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Criteria for Chemical Reaction Equilibrium

The general criterion for thermodynamic equilibrium was derived in section 6.3 as: 
 

Criteria for Chemical Reaction Equilibrium | Additional Documents & Tests for Civil Engineering (CE)                        ...(6.36b)
As already explained, the above equation implies that if a closed system undergoes a process of change while being under thermal and mechanical equilibrium, for all incremental changes associated with the compositions of each species, the total Gibbs free energy of the system would decrease.  At complete equilibrium the equality sign holds; or, in other words, the Gibbs free energy of the system corresponds to the minimum value possible under the constraints of constant (and uniform) temperature and pressure. Since the criterion makes no assumptions as to the nature of the system in terms of the number of species or phases, or if reactions take place between the species, it may also be applied to determine a specific criterion for a reactive system under equilibrium.

As has been explained in the opening a paragraph of this chapter, at the initial state of a reaction, when the reactants are brought together a state of non-equilibrium ensues as reactants begin undergoing progressive transformation to products. However, a state of equilibrium must finally attain when the rates of forward and backward reactions equalize. Under such a condition, no further change in the composition of the residual reactants or products formed occurs. However, if we consider this particular state, we may conclude that while in a macroscopic sense the system is in a state of static equilibrium, in the microscopic sense there is dynamic equilibrium as reactants convert to products and vice versa. Thus the system is subject to minute fluctuations of concentrations of each species. 

However, by the necessity of maintenance of the dynamic equilibrium the system always returns to the state of stable thermodynamic equilibrium. In a macroscopic sense then the system remains under the under equilibrium state described by eqn. 6.36b. It follows that in a reactive system at the state of chemical equilibrium the Gibbs free energy is minimum subject to the conditions of thermal and mechanical equilibrium.


The above considerations hold regardless of the number of reactants or the reactions occurring in the system. Since the reaction co-ordinate is the single parameter that relates the compositions of all the species, the variation of the total Gibbs free energy of the system as a function of the reaction coordinate may be shown schematically as in fig. 8.3; here ξ e is the value of the reaction co-ordinate at equilibrium.

Criteria for Chemical Reaction Equilibrium | Additional Documents & Tests for Civil Engineering (CE)
Fig. 8.3 Variation of system Gibbs free energy with equilibrium conversion

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FAQs on Criteria for Chemical Reaction Equilibrium - Additional Documents & Tests for Civil Engineering (CE)

1. What is chemical reaction equilibrium in civil engineering?
Ans. Chemical reaction equilibrium in civil engineering refers to the state in which the rate of forward reaction and the rate of backward reaction in a chemical reaction are equal. It is an important concept in civil engineering as it affects the behavior and stability of materials used in construction, such as concrete and steel.
2. How is chemical reaction equilibrium achieved in civil engineering?
Ans. Chemical reaction equilibrium in civil engineering can be achieved by controlling the factors that influence the rate of the forward and backward reactions. This can be done by adjusting the temperature, pressure, and concentrations of the reactants. By finding the optimal conditions, engineers can ensure that the desired equilibrium state is reached and maintained.
3. What are the factors that affect chemical reaction equilibrium in civil engineering?
Ans. Several factors can affect chemical reaction equilibrium in civil engineering, including temperature, pressure, and concentrations of the reactants. Higher temperatures generally favor the forward reaction, while lower temperatures favor the backward reaction. Similarly, higher pressures and higher concentrations of reactants can promote the forward reaction.
4. How does chemical reaction equilibrium impact the behavior of construction materials in civil engineering?
Ans. Chemical reaction equilibrium plays a crucial role in determining the behavior of construction materials in civil engineering. For example, in the case of concrete, the equilibrium between cement, water, and aggregates is essential for the strength and durability of the material. Understanding and controlling chemical reaction equilibrium allows engineers to design and construct materials that meet specific performance requirements.
5. What are some practical applications of chemical reaction equilibrium in civil engineering?
Ans. Chemical reaction equilibrium has several practical applications in civil engineering. One example is the design of concrete mixtures, where engineers strive to achieve the optimal equilibrium between cement, water, and aggregates to ensure the desired strength and durability. Another application is the corrosion protection of steel structures, where the equilibrium between oxygen, moisture, and protective coatings is crucial to prevent rust formation and maintain structural integrity.

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