Chemical Equilibrium

Chemical Equilibrium Chemical Equilibrium In a chemical reaction, chemical equilibrium is the state in which both reactants and products are present at concentrations which have no further tendency to change with time. Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction.

The reaction rates of the forward and reverse reactions are generally not zero but, being equal, there are no net changes in the concentrations of the reactant and product. This process is called dynamic equilibrium.

Forward Reaction Chemical equilibrium is a condition which means that the rate of the forward reaction is equal to the rate of the backward reaction. Equilibrium reaction is one in which there is no change in the concentration of the reactant or product.

Though concentration of the reactant and the product remains the same, the forward reaction and the backward reaction take place simultaneously.

Thus chemical equilibrium is a Dynamic one, that is, both the forward reaction and the back ward reaction take place simultaneously.

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Backward Reaction For the reaction 1, the backward reaction which is also called as the reverse reaction is B+C→A The product reacting back to form the reactant is the backward reaction. Some time the backward reaction rate is so fast that it seems as if the reaction has not occur at all because whatever product is formed, it immediately reacts to give back the reactant. So the product is not seen in the reaction itself.

Overall Reaction The overall reaction is the full equilibrium, that is, the forward reaction and the backward reaction taking place. And equilibrium condition is said to be reached when the rate of the forward reaction is equal to the rate of the backward reaction.

When the Equilibrium is reached, there is no change in the concentration of the reactant and the product. Although the concentration of the reactant and the product does not change under equilibrium condition, the concentration of the reactant and the product is not same.

They would be different.

Conditions of Equilibrium The first condition for the equilibrium is that the forward and the backward reaction rates should be equal. The pressure, temperature and the concentration of the reactant and the product do not change. There should not be any change during the equilibrium. Equilibrium is reached only in the closed vessel.

Reversibility and Equilibrium It has been established that when a chemical reaction proceeds, energy will be conserved during the chemical changes and all across the reaction when products are formed.

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But we have not found a way of predicting in which direction the reaction will go. In other words, we have not found a suitable definition of the position of equilibrium. We have discovered that for molecular systems the energy unlike the potential energy in mechanical systems does not provide a sufficient criterion for equilibrium.

We have observed that in our considerations of mechanical systems, if a change occurs in such a way that a system is always in equilibrium then the change will proceed infinitely slowly and will be capable of doing maximum amount of work.

Such a change is called reversible. The conditions which must be satisfied for a reversible change are the same conditions that must be satisfied if the system is to be in the state of equilibrium.

For a reversible change the work done by the system is a maximum. Thus, for a reversible change dw is more negative than for the equivalent irreversible spontaneous change. dU must be the same for a given change whatever way it is carried out. Therefore, for a reversible change since dwrev < dwirr we have dqrev > dqirr.

During a reversible change a system absorbs the maximum heat from its surroundings and does the maximum work on its surroundings. Observable spontaneous processes absorb less heat and do less work than the corresponding reversible process.

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