Equilibrium Constant and Reaction Quotient - AP Chemistry

According to the law of mass action, the equilibrium constant expression can always be written given the equation of the reaction itself. The equilibrium-constant expression will be written as the products over the reactants, each raised to their respectively stoichiometric coefficient. The rate law and concentrations can only be determined if there is additional data given.

Equilibrium expressions are specific for certain temperatures. Physical state, pressure, and volume do not factor into the expression.

Given the equation delta G = -RTln(Keq), plugging in the correct values for the variables will lead us to the correct answer. Using e = 2.7, R = 8.314 J/molK, T= 298 K, and G = -4955 J, we can calculate that K = 7.4.

When K is greater than 1, the products are favored and the reaction proceeds forward spontaneously. This must be true of the reaction, since it spontaneously occurs when the metal comes into contact with the water.

When Q = K, the reaction has reached equilibrium and is in a state of dynamic equilibrium, where the forward and reverse reactions are occurring at the same rate. Since there is no net change in concentration, the free energy is zero.

First, consider the formula for the equilibrium constant of arbitrary reaction:

aA+bB leftrightharpoons cC+dD

The formula for the equilibrium constant for this reaction is:

$

This is one of the properties of the law of mass action—the concentrations of pure solids and liquids don't change over the course of the reaction.

All of the statements are properties of the law of mass action, except for the answer, which is a statement of Le Chatelier's Principle.

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.

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.

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

A key point to remember is that the equilibrium constant expression only includes compounds that are in aqueous solution or in the gaseous phase. Pure solids and liquids are omitted from the expression. As a result, solid barium sulfate will not be included in the expression.

The equilibrium constant is found by the concentration of the products over the concentration of the reactants, each raised to the power of their coefficients.

In our reaction, the only reactant is a pure solid, so only the products are used in the expression.

Solids are not included in any equilibrium expression. The equilibrium expression will contain only aqueous solutions and gases. In addition, the coefficients of a balanced chemical equation correspond to exponents, not coefficients, in the equilibrium expression.

For this reaction, the only relevant product is oxygen gas and the only relevant reactant is fluorine gas.

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!

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:

We can set up the equilibrium expression by placing the products, raised to the power of their coefficients, in the numerator and reactants, raised to the power of their coefficients, in the denominator:

We are given the value of the equilibrium constant and the equilibrium concentration of . Using stoichiometric coefficients, we can determine that a variable concentration, , of will be present at equilibrium, and of will be present at equilibrium. Plugging in:

Simplify:

Now we can solve for (which is the equilibrium concentration of , as we assigned) by dividing both sides by 4 and then taking the cube root.

Rounding to 2 sig figs (to match the two numbers we were given by the problem), we get:

The correct answer gives the accurate definition of both the equilibrium constant and the reaction quotient.