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Equilibrium Concepts



Chemical reactions occur every second of the day all around the universe. Chemical reactions are found not only in industry, but also in many biological processes in the human body. For instance, the breakdown of carbohydrates into glucose and proteins into amino acids are a few examples of these biological processes. Chemical reactions can be classified into two categories: reversible and irreversible. Reversible reactions have double-headed arrows representing an equilibrium between reactants and products. Irreversible reactions have single-headed arrows representing the completion of the reaction. In a reversible reaction, products can interact or break apart to produce reactants. In an irreversible reaction, on the other hand, the reactants convert into the product, but the products cannot convert back into the reactants. A detailed look at these chemical reactions aids in our understanding of life processes.

In a closed system at equilibrium at a specific temperature, the concentrations or the partial pressures of the products and reactants can be used to calculate an equilibrium constant. The equilibrium constant can then be used to calculate how a different given closed system involving the same reaction will act at that specific temperature.

When there is a stress on a system in equilibrium, such as increasing the concentration of one of the substances or a change in pressure, the equilibrium will shift to compensate for this stress. This is called Le Chatelier's principle.

At A Glance

  • In a reversible reaction, there comes a point when the rate of the forward reaction becomes equal to the rate of the reverse reaction. The system reaches equilibrium at this condition. At equilibrium, no further change occurs in the concentrations of products or reactants unless the system is disturbed.
  • The law of mass action states that Kc is defined as the concentrations of products divided by the concentrations of reactants raised to correct powers. Kp is defined as the partial pressures of products divided by the partial pressures of reactants, raised to respective powers. The chemical coefficients become exponents in the expressions for Kc and Kp.
  • Equilibrium constants can be calculated only if the system is closed and at equilibrium.
  • When a system in equilibrium undergoes stress, such as changes in concentration, pressure, and volume, the position of equilibrium moves in an opposing direction to regain equilibrium. This is called Le Chatelier's principle. Changing temperature will establish a new equilibrium with a different Kc or Kp value.
  • When concentration of reactants increases, the shift is toward the right, and when concentration of reactants decreases, the shift is toward the left.
  • For exothermic reactions, Kc and Kp decrease with increase in temperature. For endothermic reactions, Kc and Kp increase with increase in temperature.
  • If pressure increases or volume decreases, the equilibrium shifts toward fewer moles of gas, and if pressure decreases or volume increases, the equilibrium shifts toward more moles of gas. Catalysts do not have any effect on equilibrium constants.
  • Usually solvents, pure solids, and pure liquids are ignored in calculations of equilibrium constants.
  • Equilibrium calculations can be done by writing the balanced equation, identifying the number of moles of reactants and products, designing a table to find concentrations or partial pressures, and applying the formula.