The thermal effect of a thermodynamic system appears owing to emergence in it chemical reaction, however one of its characteristics is not. This size can be determined only at observance of some conditions.
Instruction
1. The concept of thermal effect is closely connected with a concept of an enthalpy of a thermodynamic system. It is thermal energy which can be transformed to warmth at achievement of a certain temperature and pressure. This size characterizes equilibrium state of a system.
2. Any chemical reaction is always followed by allocation or absorption of a number of warmth. In this case reaction is meant as impact of reagents on system products. At the same time there is a thermal effect which is connected with change of an enthalpy of a system, and its products accept temperature reported by reagents.
3. In ideal conditions the thermal effect depends only on the nature of chemical reaction. These are such conditions under which is accepted that the system does not make any work, except expansion work, and temperatures of its products and the influencing reagents are equal.
4. Distinguish two types of course of chemical reaction: isochoric (at the constant volume) and isobaric (with a constant pressure). The formula of thermal effect looks as follows: dQ = dU + PdV where U is energy of a system, P is pressure, V – volume.
5. At isochoric process the composed PdV addresses in zero as volume does not change, so there is no expansion of a system therefore dQ = dU. At isobaric process pressure is constant, and volume increases that means that the system makes expansion work. Therefore at calculation of thermal effect to change of energy of the system the energy spent for commission of this work increases: dQ = dU + PdV.
6. PdV is a constant therefore it can be brought under the sign of differential, therefore dQ = d (U + PV). U sum + PV completely reflects a condition of a thermodynamic system and also corresponds to a condition of an enthalpy. Thus, the enthalpy is the energy spent at expansion of a system.
7. Most often calculate thermal effect of two types of reactions – formation of connections and combustion. Heat of combustion or formations – a table value therefore the thermal effect of reaction generally can be calculated, having summarized warmth of all substances participating in it.