What do equilibrium constant tell us

Chem1 Virtual Textbook. Learning Objectives How to figure out in which direction a reaction will go i. This may involve knowing equilibrium values for some of the reactants and products and determining the concentration of an unknown. Alternatively, we may be provided with the starting concentrations of reactants and products and may be asked to find the equilibrium concentrations. Kinetically Hindered Reactions Although it is by no means a general rule, it frequently happens that reactions having very large equilibrium constants are kinetically hindered, often to the extent that the reaction essentially does not take place.

Do Equilibrium Constants have Units? Strictly speaking, equilibrium expressions do not have units. These two very different values of K illustrate very nicely why reducing combustion-chamber temperatures in automobile engines is environmentally friendly. 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.

The rate of the forward reaction, A? B, is k f [A], where the little "f" after the k denotes the rate constant for the forward reaction. Likewise, the rate of the reverse reaction, B? A, is k r [B], where "r" denotes the rate constant for the reverse process.

For gases, the equilibrium constant is determined in almost the same way, except that partial pressures are used in place of concentrations. The equilibrium constant is important because it gives us an idea of where the equilibrium lies.

The larger the equilibrium constant, the further the equilibrium lies toward the products. For example, an equilibrium constant of 1. K eq is a neat constant because it allows us to determine the ratios of the concentrations of each chemical species in the equilibrium, which is pretty handy for reasons we'll see in this example:. Write the expression for the equilibrium constant, and determine what the value of K eq is given these concentrations of each of the chemical species at equilibrium:.

To figure out the expression for the chemical equilibrium, use the equation we learned for finding K c. To determine the value of K eq , all we need to do is substitute the values given to us for the concentrations in the equilibrium expression.

As a result, we get:. Once we have an equilibrium constant, we can use it to figure out what the equilibrium concentrations of the products will be given an initial concentration of the reactants.

Let's see another example:. Example : Given the reaction A? Solution : Let's walk through this problem step by step. The first step is to write the equilibrium expression for this process:.