CHEMICAL EQUILIBRIUM, Basic concepts - Physical and colloid chemistry


As a result of the study of the materials in Chapter 6, the student must: know the law of mass action;

know how calculate the chemical reaction isotherm (in accordance with the Van't Hoff equation), find equilibrium constants, isobars and isochores of chemical reactions;

master the basic concepts of chemical equilibrium, the skills of calculating equilibrium constants.

Basic concepts

Thermodynamic equilibrium is the state of a system whose characteristics (temperature, pressure, volume, concentration) do not change with time with constant external conditions. Chemical equilibrium - is a special case of thermodynamic equilibrium. At a chemical equilibrium the concentrations of all reactants do not change over time.

It should be noted that the chemical equilibrium is dynamic , i.e. corresponds to the simultaneous flow of the process in opposite directions. At the same time the rate of direct and reverse reactions are equal.

Chemical equilibrium is mobile - every infinitesimal external action on the equilibrium system causes an infinitesimal change in the state of the system; upon termination of external influence the system returns to the initial state.

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Another important property of chemical equilibrium is that the system can spontaneously come into equilibrium with two opposite sides. In other words, any non-equilibrium state is less stable, and the transition to a state from equilibrium is always connected with the need for external work.

It is known that the flow of a spontaneous process in a closed system is accompanied by a decrease in free energy (c1C & lt; 0, dF <0). Obviously, sooner or later the system will reach a minimum of free energy. The condition for the minimum of a certain function is the equality to zero of its first derivative and the positive sign of the second derivative.

In the same way, the condition of thermodynamic equilibrium in a closed system is the minimum value of the corresponding thermodynamic potential:

• isobaric-isothermal conditions (p = const, T = const):

• isochorically isothermal conditions (V = const, T = const):

The state of a system with a minimum free energy is a state of thermodynamic equilibrium.

It is known that Δ G = N - T Δ S, i.e. Δ G is determined by two components - the thermal effect Δ Н and the entropy factor T The first factor reflects an increase in the stability of the system with a decrease in internal energy; it manifests itself in the tendency toward greater aggregation of the substance, the coarsening of its particles, and the like. The second factor (entropy) reflects the tendency to intensify various dissociation processes into simpler particles under the action of their thermal motion. Both of these factors act in opposite directions, and the overall course of the reaction is determined by the predominance of one of them. The process goes on until a state is reached in which the influence of the factors becomes equal in magnitude, which corresponds to the equilibrium state. Since the change in entropy enters the equation in the composition of the product with temperature, other things being equal, the increase in temperature enhances the influence of the entropy factor.

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