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Preamble
All chemical processes involve various kinds of energy interchanges with the surrounding. Heat energy is the principal form of energy exchange between metallurgical systems and its surrounding in various extraction and refining of metals.
Thermo chemical calculations are required to perform energy balance of a process. Thermo chemical calculations are concerned with chemical changes that are accompanied by liberation or absorption of heat; for example
and
The formation of is accompanied by evolution of kcal of heat per kg mole of . Similarly kcal of heat per kg mole is evolved when carbon on combustion produces . This lecture deals with basics of calculation of heat evolved or absorbed during a chemical reaction. Suitable examples are given to illustrate the basics of calculations.
First law of thermodynamics:
In the following, only a brief account is given. The first law equation for a process occurring in a systems is
(1)
and are energy contents (internal energies) of the system in state 1 and state 2 (it must be clearly understood that thermodynamics deals with initial and final states), Q is the heat absorbed by the system from the surrounding and W is work done by the systems on the surrounding. Since and depend only on final and initial state of a systems, change in energy is independent of the path.
Since is independent of the path, is also independent of path. However Q and W represent energy exchange with the surrounding and are not thermodynamic properties of the system. For example, if for a given change in state by a path, heat Q absorbed is more than that path will also correspond to more work, W.
Some of the paths treated in thermodynamics are constant pressure, constant volume, adiabatic (Q=0) and reversible ones. Extraction of metals is mostly carried at 1 atmospheric pressure. For constant pressure processes, more useful form of first law can be derived.
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