, examines heat transfer during phase changes and chemical reactions. Colligative properties, such as boiling point elevation and osmotic pressure, reveal how the number of dissolved particles affects solution behaviour.
Statistical mechanics provides tools to interpret the properties of large numbers of molecules based on their microscopic behaviour. It connects molecular interactions with macroscopic observables such as temperature, pressure, and entropy, allowing chemists to model complex systems using only a few measurable variables. Computational chemistry enhances these investigations by using algorithms and simulations to study molecules, metal complexes, and reactions with high precision.

Key Concepts

A central theme in physical chemistry is the application of physical laws to chemical systems. Chemical species are interpreted as collections of atoms bonded together, and chemical reactions are analysed in terms of bond formation and bond breaking. Accurately predicting chemical properties requires knowledge of nuclear positions and how electrons are arranged around them, both of which are addressed through quantum theory.
Quantum chemistry is essential for determining the strength, shape, and nature of chemical bonds. It also explains how molecules absorb or emit electromagnetic radiation and how nuclear motion influences molecular behaviour. Closely allied with this field is spectroscopy, which examines the interactions between matter and electromagnetic radiation. Different types of spectroscopy—infrared, microwave, electron paramagnetic resonance, and nuclear magnetic resonance—provide detailed insight into molecular structure, rotational and vibrational states, and magnetic properties.
Chemical thermodynamics sets constraints on energy transformations in chemical systems. For example, it determines the maximum work obtainable from engines and the conditions necessary for equilibrium in mixtures. In classical thermodynamics, emphasis is placed on reversible processes and equilibrium states, though extensions of the theory permit limited descriptions of irreversible and kinetic phenomena.
Chemical kinetics complements thermodynamics by describing how fast reactions proceed. Reaction rates often depend on only a few variables, despite the enormous number of particles involved. This simplification is rooted in the statistical behaviour of matter and is formalised through statistical mechanics. These ideas enable chemists to focus on practical parameters such as concentration and temperature without monitoring individual molecular motions.

Disciplines within Physical Chemistry

Several major sub-disciplines contribute to the wider field:

• Quantum chemistry, concerned with applying quantum mechanics to chemical systems, determines electronic structures, bonding patterns, and reaction pathways.
• Spectroscopy, which explores how matter interacts with electromagnetic radiation, is vital for identifying substances and investigating molecular dynamics.
• Chemical thermodynamics, providing constraints on energy conversion and system behaviour, supports the design of reactors, engines, and industrial processes.
• Chemical kinetics, examining reaction mechanisms and rates, is crucial for optimising reaction conditions and understanding catalysis.
• Statistical mechanics, linking microscopic properties to bulk behaviour, helps predict thermodynamic variables from molecular-level information.
• Computational chemistry, using mathematical modelling and computer simulations, allows precise prediction of molecular properties, even for compounds not yet synthesised.

Together, these disciplines allow chemists to explain and predict the behaviour of matter across a wide range of conditions.

Historical Development

The term physical chemistry was first introduced by Mikhail Lomonosov in 1752, who defined it as the science explaining phenomena in complex bodies through physical experiments and chemical operations. Modern physical chemistry, however, developed significantly during the latter half of the nineteenth century. Major progress occurred in the 1860s to 1880s, particularly in thermodynamics, electrolytic solutions, and reaction kinetics.
A foundational milestone was Josiah Willard Gibbs’s 1876 publication On the Equilibrium of Heterogeneous Substances. This work introduced concepts such as Gibbs free energy, chemical potential, and the phase rule—core principles still used today. The creation of the journal Zeitschrift für Physikalische Chemie in 1887 by Wilhelm Ostwald and Jacobus Henricus van ’t Hoff helped establish physical chemistry as a recognised scientific field.
Subsequent decades saw rapid development, including the application of statistical mechanics to chemical systems and innovative studies in colloids and surfaces, with Irving Langmuir making influential contributions. The rise of quantum mechanics in the early twentieth century transformed the discipline, leading to the emergence of quantum chemistry. Linus Pauling played a prominent role in establishing the quantum mechanical description of chemical bonding.
Technological advances also shaped physical chemistry. Throughout the twentieth century, spectroscopic methods became essential tools for experimental research. Nuclear chemistry, particularly isotope separation during the mid-twentieth century, drove further advances. In more recent years, progress in astrochemistry and the development of computational algorithms for predicting physicochemical properties have expanded the scope and practical applications of the field. Modern methods allow accurate calculation of numerous properties—such as boiling points, critical temperatures, vapour pressures, and surface tensions—from molecular structures alone.

Journals and Research Literature

A range of scientific journals publish research in physical chemistry. Among the earliest is Zeitschrift für Physikalische Chemie, established in 1887. Other notable publications include Journal of Physical Chemistry A, Physical Chemistry Chemical Physics, Macromolecular Chemistry and Physics, Annual Review of Physical Chemistry, Molecular Physics, Journal of Physical Organic Chemistry, Journal of Physical Chemistry B, ChemPhysChem, Journal of Physical Chemistry C, and Journal of Physical Chemistry Letters, which consolidates shorter communications in the field.
Historical journals that once combined chemistry and physics, such as Annales de chimie et de physique, also contributed significantly to early scientific discourse.