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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.
The pure component's molar volume and molar enthalpy are equal to the corresponding partial molar quantities because there is no volume or internal energy change on mixing for an ideal solution. The molar volume of a mixture can be found from the sum of the excess volumes of the components of a mixture:
where is specific enthalpy, =: is dissipation function and is temperature. And where = (+) i.e. internal energy per unit volume equals mass density times the sum of: proper energy per unit mass, kinetic energy per unit mass, and gravitational potential energy per unit mass.
Therefore, gas volume may alternatively be expressed excluding the humidity content: V d (volume dry). This fraction more accurately follows the ideal gas law. On the contrary, V s (volume saturated) is the volume a gas mixture would have if humidity was added to it until saturation (or 100% relative humidity).
The Clapeyron equation allows us to use pressure, temperature, and specific volume to determine an enthalpy change that is connected to a phase change. It is significant to any phase change process that happens at a constant pressure and temperature.
Isotherms of an ideal gas for different temperatures. The curved lines are rectangular hyperbolae of the form y = a/x. They represent the relationship between pressure (on the vertical axis) and volume (on the horizontal axis) for an ideal gas at different temperatures: lines that are farther away from the origin (that is, lines that are nearer to the top right-hand corner of the diagram ...
When only expansion work is possible for a process we have =; this implies that the heat of reaction at constant volume is equal to the change in the internal energy of the reacting system. [ 3 ] The thermal change that occurs in a chemical reaction is only due to the difference between the sum of internal energy of the products and the sum of ...
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 ...