Chemical reactions can be classified according to the number of phases involved. A reaction that is confined to a single phase is termed homogeneous. An example of this type of reaction is the combustion of methane:
Another example of a homogeneous reaction is the transfer of electrons between ions in aqueous solution, as in the reduction of tetravalent plutonium by ferrous ions:
The medium in which the reaction occurs is designated in parentheses: g for the gas phase; aq for an aqueous solution; L for a pure, nonaqueous liquid (e.g., a molten metal); soln for a nonaqueous solution; and s for a solid phase. Discussion of aqueous ionic chemical equilibrium is deferred until Chapter 9.
Reactions that involve species present in more than one phase are termed heterogeneous. For example, in the oxidation of solid or liquid metal M to form the solid oxide MO2, each participant is present as a pure phase:
If one or more of the participants is dissolved in an inert diluent, the oxidation reaction is written as:
The purpose of the present chapter is to apply thermodynamic theory to characterize the state of equilibrium of chemical reactions. At equilibrium, the concentrations of the species involved are unchanging and the properties of the chemical reaction lead to a relation between these concentrations. This relation is known as the law of mass action.
Rather than deal with each reaction as a specific case, a generalized reaction in which molecular species A and B interact to form new molecular entities C and D is analyzed:
The coefficients a,.. ,,d are the stoichiometric numbers or balancing numbers that serve to conserve elements on the two sides of the reaction. They are usually chosen so that one of them is unity.
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