If a reaction adds energy to a system (endothermic), ΔH is positive and if a reaction subtracts energy from a system (exothermic), ΔH will be negative. This merely states that the total energy change after a reaction is equal to how much energy is present at the end subtracted by the amount we started with. In any open system, the following is true: We want to know what the change in enthalpy, ΔH, for a given reaction or process will be. In most thermodynamic applications, total enthalpy is not the quantity of interest. Where H is the total enthalpy, U is the energy of the work done in the system, p is pressure, and V is the volume of the system. This is to be seen as the specific enthalpy version of, and not to be confused with, the enthalpy equation: Where u is the specific energy, p is the pressure and v is the volume. Specific enthalpy can also be written in terms of specific energy, pressure, and specific volume such that the following equation is true: Where h is the specific enthalpy, H is the enthalpy of the system, and m is the total mass of the system. Specific enthalpy is calculated by taking the total enthalpy of the system and dividing it by the total mass of the system. The SI units for specific enthalpy are kJ/kg (kilojoules per kilogram). Specific enthalpy is used in thermodynamic equations when one wants to know the energy for a given single unit mass of a substance. Specific Enthalpy is the total energy in a system due to pressure and temperature per unit of mass in that system.
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