Any substance contains a number of heat. This heat is called an enthalpy. The enthalpy is the size characterizing energy of a system. In physics and chemistry it shows warmth of reaction. It is an alternative of internal energy, and this size is specified most often with a constant pressure when the system has some stock of energy.

## Instruction

1. In physical and chemical processes there is a transfer of heat from one body to another. It is possible, as a rule, at constant pressure and temperature. As constant pressure atmospheric usually acts. The enthalpy, as well as internal energy, is function of a state. Internal energy represents the sum of kinetic and potential energy of all system. It is a basis for the enthalpy equation. The enthalpy represents the sum of internal energy and pressure increased by system volume and is equal: H=U+pV where p is pressure in a system, V - system volume. The above-stated formula is applied to calculation of an enthalpy when all three sizes are given: pressure, volume and internal energy. However, not always the enthalpy pays off thus. Besides it, there are some more ways of calculation of an enthalpy.

2. Knowing free energy and entropy, it is possible to calculate an enthalpy. Free energy, or Gibbs's energy, represents the part of an enthalpy of a system spent for transformation into work and is equal to the difference of the enthalpy and temperature increased by entropy: ΔG=ΔH-TΔS (ΔH, ΔG, ΔS - increments of sizes) the Entropy in this formula is a measure of disorder of particles of a system. It increases at increase in temperature of T and pressure. At ΔG <0 process goes spontaneously, at ΔG> 0 - does not go.

3. Besides, the enthalpy also pays off proceeding from the equation of chemical reaction. If the equation of chemical reaction of a type of A+B=C is given, then the enthalpy can be determined by a formula: dH=dU+ΔnRT where Δn=nk-nн (nk and nn - number of moths of products of reaction and initial substances) At isobaric process the entropy is equal to change of warmth in a system: dq=dH.Pri constant pressure an enthalpy is equal: H= ∫ to CpdTB a case if enthalpy and entropy factors counterbalance each other, increment of an enthalpy is equal to the work of temperature on increment of entropy: ΔH=TΔS