It is essentially a value for the specific enthalpy change necessary to get one kilogram of water at boiling temperature (100 degrees Celsius) to change state to one kilogram of water vapor. In kilograms, this amounts to 2257 kJ/kg. From experimental data, this number is known to be 40.65 kJ/mol. This is known as the enthalpy of vaporization for water. Looking at the vaporization of water, the chemical equation is written as:įor a given mole of liquid water, there is a certain change in enthalpy that has to occur for that mole of water to change state to a gas. 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. Note that the Excel program uses SI units for the thermodynamic property values.Specific Enthalpy is the total energy in a system due to pressure and temperature per unit of mass in that system. The program is compatible with all versions of Excel.Ĭlick on this link to see a screen capture of the Excel spreadsheet for the superheated region. To use the calculator you need to have Microsoft Excel installed on your computer. You need to uncompress this file before you can access the calculator. In the subcooled and superheated region, you can specify ( P, T), ( P, v), ( P, u), ( P, h), or ( P, s), and automatically solve for the remaining thermodynamic properties.Īll the files that come with this calculator are contained in a single (compressed) file, in the "zip" format. In the saturated region, you can specify any of the six thermodynamic properties and automatically solve for the remaining five. It calculates thermodynamic properties in the subcooled region for an absolute pressure range of 5-50 MPa, using 7 data tables spread apart over this range. The Excel spreadsheet calculates thermodynamic properties in the superheated region for an absolute pressure range of 0.01-80 MPa, using 30 data tables spread apart over this pressure range. ![]() The calculator uses data points from thermodynamic property tables, and uses linear interpolation to calculate the following six thermodynamic properties: Temperature ( T), pressure ( P), specific volume ( v), energy ( u), enthalpy ( h), and entropy ( s). This calculator is programmed in Microsoft Excel. I created a steam table calculator which makes it easy to calculate the thermodynamic properties of water in the subcooled, saturated, and superheated region.
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