Equilibrium

Chemical equilibrium is the state in which the concentrations of the reactants and products in a reaction have no net change over time.

The Law of Mass Action

The Law of Mass Action is a mathematical model that explains behaviors of chemical reactions in dynamic equilibrium. Given the generic reaction,

aA(g)  +  bB(g)   ↔  cC(g)  +  cD(g) ,

Kc is known as the equilibrium constant for a reaction. An equation with a large equilibrium constant favors products , and an equation with a small equilibrium constant favors reactants. The equilibrium constant only changes when temperature changes. Increasing temperature increases Kc if the reaction is endothermic and decreases it if the reaction is exothermic. On the other hand, decreasing temperature decrease Kc if the reaction is endothermic and increases it if the reaction is exothermic. Because their concentrations are constant, liquids and solids are excluded from the law of mass action.

When the reactants and products in a chemical equation are gases, the law of mass action equation can be written in terms of partial pressures as opposed to molarities - denoted with an equilibrium constant Kp. The above equation can thus be expressed in terms of partial pressures as

Reversing a chemical equation will reciprocal the equilibrium constant. Changing the equation’s coefficients by a certain factor changes the equilibrium constant by the power of that factor.

If two equations with equilibrium constants KA and KB are added together, then the equilibrium constant for the resulting equation, KC, is given by their product (KC =   KAKB).

Le Châtelier’s Principle

Le Châtelier’s Principle can be used to predict the effect of a change in conditions on equilibrium. It states that any change in the equilibrium reaction prompts an opposing reaction in the responding system.  A shift right is an equilibrium shift towards products. A shift left is an equilibrium shift towards reactants.

Changing Volume

Increasing volume would shift the equilibrium to the side with the most moles of gas. This shift makes sense, since, as Le Châtelier’s Principle regulates, the system reduces the stress of greater volume by shifting to the side with greater moles. On the other hand, decreasing volume would shift the equilibrium to the side with the least moles of gas. Thus, if a reaction retains the same moles of gas on both sides of the equation, volume changes cannot shift equilibrium.

Changing Temperature

Increasing temperature shifts the equilibrium in the endothermic direction. What this means is that if the net enthalpy change is positive in a reaction, increasing temperature would shift the equilibrium to the right. To the contrary, if the net enthalpy change is negative in a reaction, increasing temperature would shift the equilibrium to the left. Of course, decreasing temperature would produce the opposite effects as listed above in their respective situations.