Enthalpy Change

Chemists ordinarily use a property known as enthalpy (H) to describe the thermodynamics of chemical and physical processes. Enthalpy is defined as the sum of a system’s internal energy (E) and the mathematical product of its pressure (P) and volume (V): Enthalpy is a state function. Enthalpy values for specific substances cannot be measured directly; only enthalpy changes for chemical or physical processes can be determined. For processes that take place at constant pressure (a common...

Source: Ketron Mitchell-Wynne, PhD, Asantha Cooray, PhD, Department of Physics & Astronomy, School of Physical Sciences, University of California, Irvine, CA When a pot of water is placed on a hot stove, heat is said to "flow" from the stove to the water. When two or more objects are placed into thermal contact with each other, heat spontaneously flows from the hotter objects to the colder ones, or in the direction that tends to equalize the temperature between the objects. For example,...

Hess’s law can be used to determine the enthalpy change of any reaction if the corresponding enthalpies of formation of the reactants and products are available. The main reaction may be divided into stepwise reactions : (i) decompositions of the reactants into their component elements, for which the enthalpy changes are proportional to the negative of the enthalpies of formation of the reactants, −ΔHf°(reactants), followed by (ii) re-combinations of the elements (obtained in step 1) to give...

There are two criteria that favor, but do not guarantee, the spontaneous formation of a solution: A decrease in the internal energy of the system (an exothermic change, as discussed in the previous chapter on thermochemistry) An increased dispersal of matter in the system (which indicates an increase in the entropy of the system, as you will learn about in the later chapter on thermodynamics) In the process of dissolution, an internal energy change often, but not always, occurs as heat is...

Enthalpy of Reaction Thermodynamics is the study of heat energy and other types of energy, such as work. The laws of thermodynamics are used across all known science fields and have applications ranging from biology to physics. Three Laws of Thermodynamics There are three principle laws of thermodynamics that describe the interactions occurring within the universe, regardless of scale. The first is the law of conservation of energy, which describes that the total energy of an isolated system is...