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.
Reactants and products in the solid and liquid phases, even if they are involved in the reaction, are not included in the K eq expression, as these species have activities of 1.
The activity for solids and liquids is 1, so they essentially have a constant concentration of 1, and thereby have no effect on the K eq expression. There is an equilibrium reaction that occurs as the carbon dioxide reacts with the water to form carbonic acid H 2 CO 3. Today, CO 2 can be pressurized into soft drinks, establishing the equilibrium shown above. Once you open the beverage container, however, a cascade of equilibrium shifts occurs.
First, the CO 2 gas in the air space on top of the bottle escapes, causing the equilibrium between gas-phase CO 2 and dissolved or aqueous CO 2 to shift, lowering the concentration of CO 2 in the soft drink. The lowered carbonic acid concentration causes a shift of the final equilibrium. As long as the soft drink is in an open container, the CO 2 bubbles up out of the beverage, releasing the gas into the air Figure 3.
With the lid off the bottle, the CO 2 reactions are no longer at equilibrium and will continue until no more of the reactants remain. Let us consider the evaporation of bromine as a second example of a system at equilibrium. An equilibrium can be established for a physical change—like this liquid to gas transition—as well as for a chemical reaction. Figure 4 shows a sample of liquid bromine at equilibrium with bromine vapor in a closed container. When we pour liquid bromine into an empty bottle in which there is no bromine vapor, some liquid evaporates, the amount of liquid decreases, and the amount of vapor increases.
If we cap the bottle so no vapor escapes, the amount of liquid and vapor will eventually stop changing and an equilibrium between the liquid and the vapor will be established. If the bottle were not capped, the bromine vapor would escape and no equilibrium would be reached.
A reaction is at equilibrium when the amounts of reactants or products no longer change. Chemical equilibrium is a dynamic process, meaning the rate of formation of products by the forward reaction is equal to the rate at which the products re-form reactants by the reverse reaction. When a system has reached equilibrium, no further changes in the reactant and product concentrations occur; the reactions continue to occur, but at equivalent rates. The concept of equilibrium does not imply equal concentrations, though it is possible.
Skip to content Chapter Fundamental Equilibrium Concepts. A phase equilibrium occurs when a substance is in equilibrium between two states.
For example, a stoppered flask of water attains equilibrium when the rate of evaporation is equal to the rate of condensation. A solution equilibrium occurs when a solid substance is in a saturated solution. At this point, the rate of dissolution is equal to the rate of recrystallization. Although these are all different types of transformations, most of the rules regarding equilibrium apply to any situation in which a process occurs reversibly.
Read the material at ChemGuide. Skip to main content. Search for:. Chemical Equilibrium Learning Objectives Define chemical equilibrium. List conditions for equilibrium. Pull hard! Figure 2. Equilibrium between reactants and products. Summary The concept of chemical equilibrium is described. Conditions for chemical equilibrium are listed. Practice Read the material at ChemGuide. What is a dynamic system? What happens if you change the relative rates of the forward and back reactions?
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