The physico-chemical processes is the transfer of heat from one body to another. It is possible, usually at constant pressure and temperature. In the role of constant pressure is usually atmospheric. Enthalpy, like internal energy is a state function.Internal energy is the sum of kinetic and potential energies of the whole system. It is the basis for the equation of enthalpy. Enthalpy is the sum of internal energy and pressure multiplied by the volume of the system and is equal to:H=U+pV, where p is the system pressure, V - volume of the system.The above formula is used to calculate the enthalpy in the case where all the three quantities: pressure, volume and internal energy. However, it is not always the enthalpy is calculated as follows. Besides him, there are several ways of calculating enthalpy.
Knowing the free energy and entropy, we can calculate the enthalpy. Free energy, or Gibbs free energy, represents the part of enthalpy of the system that is spent on turning in work, and equal to the difference between the enthalpy and temperature multiplied by the entropy:ΔG=ΔH-TΔS (ΔH, ΔG, ΔS - increment values)the Entropy in this formula is a measure of the disorder of particles in the system. It increases with increasing temperature T and pressure. When ΔG<0 process is spontaneous when ΔG>0 - is not.
In addition, enthalpy is also calculated based on equations of chemical reactions. If the equation of a chemical reaction of the form A+B=C, then the enthalpy can be determined by the formula:dH=dU+ΔnRT, where Δn=nk PN nk PN and the number of moles of reaction products and starting materials)In the Isobaric process equals the entropy change of heat in the system: dq=dH.At constant pressure enthalpy is equal to:H=∫СpdTВ if the enthalpy and entropy factors balance each other, the increment of enthalpy is equal to the product of temperature on the increment of entropy is:ΔH=TΔS
Advice 2: How to calculate thermal effect
The thermal effect of a thermodynamic system appears due to chemical reactions, however, one of its characteristics is not. This value can only be determined when some conditions are met.
The concept of thermal effectand is closely related to the concept of enthalpy of a thermodynamic system. This thermal energy which can be converted into heat at a certain temperature and pressure. This value characterizes the equilibrium state of the system.
Any chemical reaction is always accompanied by the absorption or release some amount of heat. In this case, the reaction refers to the impact of reagents on the system products. Thus, there is a thermal effectthat is associated with the enthalpy change of the system and its products are taking the temperature reported by the agents.
In ideal conditions, the thermal effect depends on the nature of chemical reactions. These are the conditions under which it is assumed that the system does no work other than work of expansion, and the temperature of its products and influencing the reactants are equal.
There are two types of course of the chemical reaction: isochoric (at constant volume) and Isobaric (at constant pressure). Formula thermal effectas follows:dQ = dU + PdV, where U is the energy of the system, P – pressure, V – volume.
In isochoric process PdV term vanishes because the volume doesn't change, does not expand the system, so dQ = dU. In the Isobaric process the pressure is constant and volume increases, which means that the system does work of expansion. Therefore, when calculating the thermal effectand the energy change of the system plus the energy spent on committing this work: dQ = dU + PdV.
PdV is constant, so it can be made under the sign of the differential, hence dQ = d(U + PV). The sum U + PV fully reflects the state of a thermodynamic system, and corresponds to the enthalpy. Thus, enthalpy is the energy expended in the expansion of the system.
Most commonly count thermal effect of two types of reactions is the formation of compounds and combustion. The heat of combustion or formation – table value, therefore the heat effect of reaction in the General case can be calculated by summing the warmth of all the participating substances.