Equilibrium - High School Chemistry

The equilibrium constant is given by the concentraton of products over the concentration of reactants. If it lies to the right, it means that it favors the forward reaction, and thus the reaction is "more complete" or closer to completion.

This is one of the properties of the law of mass action. The equilbrium constant for a reaction written in reverse is 1/K.

The only factor that changes the equilibrium constant is temperature. Changes in concentration of reactants or products by any means (whether addition, taking away solvent, or adding a chemical that will cause side reactions) will remove the system from equilibrium, but will not change the equilibrium constant.

When writing the equilibrium expression for an insoluble salt, remember that pure solids and liquids are not included in the expression. Also, the coefficients for the compounds in the balanced reaction become the exponents for the compounds in the equilibrium expression.

Given a generalized chemical reaction, we can determine the equilibrium constant expression.

In our reaction, the reactant is a pure solid and is not included in the equilibrium calculation.

The equilibrium constant gives the ratio of product concentration to reactant concentration that results in equal forward and reverse rates of reaction. Essentially, the equilibrium constant defines the parameters of equilibrium for a given reaction. Altering the concentration of reactants or products will change the state of the reaction, driving it to produce more reactants or products in order to accommodate the change (Le Chatelier's principle), but will not alter the equilibrium parameters. Adding an enzyme will increase the rate of reaction by decreasing the activation energy, but will still not affect the equilibrium ratios.

The main factor that is capable of altering the equilibrium constant is change in temperature, which alters the amount of energy available to the system and affects the balance of equilibrium.

The general formula for the equilibrium constant of a reaction is:

For gaseous reactions, however, partial pressure can be substituted for concentration. For the given reaction, this means our equilibrium constant will be equal to the partial pressure of the products divided by the partial pressure of the reactants.

When writing the equilibrium expression for a reaction, remember that the products are on the top of the expression and the reactants are on the bottom. Any coefficients in the balanced reaction become the exponents in the equilibrium constant.

A general formula and corresponding equilibrium expression are given here:

Our equation has only one product, but follows the same format.

Note that pure solids and liquids are not included in the equilibrium expression, but pure gases (such as nitrogen) are. This is because the pressure of pure gases can affect equilibrium, but manipulating pure solids or liquids cannot.

When writing the equilibrium expression for a reaction, remember that the products are on the top of the expression and the reactants are on the bottom. Any coefficients in the balanced reaction become the exponents in the equilibrium constant.

A general formula and corresponding equilibrium expression are given here:

Our equation follows the same format. Note, however, that pure solids and liquids are not included in the equilibrium expression, but pure gases (such as hydrogen) are. This is because the pressure of pure gases can affect equilibrium, but manipulating pure solids or liquids cannot.

For any equilibrium expression, solids and liquids are excluded because their concentrations are presumed to be constant during the reaction. The equilibrium expression is defined as the ratio of the concentration of products divided by the concentration of the reactants. Each reactant or product is raised to the power corresponding to its coefficient in the balanced chemical equation.

Using an arbitrary example:

Compare this to our question. Remember to exclude the solid magnesium!

When creating the solubility product expression for a salt, keep in mind that salts are solids, and are therefore not included in the expression. Also, the coefficients in the balanced reaction become exponents in the expression.

For a general reaction, the solubility product constant would be given as follows:

Aluminum hydroxide will dissociate based on the following balanced reaction:

As a result, the solubility product expression is:

The equilibrium constant, , is the ratio of the concentration of products raised to their coefficients, over the concentration of reactants raised to their coefficients. To find its value, we first need to balance the equation and then consider only the products and reactants that actually have concentrations (i.e. aqueous and gaseous species). Liquids and solids can be omitted from the calculation.

We note that chlorine only appears once on the right, so we add a 2 coefficient to balance the equation:

Since is a solid (most hydroxides precipitate, unless they are paired with alkali metals, barium, or calcium) it will not be included in the equilibrium constant calculation.

The chloride ion concentration is raised to the second power because of the 2 coefficient that we added. So, putting products (species on the right side, other than the solid) on the top and reactants (species on the left) on bottom, we get:

The only factor that changes the equilibrium constant is temperature. Changes in concentration of reactants or products by any means (whether addition, taking away solvent, or adding a chemical that will cause side reactions) will remove the system from equilibrium, but will not change the equilibrium constant.

The only factor that changes the equilibrium constant is temperature. Changes in concentration of reactants or products by any means (whether addition, taking away solvent, or adding a chemical that will cause side reactions) will remove the system from equilibrium, but will not change the equilibrium constant.

The reaction quotient, or Q, of the above reaction is equal to the products over the reactants. Q is calculated in the same manner as Keq, but does not require that the reaction be at equilibrium.

By placing these initial concentrations in the equilibrium expression, we can compare the reaction quotient, , to the equilibrium constant, .

The reaction quotient for this reaction is:

Note that the reactant is a pure solid, and will not be included in this expression. By setting this equal to the equilibrium constant, we can see whether or is larger in value.

We can predict how a solution will change based on the value of . When is less than , the reaction will shift to the right. If is greater than , the reaction will shift to the left. If is equal to , the solution is at equilibrium.

Since the value for is greater than the equilibrium constant in this instance, the reaction will shift to the left.

Le Chatelier's principle states that changes in pressure are attributable to changes in volume. If we increase the volume, the reaction will shift toward the side that has more moles of gas. If we decrease the volume, the reaction will shift toward the side that has less moles of gas. Since the product side has only two moles of gas, compared to the reactant side with four moles, the reaction would shift toward the product side, and more NH3 would form.

Chemical equilibrium is defined as a state when the rates of the forward and reverse reactions are equal. Different reactions will have different concentrations of products are reactants, and concentration can vary depending on the process. Chemical equilibrium, however, is always the point at which there is no bias towards creating products or reactants.

If the forward reaction rate and reverse reaction rate are equal, then there is no net change in concentration of the reactants or products. This makes them appear to be stable.

When a reaction is at equilibrium, the concentrations of reactants and products are no longer increasing or decreasing; however, this does not mean that the forward and reverse reactions have stopped taking place. They are simply taking place at the same rate so that the concentrations of products and reactants do not change. Equilibrium is a dynamic process. Reactions are still taking place, but they offset one another in terms of rate. Essentially, every product that is created is immediately converted back to reactant by the reverse process, making the concentration of reactants and products appear constant.

Le Chatelier's principle states that a system will shift in a particular direction in order to reduce introduced stress to the system. In other words, if something is added to one side of the equation, the other side will consequently increase in order to restore equilibrium.

The above reaction is exothermic, meaning that heat is released from the system. As a result, heat is considered a product.

By removing nitrogen or hydrogen gas, the system would shift to the left in order to create more reactants. If temperature is decreased, the system will create more heat by shifting to the right. Not only will this create more heat, but it will also create more ammonia, as they are both products of the reaction.