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Systems do not contain work, but can perform work, and likewise, in formal thermodynamics, systems do not contain heat, but can transfer heat. Informally, however, a difference in the energy of a system that occurs solely because of a difference in its temperature is commonly called heat , and the energy that flows across a boundary as a result ...
The law states that the total enthalpy change during the complete course of a chemical reaction is independent of the sequence of steps taken. [2] [3] Hess's law is now understood as an expression of the fact that the enthalpy of a chemical process is independent of the path taken from the initial to the final state (i.e. enthalpy is a state ...
The first and second law of thermodynamics are the most fundamental equations of thermodynamics. They may be combined into what is known as fundamental thermodynamic relation which describes all of the changes of thermodynamic state functions of a system of uniform temperature and pressure.
Enthalpy (/ ˈ ɛ n θ əl p i / ⓘ) is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. [1] It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant external pressure, which is conveniently provided by the large ambient atmosphere.
Internal energy (U) is the capacity to do work plus the capacity to release heat. Gibbs energy [2] (G) is the capacity to do non-mechanical work. Enthalpy (H) is the capacity to do non-mechanical work plus the capacity to release heat. Helmholtz energy [1] (F) is the capacity to do mechanical work plus non-mechanical work.
The above derivation uses the first and second laws of thermodynamics. The first law of thermodynamics is essentially a definition of heat, i.e. heat is the change in the internal energy of a system that is not caused by a change of the external parameters of the system.
In mathematics and physics, the heat equation is a parabolic partial differential equation. The theory of the heat equation was first developed by Joseph Fourier in 1822 for the purpose of modeling how a quantity such as heat diffuses through a given region. Since then, the heat equation and its variants have been found to be fundamental in ...
Enthalpy, heat content and heat capacity [ edit ] It is very difficult to measure the absolute amount of any thermodynamic quantity involving the internal energy (e.g. enthalpy ), since the internal energy of a substance can take many forms, each of which has its own typical temperature at which it begins to become important in thermodynamic ...