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Reaction equilibrium can be defined for reversible reactions. In these reactions, over time, a state is reached when there is no further change in the concentration of reactants or products in the system.

The rate of forward and backward reactions becomes equal, called the equilibrium state.

The value of the ratio of the concentration of the products to the reactants in the system (reaction quotient) at equilibrium is called the equilibrium constant of the reaction and is constant for a temperature.

According to the Le-Chatelier principle, the system adjusts itself to re-attain the equilibrium when equilibrium is disturbed. The le-Chatelier principle is mainly used to carry out reactions in maximum yield. The adjustments are made to counteract the effect of the disturbance in the system. It is done mainly by shifting the reaction to the reactant or product side.

The temperature and pressure conditions are adjusted so that the equilibrium is shifted toward the product side, thus producing maximum products.

For a system in equilibrium, the value of the reaction quotient will be equal to its equilibrium constant. A disturbance in equilibrium includes any change in the determining quantities of the equilibrium like temperature, pressure, and concentration of the species in the system.

This disturbance causes a change in the value of the reaction quotient, and the system adjusts itself to make the reaction quotient equal to the equilibrium constant.

Change in temperature 鈥� If a reaction is exothermic, its backward reaction will be endothermic. If the temperature of a system in equilibrium is changed, its equilibrium is disturbed. An increase in temperature is counteracted by favoring the endothermic reaction and a decrease in temperature by favoring the exothermic reaction.

Change in pressure 鈥� An increase in pressure causes the reaction to shift to the side having a smaller number of moles. A decrease in pressure shifts the reaction to the side having more number of moles.

Change in concentration 鈥� An increase in reaction concentration or decrease in product concentration favors forward reaction, and an increase in product concentration or decrease in reactant concentration favors backward reaction.