What do equilibrium constant tell you

Oxygen moves to the cells attached to hemoglobin, a protein found in the red cells. In cases of carbon monoxide poisoning, CO binds much more strongly to the hemoglobin, blocking oxygen attachment and lowering the amount of oxygen reaching the 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:.

Consider the hypothetical reversible reaction in which reactants A and B react to form products C and D. This equilibrium can be shown below, where the lower case 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 concentrations for any given reaction at equilibrium maintains a constant value. Since we know that the equilibrium constant is 1. Now, solving for x won't be a lot of fun because we'll need to use the quadratic equation.

Let's face it, we don't really want to memorize the quadratic equation, much less use it. Fortunately, there's a shortcut we can use to get around using the quadratic equation when the K eq values are very small. If K eq is small, then we can safely guess that the amount of product formed x will be very, very small when compared to the initial quantity of the reactant.

This is because a very small K eq value means that very little product has been formed. As a result, we can omit the "x" in the denominator to simplify the [1. Our new and easier expression to solve is transformed into:. Let's see if this was a good assumption.

If the amount of product formed was 1. As a result, our assumption was valid and we saved ourselves a lot of mathematical heartache and toil. I love it when I save myself toil! All rights reserved including the right of reproduction in whole or in part in any form. To order this book direct from the publisher, visit the Penguin USA website or call This equation is a mathematical statement of the law of mass action : When a reaction has attained equilibrium at a given temperature, the reaction quotient for the reaction always has the same value.

Evaluating a Reaction Quotient Gaseous nitrogen dioxide forms dinitrogen tetroxide according to this equation:. When 0. The equilibrium constant is 1. However, it is common practice to omit units for K c values computed as described here, since it is the magnitude of an equilibrium constant that relays useful information.

As will be discussed later in this module, the rigorous approach to computing equilibrium constants uses dimensionless quantities derived from concentrations instead of actual concentrations, and so K c values are truly unitless. What is the value of the equilibrium constant, K c?

The magnitude of an equilibrium constant is a measure of the yield of a reaction when it reaches equilibrium. A large value for K c indicates that equilibrium is attained only after the reactants have been largely converted into products. A small value of K c —much less than 1—indicates that equilibrium is attained when only a small proportion of the reactants have been converted into products. Once a value of K c is known for a reaction, it can be used to predict directional shifts when compared to the value of Q c.

A system that is not at equilibrium will proceed in the direction that establishes equilibrium. The data in Figure 2 illustrate this. This value is 0.

It is important to recognize that an equilibrium can be established starting either from reactants or from products, or from a mixture of both. For example, equilibrium was established from Mixture 2 in Figure 2 when the products of the reaction were heated in a closed container. In fact, one technique used to determine whether a reaction is truly at equilibrium is to approach equilibrium starting with reactants in one experiment and starting with products in another.

If the same value of the reaction quotient is observed when the concentrations stop changing in both experiments, then we may be certain that the system has reached equilibrium. Predicting the Direction of Reaction Given here are the starting concentrations of reactants and products for three experiments involving this reaction:.

Determine in which direction the reaction proceeds as it goes to equilibrium in each of the three experiments shown. Check Your Learning Calculate the reaction quotient and determine the direction in which each of the following reactions will proceed to reach equilibrium.

In Example 2 , it was mentioned that the common practice is to omit units when evaluating reaction quotients and equilibrium constants. It should be pointed out that using concentrations in these computations is a convenient but simplified approach that sometimes leads to results that seemingly conflict with the law of mass action.

For example, equilibria involving aqueous ions often exhibit equilibrium constants that vary quite significantly are not constant at high solution concentrations. This may be avoided by computing K c values using the activities of the reactants and products in the equilibrium system instead of their concentrations. The activity of a substance is a measure of its effective concentration under specified conditions.

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.