Thermochemistry in the context of Enthalpy

In chemistry and thermodynamics, calorimetry (from Latin calor 'heat' and Greek μέτρον (metron) 'measure') is the science or act of measuring changes in state variables of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reactions, physical changes, or phase transitions under specified constraints. Calorimetry is performed with a calorimeter. Scottish physician and scientist Joseph Black, who was the first to recognize the distinction between heat and temperature, is said to be the founder of the science of calorimetry.

Indirect calorimetry calculates heat that living organisms produce by measuring either their production of carbon dioxide and nitrogen waste (frequently ammonia in aquatic organisms, or urea in terrestrial ones), or from their consumption of oxygen. Lavoisier noted in 1780 that heat production can be predicted from oxygen consumption this way, using multiple regression. The dynamic energy budget theory explains why this procedure is correct. Heat generated by living organisms may also be measured by direct calorimetry, in which the entire organism is placed inside the calorimeter for the measurement.

Pierre Eugène Marcellin Berthelot (French: [bɛʁtəlo]; 25 October 1827 – 18 March 1907) was a French chemist and Republican politician noted for the Thomsen–Berthelot principle of thermochemistry. He synthesized many organic compounds from inorganic substances, providing a large amount of counter-evidence to the theory of Jöns Jakob Berzelius that organic compounds required organisms in their synthesis.Berthelot was convinced that chemical synthesis would revolutionize the food industry by the year 2000, and that synthesized foods would replace farms and pastures. "Why not", he asked, "if it proved cheaper and better to make the same materials than to grow them?"

He was considered "one of the most famous chemists in the world." Upon being appointed to the post of Minister of Foreign Affairs for the French government in 1895, he was considered "the most eminent living chemist" in France. In 1901, he was elected as one of the "Forty Immortals" of the Académie française.He gave all his discoveries not only to the French government but to humanity.

A calorimeter is a device used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimeters and accelerated rate calorimeters are among the most common types. A simple calorimeter just consists of a thermometer attached to a metal container full of water suspended above a combustion chamber. It is one of the measurement devices used in the study of thermodynamics, chemistry, and biochemistry.

To find the enthalpy change per mole of a substance A in a reaction between two substances A and B, the substances are separately added to a calorimeter and the initial and final temperatures (before the reaction has started and after it has finished) are noted. Multiplying the temperature change by the mass and specific heat capacities of the substances gives a value for the energy given off or absorbed during the reaction. Dividing the energy change by how many moles of A were present gives its enthalpy change of reaction.$${\displaystyle q=C_{\text{v}}(T_{f}-T_{i}),}$$ where q is the amount of heat according to the change in temperature measured in joules and C_v is the heat capacity of the calorimeter which is a value associated with each individual apparatus in units of energy per temperature (joules/kelvin).

In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change ΔH⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines as "... a reaction for which the overall standard Gibbs energy change ΔG⚬ is negative." A strongly exothermic reaction will usually also be exergonic because ΔH⚬ makes a major contribution to ΔG⚬. Most of the spectacular chemical reactions that are demonstrated in classrooms are exothermic and exergonic. The opposite is an endothermic reaction, which usually takes up heat and is driven by an entropy increase in the system.

The joule per mole (symbol: J·mol or J/mol) is the unit of molar energy, which is the ratio of energy to amount of substance in the International System of Units (SI).

The energy concerned may be thermodynamic energy, enthalpy, etc. For example, the Gibbs free energy of a compound in the area of thermochemistry is often quantified with the unit kilojoule per mole (symbol: kJ·mol or kJ/mol).

In chemical thermodynamics, an endergonic reaction (from Greek ἔνδον (endon) 'within' and ἔργον (ergon) 'work'; also called a heat absorbing nonspontaneous reaction or an unfavorable reaction) is a chemical reaction in which the standard change in free energy is positive, and an additional driving force is needed to perform this reaction. In layman's terms, the total amount of useful energy is negative (it takes more energy to start the reaction than what is received out of it) so the total energy is a net negative result, as opposed to a net positive result in an exergonic reaction. Another way to phrase this is that useful energy must be absorbed from the surroundings into the workable system for the reaction to happen.

Under constant temperature and constant pressure conditions, this means that the change in the standard Gibbs free energy would be positive,

In thermochemistry, the Thomsen–Berthelot principle is a hypothesis in the history of chemistry which argued that all chemical changes are accompanied by the production of heat and that processes which occur will be ones in which the most heat is produced. This principle was formulated in slightly different versions by the Danish chemist Julius Thomsen in 1854 and by the French chemist Marcellin Berthelot in 1864. This early postulate in classical thermochemistry became the controversial foundation of a research program that would last three decades.

This principle came to be associated with what was called the thermal theory of affinity, which postulated that the heat evolved in a chemical reaction was the true measure of its affinity.