Thermochemistry
Thermochemistry is the branch of chemistry that examines the heat and energy changes associated with physical transformations and chemical reactions. It encompasses critical concepts such as calories, enthalpy, entropy, and heat capacities, focusing on how energy transfers affect various processes. Central to thermochemistry are two foundational laws: Lavoisier and Laplace's law, which states that energy changes in a transformation and its reverse are equal and opposite, and Hess's law, which indicates that the total energy change is the same regardless of the pathway taken in a reaction.
Thermochemistry differentiates between endothermic processes, which absorb heat (like photosynthesis and the evaporation of water), and exothermic processes, which release heat (such as combustion and the condensation of water vapor). The field also employs calorimetry to measure these heat changes, often using advanced tools like differential scanning calorimeters to assess the thermal properties of materials. By studying how energy interacts within a system, thermochemistry provides essential insights applicable in various scientific and industrial contexts.
Subject Terms
Thermochemistry
Summary: The study of the heat and energy associated with physical transformations and chemical reactions, thermochemistry focuses on the principles and processes involving calories, enthalpy, entropy, the heat of combustion, the heat of formation, and heat capacity.
Thermochemistry is the study of the heat and energy associated with physical transformations and chemical reactions. Thermochemistry studies calories, enthalpy, entropy, the heat of combustion, the heat of formation, and heat capacity. In particular, thermochemistry is founded on two laws: Antoine-Laurent de Lavoisier and Pierre-Simon Laplace’s law, formulated in 1780, states that the energy change accompanying any transformation is equal and opposite to the energy change accompanying the reverse of that process. Germain Henri Hess’s law, formulated in 1840, says that the energy change accompanying any transformation is the same, regardless of the number of steps involved in the process of the transformation. These laws later influenced the formulation of the first law of thermodynamics (by Rudolf Clausius), in 1850, which stated that the internal energy of a system is equal to the heat supplied to the system minus the work done by the system.
Thermochemistry studies systems, such as specific chemical processes or objects. A system refers to the part of the world that is being studied, with everything else considered as its environment. A system may be closed, in which case it can exchange energy with its environment, not matter; it may be open, in which case both matter and energy can be exchanged; or it may be isolated, meaning that neither energy nor matter may be exchanged. Processes may be described as adiabatic (without heat exchange), isobaric (without pressure change), or isothermal (without temperature change).
Thermochemistry pays particular attention to the energy exchanges of phase changes, chemical reactions, and the formation of solutions. It studies four major state functions: internal energy (the total energy contained by a system and necessary to create the system), enthalpy (internal energy plus the energy required to make room for a system by displacing its environment), entropy (a measure of energy dispersal at a specific temperature), and Gibbs free energy (the work obtainable from the system).
Endothermic and Exothermic Processes
The two types of reactions studied by thermochemistry are endothermic and exothermic.
Endothermic reactions absorb heat from the surroundings, as occurs when a chemical cold pack is activated (by breaking a seal separating two chemicals, such as ammonium chloride with water) to initiate a chemical reaction that absorbs heat in order to turn it into chemical bond energy. Photosynthesis is another endothermic process, whereby plants create chemical energy (in the form of carbohydrates) by exposure to carbon dioxide, light, and water.
Perhaps the best-known endothermic process is the evaporation of water. Because a molecule must have sufficient kinetic energy to overcome liquid-phase intermolecular forces and kinetic energy is proportional to temperature, the warmest molecules evaporate first, which lowers the average temperature of the molecules remaining in a process called evaporative cooling. Humans sweat because it cools us off: Heat from the body’s surface is transferred to the sweat, which then “carries it away” by evaporating.
Exothermic processes release heat from a system. The burning of a candle and the combustion of fuels such as wood, coal, and petroleum are obvious exothermic processes. The condensation of water vapor is also an exothermic process, as is the setting of cement, concrete, and epoxy. In chemical reactions, heat released by an exothermic reaction is usually absorbed in the form of electromagnetic energy, and as kinetic energy is lost via reacting electrons, light is released.
Calorimetry
Calorimetry is the process of measuring heat changes. Modern thermochemists use the differential scanning calorimeter, which measures the amount of heat required to increase the temperature of a sample being tested and a reference with a well-defined heat capacity, as a function of temperature. As the sample goes through phase transitions and other physical transformations, either more or less heat will be required to keep it at the same temperature as the reference material, depending on whether the process is exothermic or endothermic. A sample undergoing exothermic processes will require less heat, whereas a sample undergoing endothermic processes will require more. The difference in heat flow is observed while maintaining the sample and the reference at the same temperature. Differential scanning calorimetry is widely used in industry as a way to test the purity of sample materials.
Bibliography
Han, Zhennan, et al. "Engineering Thermochemistry to Cope with Challenges in Carbon Neutrality." Journal of Cleaner Production, vol. 416, 2023, doi.org/10.1016/j.jclepro.2023.137943. Accessed 6 Aug. 2024.
McCray, Tama. An Introduction to Thermochemistry. Delhi: White Word Publications, 2012.
Mirskiy, Anton G. Thermochemistry and Advances in Chemistry Research. New York: Nova Science Publishers, 2009.
Perrot, Pierre. A to Z of Thermodynamics. New York: Oxford University Press, 1998.
Stacy, Angelica M., Janice A. Coonrod, and Jennifer Claesgens. Fire: Energy and Thermochemistry. Emeryville, CA: Key Curriculum Press, 2005.