On the other hand, if the forward reaction is slow and the reverse reaction is fast, the equilibrium favors the reactants. It probably won't be a big surprise to you to find that scientists have figured out a way to quantify the position of an equilibrium.
This was done by the "law of mass action. What the law of mass action says is simple. Let's say that we have a reaction taking place in solution with the following equation:.
We can relate the equilibrium constant of a chemical equilibrium to the rates of the forward and reverse reactions. For example, consider the process A? 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:. Our next step is to figure out what the concentrations of each species will be at equilibrium. We do this by setting up a chart that shows the initial concentrations of all species, how the concentrations of each will change, and what the final concentrations of each species will be.
For this process, the chart is given next don't panic, we'll explain how we got all these values in a minute :. The equations to represent this are always written with the metal ion and ligand on the reactant side and the complex on the product side, as shown below.
The equilibrium constant expressions are shown below, and these are known as formation constants K f. Since all soluble forms of lead are toxic, this increase in lead concentration is a potential problem. We can now couple these reactions into our scheme that describes the solubility of lead phosphate in this solution.
This is now quite a complicated set of simultaneous reactions that take place. Our goal in the equilibrium unit of this course will be to develop the facility to handle these types of complicated problems. Before we get started into this process, there are a couple of other general things to know about chemical equilibrium.
Consider the general reaction shown below. One way of describing equilibrium is to say that the concentrations do not change. The concentrations of the species in this solution represent a macroscopic parameter of the system, and so at the macroscopic level , this system is static. Another way of describing equilibrium is to say that for every forward reaction there is a corresponding reverse reaction. This means at the microscopic level that As and Bs are constantly converting to Cs and Ds and vice versa, but that the rate of these two processes are equal.
At the microscopic level , a system at equilibrium is dynamic. Unless you have taken physical chemistry, I am fairly certain that everything you have learned until this point has taught you that the following expression can be used to describe the equilibrium state of this reaction.
Well it turns out that this expression is not rigorously correct. Instead of the concentrations of reagents, the actual terms we need in an equilibrium constant expression are the activities of the substances. The expression shown below is the correct form of the equilibrium constant, in which a A represents the activity of substance A. For that A species to react with a B, another A species must move out of the way.
If the correct form of the equilibrium constant expression uses the activities of the chemicals, why have you always been taught to use concentrations? It turns out that in most situations we do not have reliable procedures to accurately calculate the activities of substances. If we did, we would almost certainly use the correct form of the expression. Since we do not know how to evaluate the activities of substances under most circumstances, we do the next best thing and use concentrations as an approximation.
This means that all equilibrium calculations are at best approximations some better than others. In other words, equilibrium calculations usually provide estimations of the situation, but not rigorously correct answers. Because the entire premise is based on an approximation, this will often allow us to make other approximations when we perform equilibrium calculations. These approximations will usually involve ignoring the contributions of minor constituents of the solution.
One last thing we ought to consider is when the approximation of using concentration instead of activity is most valid. Perhaps a way to see this is to consider a solution that has lots of A the concentration of A is high and only a small amount of B the concentration of B is low.
Inactivity results if a similar species is in the way of the two reactants getting together. Since the concentration of B is low, there is very little probability that one B would get in the way of another and prevent it from encountering an A.
Concentration is a better approximation of activity at low concentrations. The example I have shown with A and B implies there is no solvent, but this trend holds as well if the substances are dissolved in a solvent. Notice as well that the activity can never be higher than the concentration, but only lower. How low a concentration do we need to feel fully comfortable in using the approximation of concentrations for activities?
A general rule of thumb is if the concentrations are less than 0.
0コメント