Physical chemistry

• 1. Introduction

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  Physical chemistry got its name from a lecture hall in 1752. Mikhail Lomonosov stood before students at Petersburg University and announced a course he called, in Russian, a course in true physical chemistry. He gave it a definition that still holds. The new science would explain, through physical experiments, the reasons for what happens inside complex bodies during chemical operations. It was a promise to bring the tools of physics to bear on the messy reality of matter. How do you predict whether a reaction will happen at all? How fast will it go? Why can a mixture of countless particles be captured by a handful of numbers like pressure and temperature? And how did a field named in the eighteenth century only truly come alive more than a hundred years later? Those are the questions this documentary follows, from heterogeneous substances to spectroscopy to the practical magic of predicting a boiling point from structure alone.

• 2. Physics For Chemists

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  Motion, energy, force, time. These are the raw materials physical chemistry borrows to study chemical systems, alongside thermodynamics, quantum chemistry, statistical mechanics, analytical dynamics and chemical equilibria. The field examines both the macroscopic and the microscopic, the bulk and the particle. Unlike chemical physics, physical chemistry is predominantly a supra-molecular science. Most of its founding principles relate to the bulk of a material rather than to molecular or atomic structure alone. Chemical equilibrium and colloids are the textbook examples of this preference for the collective over the single particle. The relationships it tries to understand are wide-ranging. Intermolecular forces shape physical properties such as plasticity, tensile strength, and the surface tension of liquids. The identity of ions governs the electrical conductivity of materials. Surface science meets biology in the electrochemistry of cell membranes. At its heart sits a single classical idea. All chemical compounds can be described as groups of atoms bonded together, and reactions are simply the making and breaking of those bonds. Predicting a compound's properties from how its atoms bond is one of the field's major goals. To do that precisely, you must know both where the nuclei sit and how the electrons are distributed around them.

• 3. Will It React

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  Spontaneity is the question that chemical thermodynamics was built to answer. Which reactions can happen on their own, and which properties are even possible for a given chemical mixture? Thermodynamics sets limits. It tells you how far a reaction can proceed, or how much energy can be converted into work in an internal combustion engine. It also links properties together, connecting something like the thermal expansion coefficient to the rate of change of entropy with pressure for a gas or a liquid. Engineers lean on it constantly. Thermodynamics can judge whether a reactor or engine design is feasible, or check whether experimental data are even valid. It has limits of its own. Classical thermodynamics is mostly concerned with systems in equilibrium and with reversible changes. It is less concerned with what actually does happen away from equilibrium, or how fast. Quasi-equilibrium and non-equilibrium thermodynamics can describe irreversible changes, but only to a limited extent. Thermochemistry handles a narrower slice of this world, the transfer of heat between a system and its surroundings during a phase change or a reaction. The study of colligative properties tracks how the number of species in a solution changes its behavior. And the phase rule ties together the number of phases, the number of components, and the degrees of freedom in a single relationship.

• 4. How Fast

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  Transition states are the gatekeepers of chemical kinetics, the branch of physical chemistry that asks which reactions occur and how quickly. For reactants to become products, most chemical species must climb through a transition state that is higher in energy than either the reactants or the products. That energy barrier slows everything down. In general, the higher the barrier, the slower the reaction. Most reactions are not single events. They occur as a sequence of elementary reactions, each with its own transition state to clear. The central questions of kinetics are practical ones. How does the rate depend on temperature and on the concentrations of reactants and catalysts? How can catalysts and reaction conditions be engineered to optimize that rate? There is something almost surprising hidden in this. Reaction speed can often be pinned down with just a few concentrations and a temperature, without knowing the position and speed of every molecule. That convenience is a special case of a deeper truth the next chapter explains.

• 5. The Power Of The Average

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  Six times ten to the twenty-third. That number, the Avogadro constant, hints at why physical chemistry can be so economical with information. Everything happening in a mixture of that many particles can often be captured by just a few variables, pressure, temperature, and concentration. To the extent an engineer needs to know, the swarm reduces to a handful of numbers. The precise reasons live in statistical mechanics, a specialty within physical chemistry that it shares with physics. Statistical mechanics does more than explain this collapse of complexity. It provides ways to predict the properties we see in everyday life directly from molecular properties, without leaning on empirical correlations based on chemical similarities. Quantum chemistry sits alongside it as another bridge between scales. It applies quantum mechanics to chemical problems and provides tools to determine how strong bonds are and what shape they take, how nuclei move, and how a compound absorbs or emits light. Closely related is spectroscopy, the sub-discipline concerned specifically with how electromagnetic radiation interacts with matter. Together these fields let chemists calculate the energy of electron movement in molecules and metal complexes.

• 6. From Gibbs To Pauling

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  Eighteen seventy-six is the year modern physical chemistry found one of its cornerstones. That was when Josiah Willard Gibbs published his paper, On the Equilibrium of Heterogeneous Substances. It introduced Gibbs energy, chemical potentials, and the Gibbs phase rule in one stroke. The modern field had been taking shape since the 1860s to 1880s, growing out of work on chemical thermodynamics, electrolytes in solutions, and chemical kinetics. In 1887 the field gained its first dedicated outlet. The German journal Zeitschrift für Physikalische Chemie was founded that year by Wilhelm Ostwald and Jacobus Henricus van 't Hoff. Together with Svante August Arrhenius, these were the leading figures of the late nineteenth and early twentieth centuries. All three were awarded the Nobel Prize in Chemistry between 1901 and 1909. The decades that followed pushed in new directions. Statistical mechanics was applied to chemical systems, and Irving Langmuir made many contributions to colloids and surface chemistry. From the 1930s, quantum mechanics grew into quantum chemistry, where Linus Pauling was one of the leading names. Theory advanced in step with experiment. The rise of spectroscopy, including infrared, microwave, electron paramagnetic resonance and nuclear magnetic resonance, was probably the most important experimental development of the twentieth century.

• 7. Boiling Points From Blueprints

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  More than twenty physicochemical properties can now be calculated from chemical structure alone. Boiling point, critical point, surface tension, vapor pressure, and others can be worked out precisely even if the molecule has never been synthesized. This is the world of additive physicochemical properties, and it is where much of the practical importance of contemporary physical chemistry lies. The methods carry the names of their inventors, the Group contribution method, the Lydersen method, the Joback method, and Benson group increment theory, alongside the quantitative structure, activity relationship. Their reach was widened by discoveries elsewhere. Nuclear chemistry, especially isotope separation before and during World War II, fed the field, as did more recent work in astrochemistry. The journals trace the same expansion. The Journal of Physical Chemistry ran from 1896 before splitting into separate A, B, and C editions, and titles like Physical Chemistry Chemical Physics carry a lineage of Faraday Transactions reaching back to 1905. Even older are the combined chemistry and physics journals such as Annales de chimie et de physique, started in 1789. From a lecture in Petersburg to an unsynthesized molecule whose boiling point is already known, physical chemistry has spent its history turning the principles of physics into answers chemists can use.