Chemical Equilibrium. Reversible reactions. Reversible reactions. Some reactions proceed to completion. 2 NO 2 (g) N 2 O 4 (g)

Transcription

1 Chemical Equilibrium 2 2 (g) 2 4 (g) Some s proceed to completion A chemical is said to go to completion if the proceeds until all of the limiting reactant or reactants is used up Examples: Dissolution of a soluble salt H 2 acl(s) a + (aq) + Cl - (aq) Dissociation of a strong acid H 3 (l) + H 2 (l) H 3 + (aq) (aq) Combustion CH 4 (g) (g) C 2 (g) + 2 H 2 (g) Reversible s Many s do not proceed to completion -- instead they approach an equilibrium state in which both reactants and products are present This occurs because the is reversible -- the products formed by the can themselves react to form the original reactants Reversible s In a reversible, the products formed by the can themselves react to form the original reactants Example: 2 24 combination / decomposition combination 2 2 (g) 2 4 (g) forward Example: itrogen dioxide / dinitrogen tetroxide combination 2 2 (g) 2 4 (g) red-brown colorless decomposition 2 2 (g) 2 4 (g) red-brown colorless 2 4 (g) decomposition 2 2 (g) 2 2 (g) 2 4 (g) reverse combined equation for forward and reverse s

2 Definition of equilibrium General definition: equilibrium -- a state of balance The concept of equilibrium The system is in a state of static equilibrium -- no water is flowing and the volume of water in both tanks is constant Pump #1 FF Technical definition: equilibrium -- a dynamic state in which two or more opposing processes are taking place at the same time and at the same rate Tank A Pump #2 FF Tank B The concept of equilibrium The system is in a state of dynamic equilibrium -- water is flowing in and out of both tanks, but the volume of water in both tanks is constant The concept of equilibrium ote: For the system to be at equilibrium, the amounts of water in each tank can be different -- but the flow rates have to be the same Pump #1 5 gal / min Pump #1 5 gal / min Tank A Pump #2 5 gal / min Tank B Tank A Pump #2 5 gal / min Tank B 65 gallons

3 Definition of equilibrium Examples of chemical equilibrium Many chemical s Chemical definition: chemical equilibrium -- for a reversible, a dynamic state in which the rate of the forward is exactly equal to the rate of the reverse 2 2 (g) 2 4 (g) ote: The concentrations of reactants and products do not have to be equal Initial: 1.00 mol 0 mol Equilibrium at 298 K: mol 0.48 mol At equilibrium, the rate of the forward is equal to the rate of the reverse (but the concentrations of products and reactants are not equal) Examples of chemical equilibrium Reversible chemical 2 2 (g) 2 4 (g) At equilibrium: Rate of forward = Rate of reverse Reaction rates Every chemical has a rate, or speed, at which it proceeds -- some are fast, some are very slow The rate of a is dependent of the chemical and physical properties of the reactants (and catalysts, if any are present) 2 molecules are combining to form 24 at the same rate that 24 molecules are decomposing to form 2 The rate of a varies under different conditions -- concentration of reactants -- temperature -- volume (if gases are involved) The rate of a is proportional to the concentration of the reactants

4 Rates of forward and reverse s Example: Formation / decomposition of hydrogen iodide H 2 (g) + I 2 (g) 2 HI(g) Rates of forward and reverse s The concentration of reactants decreases with time, decreasing the rate of the forward As the forward proceeds, the concentration of the reactants decreases -- the rate of the forward therefore decreases As the forward proceeds, the concentration of the products increases -- the rate of the reverse therefore increases Eventually, a point is reached where the rate of the forward is equal to the rate of the reverse -- when this occurs, the system has reached a state of equilibrium The concentration of products increases with time, increasing the rate of the reverse For a system at equilibrium: the rates of the forward and reverse s are equal the concentrations of the reactants and products are constant The equilibrium constant (K eq ) is a value representing the unchanging concentrations of the reactants and the products in a chemical at equilibrium For the general : a A + b B c C + d D The expression for the equilibrium constant is: [C] c [D] d [A] a [B] b The quantities in brackets are the concentrations of each substance in moles / liter The superscripts a, b, c, and d are the coefficients from the balanced chemical equation For any, the value of K eq will vary with temperature (25 C is assumed unless stated otherwise)

5 For the general : a A + b B c C + d D The expression for the equilibrium constant is: [C] c [D] d [A] a [B] b Reaching equilibrium For the : 2 S S3 Flasks filled initially with only S2 and 2 or only S3 will both reach the same equilibrium state Initial conditions Equilibrium Initial conditions IMPRTAT Pure solids and liquids are not included in the expression for the equilibrium constant -- i.e., only gases and aqueous substances appear in the expression for the equilibrium constant All S2 and 2 o S3 Mostly S3 Small amounts of S2 and 2 All S3 o S2 and 2 Equilibrium can favor the products or reactants For the : 2 S S3 Equilibrium is reached after most of the forward has occurred The equilibrium for this favors the products Equilibrium can favor the products or reactants For the : 2 S S3 Equilibrium is reached after most of the forward has occurred The equilibrium for this favors the products Concentration ( mol / L ) S2 2 Forward Reverse S3 S2 2 S3 Concentration ( mol / L ) S2 2 Forward Reverse S3 S2 2 S3 Initial Conditions: All S2 and 2 o S3 Equilibrium: Small amounts of S2 and 2 Mostly S3 Initial Conditions: o S2 or 2 All S3 Equilibrium: Small amounts of S2 and 2 Mostly S3

6 Equilibrium can favor the products or reactants 2 S S3 [S 3 ] 2 [S 2 ] 2 [ 2 ] Large amounts Small amounts For the : CCl2 C + Cl2 Equilibrium is reached after very little of the forward has occurred The equilibrium for this favors the reactants Equilibrium is reached after most of the forward has occurred [S 3 ] 2 >> [S 2 ] and [ 2 ] The equilibrium for this favors the products Concentration ( mol / L ) CCl2 Forward Reverse C Cl2 CCl2 C Cl2 Keq >> 1 Initial Conditions: All CCl2 o C or Cl2 Equilibrium: Mostly CCl2 Small amounts of C and Cl2 Equilibrium can favor the products or reactants For the : CCl2 C + Cl2 Equilibrium is reached after very little of the forward has occurred The equilibrium for this favors the reactants CCl2 C + Cl2 [C] [Cl 2 ] [CCl 2 ] Small amounts Large amount Concentration ( mol / L ) CCl2 Forward Reverse C Cl2 CCl2 C Cl2 Equilibrium is reached after very little of the forward has occurred [C] and [Cl 2 ] << [CCl 2 ] The equilibrium for this favors the reactants Initial Conditions: o CCl2 All C and Cl2 Equilibrium: Mostly CCl2 Small amounts of C and Cl2 Keq << 1

7 Value of equilibrium constant indicates whether products or reactants are favored K very small a A + b B c C + d D Products favored: Large Keq Reactants favored: Small Keq Products/reactants roughly equal: Keq! 1 Virtually no occurs (only reactants present at equilibrium) K = 10-3 K = 1 K = 10 3 More reactants present than products at equilibrium More products present than reactants at equilibrium [C] c [D] d [A] a [B] b K very large Reaction goes to completion (only products present at equilibrium) Example: Write the equilibrium constant expression for: 3 H 2 (g) + 2 (g) 2 H 3 (g) The expression for the equilibrium constant is: [H 3 ] 2 [H 2 ] 3 [ 2 ] Example: Consider the following : PCl 5 (g) PCl 3 (g) + Cl 2 (g) Calculate the value of K eq for the based on the following equilibrium concentrations of products and reactants at 300 C: [PCl 5 ] = mol/l Example: Consider the following : PCl 5 (g) PCl 3 (g) + Cl 2 (g) Calculate the value of K eq for the based on the following equilibrium concentrations of products and reactants at 300 C: Step 1: Write the expression for the equilibrium constant [PCl 3 ] = 0.97 mol/l [Cl 2 ] = 0.97 mol/l [PCl 3 ] [Cl 2 ] [PCl 5 ]

8 Example: Consider the following : PCl 5 (g) PCl 3 (g) + Cl 2 (g) Calculate the value of K eq for the based on the following equilibrium concentrations of products and reactants at 300 C: Step 2: Plug in the concentration values and solve for K eq [PCl 3 ] [Cl 2 ] [PCl 5 ] = (0.97)(0.97) (0.030) = 31 Which of these s proceeds to completion? Which of these s has more products than reactants present at equilibrium? Which of these s has more reactants than products at equilibrium? 3 H 2 (g) + 2 (g) 2 H 3 (g) K eq (at 25 C) 5.9 x 10 5 H 2 (l) H+(aq) + H (aq) 1.0 x C(g) + 2 H 2 (g) CH 3 H(g) 10.5 Homework assignment Chapter 7 Problems: 7.52, 7.54, 7.55, 7.56, 7.57, 7.58, 7.59, 7.60, 7.61