How is equilibrium established

As we have established, the rates of the forward and reverse reactions are the same at equilibrium, and so the concentrations of all of the substances are constant. Since that is the case, it stands to reason that a ratio of the concentration for any given reaction at equilibrium maintains a constant value. Each concentration is raised to the power of its coefficient in the balanced chemical equation.

For the general reaction above, the equilibrium constant expression is written as follows:. The value of the equilibrium constant for any reaction is only determined by experiment. As detailed in the above section, the position of equilibrium for a given reaction does not depend on the starting concentrations and so the value of the equilibrium constant is truly constant.

It does, however, depend on the temperature of the reaction. This is because equilibrium is defined as a condition resulting from the rates of forward and reverse reactions being equal. If the temperature changes, the corresponding change in those reaction rates will alter the equilibrium constant. Allison Soult , Ph. Department of Chemistry, University of Kentucky. Learning Outcomes Explain chemical equilibrium. Calculate and compare Q and K values.

Conditions for Equilibrium and Types of Equilibrium It may be tempting to think that once equilibrium has been reached, the reaction stops. The system must be closed, meaning no substances can enter or leave the system. Reversible reactions will always result in a mixture of reactants and products being formed. While this isn't a major problem with the Haber process, it can often result in expensive reactant molecules not being completely converted into products.

As the forward reaction slows down, the reverse reaction will speed up until they are both taking place at the same rate. This is called the equilibrium position. At equilibrium the concentration of reactant and products remain constant but NOT necessarily equal.

Equilibrium can only be obtained in a closed system where the reaction is carried out in a sealed container and none of the reactants or products are lost. Concentration vs. Note that over time, the curves level out, or plateau, and the concentrations of A and B are no longer changing. This is the point at which the system has reached chemical equilibrium.

While there are various factors that can increase or decrease the amount of time it takes for a given system to reach equilibrium, the equilibrium position itself is unaffected by these factors. For instance, if a catalyst is added to the system, the reaction will proceed more quickly, and equilibrium will be reached faster, but the concentrations of both A and B will be the same at equilibrium for both the catalyzed and the uncatalyzed reaction.

The equilibrium state is one in which there is no net change in the concentrations of reactants and products. Despite the fact that there is no apparent change at equilibrium, this does not mean that all chemical reaction has ceased. Nothing could be further from the truth; at equilibrium, the forward and reverse reactions continue, but at identical rates, thereby leaving the net concentrations of reactants and products undisturbed.

Equilibrium lesson : This lesson introduces equilibrium as a condition where the forward and reverse reaction rates are equal. We also explain how the concentrations remain constant at equilibrium. The relationship between forward and reverse reactions in dynamic equilibrium can be expressed mathematically in what is known an equilibrium expression, or K eq expression. Most often, this expression is written in terms of the concentrations of the various reactants and products, and is given by:.

Species in brackets represent the concentrations of products, which are always in the numerator, and reactants, which are always in the denominator. Each of the concentrations is raised to a power equal to the stoichiometric coefficient for each species.

Assuming this reaction is an elementary step, we can write the rate laws for both the forward and reverse reactions:. Rearranging this equation and separating the rate constants from the concentration terms, we get:. Notice that the left side of the equation is the quotient of two constants, which is simply another constant.

We simplify and write this constant as K eq :. Keep in mind that the only species that should be included in the K eq expression are reactants and products that exist as gases or are in aqueous solution.