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CHAPTER 15. Chemical Equilibrium. Chapter Goals. Basic Concepts The Equilibrium Constant Variation of K c with the Form of the Balanced Equation The Reaction Quotient Uses of the Equilibrium Constant, K c Disturbing a System at Equilibrium: Predictions
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CHAPTER 15 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 Practical Application of Equilibrium
Chapter Goals • Disturbing a System at Equilibrium: Calculations • Partial Pressures and the Equilibrium Constant • Relationship between Kp and Kc • Heterogeneous Equilibria • Relationship between Gorxn and the Equilibrium Constant • Evaluation of Equilibrium Constants at Different Temperatures
Basic Concepts • Reversiblereactions 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 that 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 • This movie depicts a dynamic equilibrium.
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.
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
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 :
Similarly, for the general reaction: we can define a constant The Equilibrium 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 • Equilibrium constants are dimensionless because they actually involve a thermodynamic quantity called activity. • Activities are directly related to molarity
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
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!
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. Variation of Kc with the Form of the Balanced Equation
Variation of Kc with the Form of the Balanced Equation • Large equilibrium constants indicate that most of the reactants are converted to products. • Small equilibrium constants indicate that only small amounts of products are formed.
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 • 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?
