17 Chemical Equilibrium

Chapter Goals Basic Concepts The Equilibrium Constant Variation of Kc with the Form of the Balanced Equation The Reaction Quotient Uses of the Equilibrium Constant, Kc Disturbing a System at Equilibrium: Predictions The Haber Process: A Commercial Application of Equilibrium

Basic Concepts Reversible reactions do not go to completion. They can occur in either direction Symbolically, this is represented as:

Basic Concepts Chemical equilibrium exists when two opposing reactions occur simultaneously at the same rate. A chemical equilibrium is a reversible reaction - the forward reaction rate is equal to the reverse reaction rate. Chemical equilibria are dynamic equilibria. Molecules are continually reacting, even though the overall composition of the reaction mixture does not change.

Basic Concepts One example of a dynamic equilibrium can be shown using radioactive 131I as a tracer in a saturated PbI2 solution.

Basic Concepts Graphically, this is a representation of the rates for the forward and reverse reactions for this general reaction.

Basic Concepts One of the fundamental ideas of chemical equilibrium is that equilibrium can be established from either the forward or reverse direction.

Basic Concepts

Basic Concepts

The Equilibrium Constant For a simple one-step mechanism reversible reaction such as: The rates of the forward and reverse reactions can be represented as:

The Equilibrium Constant When system is at equilibrium: Ratef = Rater

The Equilibrium Constant Because the ratio of two constants is a constant we can define a new constant as follows :

The Equilibrium Constant Similarly, for the general reaction: we can define a constant

The Equilibrium Constant Kc is the equilibrium constant. Kc is defined for a reversible reaction at a given temperature as the product of the equilibrium concentrations (in M) of the products, each raised to a power equal to its stoichiometric coefficient in the balanced equation, divided by the product of the equilibrium concentrations (in M) of the reactants, each raised to a power equal to its stoichiometric coefficient in the balanced equation.

The Equilibrium Constant Example 17-1: Write equilibrium constant expressions for the following reactions at 500oC. All reactants and products are gases at 500oC.

The Equilibrium Constant

The Equilibrium Constant

The Equilibrium Constant Equilibrium constants are dimensionless because they actually involve a thermodynamic quantity called activity. Activities are directly related to molarity

The Equilibrium Constant Example 17-2: One liter of equilibrium mixture from the following system at a high temperature was found to contain 0.172 mole of phosphorus trichloride, 0.086 mole of chlorine, and 0.028 mole of phosphorus pentachloride. Calculate Kc for the reaction. Equil []’s 0.028 M 0.172 M 0.086 M You do it!

The Equilibrium Constant Example 17-3: The decomposition of PCl5 was studied at another temperature. One mole of PCl5 was introduced into an evacuated 1.00 liter container. The system was allowed to reach equilibrium at the new temperature. At equilibrium 0.60 mole of PCl3 was present in the container. Calculate the equilibrium constant at this temperature.

The Equilibrium Constant Example 17-4: At a given temperature 0.80 mole of N2 and 0.90 mole of H2 were placed in an evacuated 1.00-liter container. At equilibrium 0.20 mole of NH3 was present. Calculate Kc for the reaction. You do it!

Variation of Kc with the Form of the Balanced Equation The value of Kc depends upon how the balanced equation is written. From example 17-2 we have this reaction: This reaction has a Kc=[PCl3][Cl2]/[PCl5]=0.53

Variation of Kc with the Form of the Balanced Equation Example 17-5: Calculate the equilibrium constant for the reverse reaction by two methods, i.e, the equilibrium constant for this reaction. Equil. []’s 0.172 M 0.086 M 0.028 M The concentrations are from Example 17-2.

The Reaction Quotient The mass action expression or reaction quotient has the symbol Q. Q has the same form as Kc The major difference between Q and Kc is that the concentrations used in Q are not necessarily equilibrium values.

The Reaction Quotient Why do we need another “equilibrium constant” that does not use equilibrium concentrations? Q will help us predict how the equilibrium will respond to an applied stress. To make this prediction we compare Q with Kc.

The Reaction Quotient

The Reaction Quotient Example 17-6: The equilibrium constant for the following reaction is 49 at 450oC. If 0.22 mole of I2, 0.22 mole of H2, and 0.66 mole of HI were put into an evacuated 1.00-liter container, would the system be at equilibrium? If not, what must occur to establish equilibrium?

Uses of the Equilibrium Constant, Kc Example 17-7: The equilibrium constant, Kc, is 3.00 for the following reaction at a given temperature. If 1.00 mole of SO2 and 1.00 mole of NO2 are put into an evacuated 2.00 L container and allowed to reach equilibrium, what will be the concentration of each compound at equilibrium?

Uses of the Equilibrium Constant, Kc Example 17-8: The equilibrium constant is 49 for the following reaction at 450oC. If 1.00 mole of HI is put into an evacuated 1.00-liter container and allowed to reach equilibrium, what will be the equilibrium concentration of each substance?

Disturbing a System at Equilibrium : Predictions LeChatelier’s Principle - If a change of conditions (stress) is applied to a system in equilibrium, the system responds in the way that best tends to reduce the stress in reaching a new state of equilibrium. We first encountered LeChatelier’s Principle in Chapter 14. Some possible stresses to a system at equilibrium are: Changes in concentration of reactants or products. Changes in pressure or volume (for gaseous reactions) Changes in temperature.

Disturbing a System at Equilibrium : Predictions Changes in Concentration of Reactants and/or Products Also true for changes in pressure for reactions involving gases. Look at the following system at equilibrium at 450oC.

Disturbing a System at Equilibrium : Predictions Changes in Concentration of Reactants and/or Products Also true for changes in pressure for reactions involving gases. Look at the following system at equilibrium at 450oC.

Disturbing a System at Equilibrium : Predictions Changes in Concentration of Reactants and/or Products Also true for changes in pressure for reactions involving gases. Look at the following system at equilibrium at 450oC.

Disturbing a System at Equilibrium : Predictions Changes in Volume (and pressure for reactions involving gases) Predict what will happen if the volume of this system at equilibrium is changed by changing the pressure at constant temperature:

Disturbing a System at Equilibrium : Predictions

Disturbing a System at Equilibrium : Predictions

Disturbing a System at Equilibrium : Predictions Changing the Temperature

Disturbing a System at Equilibrium : Predictions Changing the Reaction Temperature Consider the following reaction at equilibrium:

Disturbing a System at Equilibrium : Predictions Introduction of a Catalyst Catalysts decrease the activation energy of both the forward and reverse reaction equally. Catalysts do not affect the position of equilibrium. The concentrations of the products and reactants will be the same whether a catalyst is introduced or not. Equilibrium will be established faster with a catalyst.

Disturbing a System at Equilibrium : Predictions Example 17-9: Given the reaction below at equilibrium in a closed container at 500oC. How would the equilibrium be influenced by the following?

Disturbing a System at Equilibrium : Predictions Example 17-10: How will an increase in pressure (caused by decreasing the volume) affect the equilibrium in each of the following reactions?

Disturbing a System at Equilibrium : Predictions Example 17-11: How will an increase in temperature affect each of the following reactions?

The Haber Process: A Practical Application of Equilibrium The Haber process is used for the commercial production of ammonia. This is an enormous industrial process in the US and many other countries. Ammonia is the starting material for fertilizer production. Look at Example 17-9. What conditions did we predict would be most favorable for the production of ammonia?

The Haber Process: A Practical Application of Equilibrium

The Haber Process: A Practical Application of Equilibrium

The Haber Process: A Practical Application of Equilibrium This diagram illustrates the commercial system devised for the Haber process.

Disturbing a System at Equilibrium: Calculations To help with the calculations, we must determine the direction that the equilibrium will shift by comparing Q with Kc. Example 17-12: An equilibrium mixture from the following reaction was found to contain 0.20 mol/L of A, 0.30 mol/L of B, and 0.30 mol/L of C. What is the value of Kc for this reaction?

Disturbing a System at Equilibrium: Calculations

Disturbing a System at Equilibrium: Calculations If the volume of the reaction vessel were suddenly doubled while the temperature remained constant, what would be the new equilibrium concentrations? Calculate Q, after the volume has been doubled

Disturbing a System at Equilibrium: Calculations Since Q<Kc the reaction will shift to the right to re-establish the equilibrium. Use algebra to represent the new concentrations.

Disturbing a System at Equilibrium: Calculations Since Q<Kc the reaction will shift to the right to re-establish the equilibrium. Use algebra to represent the new concentrations.

Disturbing a System at Equilibrium: Calculations Since Q<Kc the reaction will shift to the right to re-establish the equilibrium. Use algebra to represent the new concentrations.

Disturbing a System at Equilibrium: Calculations Since Q<Kc the reaction will shift to the right to re-establish the equilibrium. Use algebra to represent the new concentrations.

Disturbing a System at Equilibrium: Calculations

Disturbing a System at Equilibrium: Calculations

Disturbing a System at Equilibrium: Calculations

Disturbing a System at Equilibrium: Calculations Example 17-13: Refer to example 17-12. If the initial volume of the reaction vessel were halved, while the temperature remains constant, what will the new equilibrium concentrations be? Recall that the original concentrations were: [A]=0.20 M, [B]=0.30 M, and [C]=0.30 M. You do it!

Disturbing a System at Equilibrium: Calculations Example 17-14: A 2.00 liter vessel in which the following system is in equilibrium contains 1.20 moles of COCl2, 0.60 moles of CO and 0.20 mole of Cl2. Calculate the equilibrium constant. You do it!

Disturbing a System at Equilibrium: Calculations An additional 0.80 mole of Cl2 is added to the vessel at the same temperature. Calculate the molar concentrations of CO, Cl2, and COCl2 when the new equilibrium is established. You do it!

Group Question If you are having trouble getting a fire started in the barbecue grill, a common response is to blow on the coals until the fire begins to burn better. However, this has the side effect of dizziness. This is because you have disturbed an equilibrium in your body. What equilibrium have you affected?