Effect of Concentration
According to Le Chatelier's principle, if an external stress is applied at equilibrium, the system adjusts to offset the stress and reaches a new equilibrium position. This stress can be a change in the temperature, pressure, volume, or concentration. For example, when more reactants are added to a chemical system or if the products formed are removed, the equilibrium will shift to the right, toward the side of the products. If more products are added or if reactants are removed, the equilibrium will shift to the left, toward the side of reactants.
Thus, reversible reactions are self-directive. That is, even if they are thrown out of balance, they tend to naturally shift and regain their balance. This tendency to regain equilibrium offers an important result that can be implemented in manufacturing of industrial chemicals. Removing products can trigger production of more products. For example, in the contact process, a process that is used in production of sulfuric acid on an industrial scale, sulfur trioxide is the product. Sulfur trioxide is continually removed so that the reaction continues.Another example is in the Haber-Bosch process, used in the synthesis of ammonia. Nitrogen and hydrogen are made to react to form ammonia gas. This is a reversible reaction with a certain value of equilibrium. The ammonia gas often dissociates again into nitrogen and hydrogen.
Effect of Temperature
A temperature change can affect the equilibrium constant. This naturally changes the equilibrium. Depending on whether the reaction is endothermic or exothermic, the left or right side of the reversible reaction may become more favorable. An exothermic reaction releases heat, and an endothermic reaction absorbs heat from the surroundings. A useful way to understand how heat affects a reversible reaction is to treat heat as a reactant or a product. An exothermic reaction has heat as a product. An endothermic reaction has heat as a reactant:
Exothermic reaction. Enthalpy , the change in heat in a system, is negative.
Endothermic reaction. Enthalpy , the change in heat in a system, is positive.
In an exothermic reaction, increasing temperature will reduce Kc and Kp, favoring the reactants' side. Reducing temperature will increase Kc and Kp, favoring the products' side.
In an endothermic reaction, increasing temperature will increase Kc and Kp, favoring the products' side. Reducing temperature will decrease Kc and Kp, favoring the reactants' side.
Effect of Temperature on Kc and Kp
|Temperature increase||Equilibrium shifts RIGHT, and there are more PRODUCTS.||Equilibrium shifts LEFT, and there are more REACTANTS.|
|Temperature decrease||Equilibrium shifts LEFT, and there are more REACTANTS.||Equilibrium shifts RIGHT, and there are more PRODUCTS.|
The decomposition of dinitrogen tetroxide into nitrogen dioxide is a reversible, endothermic reaction. At a temperature of 100°C, both Kc and Kp values are low. When heat is supplied, Kc and Kp values change dramatically. Heat causes the system to shift toward the product side, and there is a greater yield of nitrogen dioxide.See the following example reaction:
Effect of Temperature on Kc and Kp for the Reaction
Effect of Pressure/Volume and Catalysts
Pressure and volume changes are often related. An increase in volume without changing the amount of gas in a system will result in a pressure decrease. A decrease in volume will result in a pressure increase. Adding or removing gases from a system may change pressure without changing volume.
If pressure is increased, the equilibrium will shift to the side of the reaction that has fewer moles of gas. In the same way, if the pressure is decreased, the shift will be to the side that favors a greater number of moles of gas. If both sides have the same number of moles of gas, a change in pressure will not affect equilibrium.Again, consider the synthesis of ammonia by the Haber-Bosch process in a closed system.
In a closed system, if pressure is increased, the equilibrium shifts toward the side with fewer moles of gas—the products' side, in this example. If pressure is decreased, the equilibrium shifts toward the side with more moles of gas—the reactants' side, in this example.
Effect of Pressure on a Closed Equilibrium System
|Change in Pressure||System Reaction|
|Increase||Moves toward side with fewer moles of gas|
|Decrease||Moves toward side with more moles of gas|
Summary of Stresses on Equilibrium Systems
|Concentration increase||Away from concentration increase|
|Concentration decrease||Toward concentration decrease|
|Pressure increase (for gases)||Toward the side with fewer moles of gas|
|Temperature increase||Away from the side where energy is released|
|Temperature decrease||Toward the side where energy is released|