# How to solve equilibrium constant problems

Chapter 15.3: Solving Equilibrium Problems

Strategy: Construct a table and enter the initial partial pressures, the changes in the partial pressures that occur during the Write the equilibrium equation for the reaction. Then substitute values from the table to solve for the change in Calculate the partial pressure of NO. Check your. Techniques for Solving Equilibrium Problems. If Possible, Take the Square Root of Both Sides; Using the Quadratic Equation; Solving Equations Containing x 3, x 4, etc. The Method of Successive Approximations; Assume That the Change is Small; If Possible, Take the Square Root of Both Sides.

There are two fundamental kinds of equilibrium problems: 1 those in which we are given the concentrations of the reactants and the products at equilibrium or, more often, information that allows us to calculate these concentrationsand we are asked to calculate the equilibrium constant for the reaction; and 2 those in which we are given the equilibrium constant and the initial concentrations of reactants, and we are asked to calculate the concentration of one or more substances at equilibrium.

In this section, we describe methods for solving both kinds of problems. We saw in the exercise in Example 6 in Section Remember that equilibrium constants are unitless.

A more complex example of this type of problem is the conversion of n -butane, an additive used to increase how to set combination lock volatility of gasoline, to isobutane 2-methylpropane. This reaction can be written as follows:. At equilibrium, a mixture of n -butane and isobutane at room temperature was found to contain 0. Substituting these concentrations into the equilibrium constant expression.

Thus the equilibrium constant for the reaction as written is 2. The reaction between gaseous sulfur dioxide and oxygen is a key step in the industrial synthesis of sulfuric acid:.

A mixture of SO 2 and O 2 was maintained at K until the system reached equilibrium. The equilibrium mixture contained 5. Calculate K and K p at this temperature. Given: balanced equilibrium equation and composition of equilibrium mixture. Asked for: equilibrium constant. Write the equilibrium constant expression for the reaction. Then substitute the appropriate equilibrium concentrations into this equation to obtain K.

Substituting the appropriate equilibrium concentrations into the equilibrium **how to solve equilibrium constant problems** expression. To solve for K pwe use Equation Hydrogen gas and iodine react to form hydrogen iodide via the reaction.

A mixture of H 2 and I 2 was maintained at K until the system reached equilibrium. The equilibrium mixture contained 1. Calculate K and K p for this reaction. Chemists are not often given the concentrations of all the substances, and they are not likely to measure the equilibrium concentrations of all the relevant substances for a particular system.

In such cases, we can obtain the equilibrium concentrations from the initial concentrations of the reactants and the balanced chemical equation for the reaction, as long as the equilibrium concentration of one of the substances is known. Example 9 shows one way to do this. The contents of the reactor were then analyzed and found to contain 0. Calculate K at this temperature.

Given: balanced equilibrium equation, amount of reactant, volume, and amount of one product at equilibrium. Asked for: K. A Write the equilibrium constant expression for the reaction.

Construct a table showing the initial concentrations, the changes in concentrations, and the final concentrations as initial concentrations plus changes in concentrations.

B How to draw crying face all possible initial concentrations from the data given and insert them in the table.

C Use the coefficients in the balanced chemical equation to obtain the changes in concentration of all other substances in the reaction. Insert those concentration changes in the table. D Obtain the final concentrations by summing the columns. Calculate the equilibrium constant for the reaction. A The first step in any such problem is to balance the chemical equation for the reaction if it is not already balanced and use it to derive the equilibrium constant expression.

In this what is a pbf file, the equation is already balanced, and the equilibrium constant expression is as follows:.

To obtain the concentrations of NOCl, NO, and Cl 2 at equilibrium, we construct a table showing what is known and what needs to be calculated. We begin by writing the balanced chemical equation at the top of the table, followed by three lines corresponding to the initial concentrations, the changes in concentrations required to get from the initial to the final state, and the final concentrations.

B Initially, the system contains 1. The initial concentrations of NO and Cl 2 are 0 M because initially no products are present. Moreover, we are told that at equilibrium the system contains 0. We insert these values into the following table:.

C We use the stoichiometric relationships given in the balanced chemical equation to find the change in the concentration of Cl 2the substance for which initial and final concentrations are known:. According to the coefficients in the balanced chemical equation, 2 mol of NO are produced for every 1 mol of Cl 2so the change in the NO concentration is as follows:. We can now calculate the equilibrium constant for the reaction:. At equilibrium, the mixture contained 0.

What is K p? The original laboratory apparatus designed by Fritz Haber and Robert Le Rossignol in for synthesizing ammonia from its elements. A metal catalyst bed, where ammonia was produced, is in the large cylinder at the left.

The Haber-Bosch process used for the industrial production of ammonia uses essentially the same process and components but on a much larger scale. To describe how to calculate equilibrium concentrations from an equilibrium constant, we first consider a system that contains only a single product and a single reactant, the conversion of n -butane to isobutane Equation If we begin with a 1.

We need to calculate the equilibrium concentrations of both n -butane and isobutane. Because it is generally difficult to calculate final concentrations directly, we focus on the change in the concentrations of the substances between the initial and the final equilibrium conditions.

This is because the balanced chemical equation for the reaction tells us that 1 mol of n -butane is consumed for every 1 mol of isobutane produced. We can then express the final concentrations in terms of the initial concentrations and the changes they have undergone. Substituting the expressions for the final concentrations of n -butane and isobutane from the table into the equilibrium equation.

Rearranging and solving for x. We obtain the final concentrations by substituting this x value into the expressions for the final concentrations of n -butane and isobutane listed in the table:.

We can check the results by substituting them back into the equilibrium constant expression to see whether they give the same K that we used in the calculation:.

This is the same K we were what old pennies are worth the most money, so we can be confident of our results.

Example 10 illustrates a common type of equilibrium problem that you are likely to encounter. The water—gas shift reaction is important in several chemical processes, such as the production of H 2 for fuel cells. If a mixture of gases that initially contains 0. Given: balanced equilibrium equation, Kand initial concentrations. Asked for: final concentrations. A Construct a table showing what is known and what needs to be calculated.

Define x as the change in the concentration of one substance. Then use the reaction stoichiometry to express what to feed a 9 month old baby changes in the concentrations of the other substances in terms of x. From the values in the table, calculate the final concentrations. B Write the equilibrium equation for the reaction. Substitute appropriate values from the table to obtain *how to solve equilibrium constant problems.* C Calculate the how to not psych yourself out concentrations of all species present.

Check your answers by substituting these values into the equilibrium constant expression to obtain K. Just as before, we will focus on the life is what you make of it in the concentrations of the various substances between the initial and final states.

We can use the stoichiometry of the reaction to express the changes in the concentrations of the other substances in terms of x. We enter the values in the following table and what is the us national debt as of today the final concentrations.

B We can now use the equilibrium equation and the given K to solve for x :. We could solve this equation with the quadratic formula, but it is far easier to solve for x by recognizing that the left side of the equation is a perfect square; that is. The quadratic formula is presented in Essential Skills 7 in Section Taking the square root of the middle and right terms.

C The final concentrations of all species in the reaction mixture are as follows:. We can check our work by inserting the calculated values back into the equilibrium constant expression:. To two significant figures, this K is the same as the value given in the problem, so our answer is confirmed. Hydrogen gas reacts with iodine vapor to give hydrogen iodide according to the following chemical equation:. In Example 10, the initial concentrations of the reactants were the same, which gave us an equation that how to polish abalone shells a perfect square and simplified our calculations.

Under these conditions, there marco biondi this is what you are usually no way to simplify the problem, and we must determine the equilibrium concentrations with other means. Such a case is described in Example In the water—gas shift reaction shown in Example 10, a sample containing 0. What is the composition of the reaction mixture at equilibrium?

Given: balanced equilibrium equation, concentrations of reactants, and K. Asked for: composition of reaction mixture at equilibrium. A Write the equilibrium equation. Construct a table showing the initial concentrations of all substances in the mixture. Complete the table showing the changes in the concentrations x and the final concentrations.

B Write the equilibrium constant expression for the reaction.

Calculating an Equilibrium Constant from Equilibrium Concentrations

Jan 02, · The equilibrium constant (K) for the chemical equation aA + bB - cC + dD can be expressed by the concentrations of A,B,C and D at equilibrium by the equation K = [C] c [D] d /[A] a [B] b For this equation, there is no dD so it is left out of the equation. K = [C] c /[A] a [B] b Substitute for this reaction K = [HI] 2 /[H 2][I 2] K = ( M) 2 /( M)( M). technique for solving equilibrium problems One thing that sometimes makes equilibrium problems difficult is solving the equilibrium constant equation for ?. However, there are several different techniques we can use: 1. Assume that the change is small compared with the initial concentrations, and neglect ? when it is added to or subtracted from an initial concentration. Aug 27, · The equilibrium constant K is the ratio of products to reactants. If K is a very small number, you would expect there to be more reactants than products. In this case, K = x 10 -4 is a small number. In fact, the ratio indicates there are times more reactants than products.

The equilibrium constant is a powerful tool for predicting how a chemical reaction will behave. We have already seen how to calculate the equilibrium constant from the concentrations of the reactants and products at equilibrium or, more often, information that allows us to calculate these concentrations , and how to predict the direction a reaction will proceed to reach equilibrium from concentrations of the reactants and products at any given moment.

But what if the equilibrium constant is known, but the concentrations of reactants and products are not? In this section, we describe methods for solving problems in which we are given the equilibrium constant and the initial concentrations of reactants, and we are asked to calculate the concentration of one or more substances at equilibrium.

If we begin with a 1. We need to calculate the equilibrium concentrations of both n-butane and isobutane. Because it is generally difficult to calculate final concentrations directly, we focus on the change in the concentrations of the substances between the initial and the final equilibrium conditions. This is because the balanced chemical equation for the reaction tells us that 1 mol of n-butane is consumed for every 1 mol of isobutane produced.

We can then express the final concentrations in terms of the initial concentrations and the changes they have undergone. Substituting the expressions for the final concentrations of n-butane and isobutane from the table into the equilibrium equation,. The water—gas shift reaction is important in several chemical processes, such as the production of H 2 for fuel cells. This reaction can be written as follows:.

If a mixture of gases that initially contains 0. Just as before, we will focus on the change in the concentrations of the various substances between the initial and final states.

We enter the values in the following table and calculate the final concentrations. C The final concentrations of all species in the reaction mixture are as follows:. We can check our work by inserting the calculated values back into the equilibrium constant expression:. To two significant figures, this K is the same as the value given in the problem, so our answer is confirmed. Hydrogen gas reacts with iodine vapor to give hydrogen iodide according to the following chemical equation:.

Under these conditions, there is usually no way to simplify the problem, and we must determine the equilibrium concentrations with other means. What is the composition of the reaction mixture at equilibrium? Asked for : composition of reaction mixture at equilibrium. The final concentrations are the sums of the initial concentrations and the changes in concentrations at equilibrium.

Thus we must expand the expression and multiply both sides by the denominator:. We can check our work by substituting these values into the equilibrium constant expression:. If a sample containing 0. In many situations it is not necessary to solve a quadratic or higher-order equation. In these cases, we can make judicious assumptions about the reaction that allow us to take "shortcuts" with our calculations. These shortcuts introduce a small amount of error into the calculation, but if the error is small enough, this is acceptable.

One shortcut situation is when the equilibrium constant is very small and the starting reactant concentration is large. A Because we are given Kp and partial pressures are reported in atmospheres, we will use partial pressures. Each substance has a final partial pressure equal to the sum of the initial pressure and the change in that pressure at equilibrium. B Substituting these values into the equation for the equilibrium constant,.

In principle, we could multiply out the terms in the denominator, rearrange, and solve the resulting quadratic equation. Substituting these expressions into our original equation,. When can we make such an assumption? Otherwise, we must use the quadratic formula or some other approach.

We can verify our results by substituting them into the original equilibrium equation:. Under certain conditions, oxygen will react to form ozone, as shown in the following equation:. When we solve this type of problem, we view the system as equilibrating from the products side of the reaction rather than the reactants side.

If a mixture of 0. A From the magnitude of the equilibrium constant, we see that the reaction goes essentially to completion. Because the initial concentration of ethylene 0. If the reaction were to go to completion, the concentration of ethane would be 0. Because the concentration of hydrogen is greater than what is needed for complete reaction, the concentration of unreacted hydrogen in the reaction mixture would be 0.

The equilibrium constant for the forward reaction is very large, so the equilibrium constant for the reverse reaction must be very small. The final equilibrium concentrations are the sums of the concentrations for the forward and reverse reactions.

B Substituting values into the equilibrium constant expression,. Once again, the magnitude of the equilibrium constant tells us that the equilibrium will lie far to the right as written, so the reverse reaction is negligible. We can verify our calculations by substituting the final concentrations into the equilibrium constant expression:.

Equilibrium constants can be used to calculate the equilibrium concentrations of reactants and products by using the quantities or concentrations of the reactants, the stoichiometry of the balanced chemical equation for the reaction, and a tabular format to obtain the final concentrations of all species at equilibrium. The calculation of final concentrations generally requires solving a quadratic equation.

In certain situations, it is possible to simplify the mathematics of the calculation by judicious assumptions about the extent of reaction. Skills to Develop To solve quantitative problems involving chemical equilibriums. From the values in the table, calculate the final concentrations. Write the equilibrium equation for the reaction. Calculate the final concentrations of all species present. Construct a table showing the initial concentrations of all substances in the mixture.

Write the equilibrium constant expression for the reaction. Calculate the final concentration of each substance in the reaction mixture. Calculation Shortcuts In many situations it is not necessary to solve a quadratic or higher-order equation.

Calculate the equilibrium concentrations. Check your answers by substituting these values into the equilibrium equation. Solution : A From the magnitude of the equilibrium constant, we see that the reaction goes essentially to completion.

Summary Equilibrium constants can be used to calculate the equilibrium concentrations of reactants and products by using the quantities or concentrations of the reactants, the stoichiometry of the balanced chemical equation for the reaction, and a tabular format to obtain the final concentrations of all species at equilibrium.

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