What is equal at equilibrium

This article explains how to write equilibrium constant expressions, and introduces the calculations involved with both the concentration and the partial pressure equilibrium constant. A homogeneous reaction is one where the states of matter of the products and reactions are all the same the word "homo" means "same". In most cases, the solvent determines the state of matter for the overall reaction.

For example, the synthesis of methanol from a carbon monoxide-hydrogen mixture is a gaseous homogeneous mixture, which contains two or more substances:. At equilibrium, the rate of the forward and reverse reaction are equal, which is demonstrated by the arrows. The equilibrium constant, however, gives the ratio of the units pressure or concentration of the products to the reactants when the reaction is at equilibrium.

The synthesis of ammonia is another example of a gaseous homogeneous mixture:. A heterogeneous reaction is one in which one or more states within the reaction differ the Greek word "heteros" means "different". For example, the formation of an aqueous solution of lead II iodide creates a heterogeneous mixture dealing with particles in both the solid and aqueous states:.

The decomposition of sodium hydrogen carbonate baking soda at high elevations is another example of a heterogeneous mixture, this reaction deals with molecules in both the solid and gaseous states:.

This difference between homogeneous and heterogeneous reactions is emphasized so that students remember that solids, pure liquids, and solvents are treated differently than gases and solutes when approximating the activities of the substances in equilibrium constant expressions. The numerical value of an equilibrium constant is obtained by letting a single reaction proceed to equilibrium and then measuring the concentrations of each substance involved in that reaction.

The ratio of the product concentrations to reactant concentrations is calculated. Because the concentrations are measured at equilibrium, the equilibrium constant remains the same for a given reaction independent of initial concentrations.

This knowledge allowed scientists to derive a model expression that can serve as a "template" for any reaction. This basic "template" form of an equilibrium constant expression is examined here. The thermodynamically correct equilibrium constant expression relates the activities of all of the species present in the reaction.

Although the concept of activity is too advanced for a typical General Chemistry course, it is essential that the explanation of the derivation of the equilibrium constant expression starts with activities so that no misconceptions occur. 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. Red blood cells transport oxygen to the tissues so they can function. In the absence of oxygen, cells cannot carry out their biochemical responsibilities. Oxygen moves to the cells attached to hemoglobin, a protein found in the red cells.

Treatment involves the patient breathing pure oxygen to displace the carbon monoxide. The equilibrium reaction shown below illustrates the shift toward the right when excess oxygen is added to the system:. This equilibrium can be shown below, where the lowercase letters represent the coefficients of each substance. 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.

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.

As such, they are omitted. Note that because it is a solid, the activity of C s is 1, and it is omitted from the final K expression. When looking at the K eq expression, we should notice that it is essentially a ratio relating the concentrations of products to the concentrations of reactants at equilibrium. If we know the value of K eq , we can draw some conclusions about the thermodynamics of the forward and reverse reactions.

These conclusions are summarized as follows:. Privacy Policy. Skip to main content. Chemical Equilibrium. Search for:. Equilibrium Equilibrium Chemical equilibrium is the state in which the forward reaction rate and the reverse reaction rate are equal.

Learning Objectives Recall the relationship between the forward and reverse reaction rates when a reaction is at equilibrium. Key Takeaways Key Points In a chemical equilibrium, the forward and reverse reactions occur at equal rates, and the concentrations of products and reactants remain constant. A catalyst speeds up the rate of a chemical reaction, but has no effect upon the equilibrium position for that reaction. Key Terms chemical equilibrium : In a chemical reaction, the state in which both reactants and products are present at concentrations that have no further tendency to change with time.