Chapter 15 Equilibrium

Equilibrium N H 2  2 NH 3 N H 2  2 NH 3 Both reactions occur, Both reactions occur, Closed system Closed system No change in No change in Equilibrium reached no matter what direction. Equilibrium reached no matter what direction.

Write: Rate law for forward Rate law for forward Rate = Rate = Rate Law for reverse Rate Law for reverse Rate = Rate =

Then… Rate f = Rate r Rate f = Rate r

So… KfKf krkr =

Or kckc =

Equilibrium Constants Function of the Function of the Always Always Allows determination of Allows determination of

Equilibrium Expression aA + bB  cC + d D K c = Law of mass action

Rules Products in Products in Brackets indicate Brackets indicate Concentration is raised Concentration is raised Pure liquids and solids Pure liquids and solids

Practice Write the equilibrium expression for: H 2(g) + I 2(g)  2 HI (g) C (s) + O 2(g)  2 CO (g)

Problem ? What is the equilibrium constant for the following reaction if the concentrations are [H 2 ]=[I 2 ]= M and [HI]= M? H 2(g) + I 2(g)  2 HI (g) Does the equilibrium lie to the left or right?

Equilibrium expression N 2 O 4(g)  2 NO 2(g) Kp=Kp=

K c vs K P These will have different values since K p = K c (RT)  n Where R = (L atm)/(K mol) And T = temperature in K And  n = change in moles of gas

Practice 2 SO 3(g) ↔ 2 SO 2(g) + O 2(g) K c = 4.08 E -3 What is the K p expression? What is the value for K p ? T = 300 K

The Meaning of K… What can you say about the amount of (concentration or pressure) reactant or product if: K is much larger than 1? K is much smaller than 1? K is equal to 1?

Practice For the reaction: For the reaction: N 2(g) + O 2(g) ↔ 2 NO (g) 1. What is the equilibrium expression? 2. If K c at 25 o C is 1.00 E -30, is this a good method to ‘fix’ nitrogen?

Practice For the reaction: For the reaction: H 2(g) + I 2(g) ↔ 2 HI (g) K c = 794 at 298K K c = 54 at 700 K Which temperature is favored to produce HI? Why?

Direction for equilibrium For the reaction: For the reaction: H 2(g) + I 2(g) ↔ 2 HI (g) K c = 794 at 298K Write the K expression. Write the K expression. Write the K expression for the reverse reaction. Write the K expression for the reverse reaction. What is the value of K for the reverse reaction? What is the value of K for the reverse reaction?

Heterogeneous Equilibria

Compare N 2 (g) + 3 H 2 (g) ↔ 2 NH 3 (g) PbCl 2(s) ↔ Pb +2 (aq) + 2 Cl - (aq)

… Definitions… Heterogeneous equilibria Homogeneous equilibria

Heterogeneous As long as there is pure solid or liquid present, The concentration of the solid/liquid

Write K c PbCl 2(s) ↔ Pb +2 (aq) + 2 Cl - (aq) Knowing what you do about solubility, would you expect the value of K to be high or low?

Practice Write the equilibrium constant expressions for K c and K p for: Write the equilibrium constant expressions for K c and K p for: CO 2(g) + H 2(g) ↔ CO (g) + H 2 O (l) SnO 2(s) + 2CO (g) ↔ Sn (s) + 2CO 2(g)

Calculations For the reaction: For the reaction: 2NO 2 Cl (g) ↔ 2NO 2(g) + Cl 2(g) Write the K expression. If [NO 2 Cl] = M and [NO 2 ] = M and [Cl 2 ] = M, what is the value of K c at 500. K? K c = 0.558

A mixture of E-3 mole of hydrogen and E-2 mole of iodine are placed in a L container at 448 degrees C and allowed to come to equilibrium. At equilibrium, [HI] = 1.87 E-3 M. Calculate Kc for the reaction: A mixture of E-3 mole of hydrogen and E-2 mole of iodine are placed in a L container at 448 degrees C and allowed to come to equilibrium. At equilibrium, [HI] = 1.87 E-3 M. Calculate Kc for the reaction: H 2(g) + I 2(g) 2HI (g)

ICE boxes ICE boxes Initial Change Equilibrium The reaction goes here… Use a little bit of algebra…

ICE BOX STEPS 1. Calculate the known initial and equilibrium concentrations 2. Calculate the change in concentration that occurs 3. Use stoichiometry to calculate the changes in concentration for all other species 4. Calculate the desired item from the ICE box data.

Sulfur trioxide decomposes at high temperature to form sulfur dioxide and oxygen. Initially, a container at 1000 K has a sulfur trioxide concentration of 6.09 E-3 M. At equilibrium the [SO 3 ] = 2.44 E-3 M. Calculate the value of K c at 1000 K. Sulfur trioxide decomposes at high temperature to form sulfur dioxide and oxygen. Initially, a container at 1000 K has a sulfur trioxide concentration of 6.09 E-3 M. At equilibrium the [SO 3 ] = 2.44 E-3 M. Calculate the value of K c at 1000 K.

Q

The reaction Quotient… If you put a mixture of 1.25 mol of hydrogen, mol of nitrogen, and 1.50 mol of ammonia in a 1.25 L container, will the reaction move to the right or left or is it at equilibrium? K c = If you put a mixture of 1.25 mol of hydrogen, mol of nitrogen, and 1.50 mol of ammonia in a 1.25 L container, will the reaction move to the right or left or is it at equilibrium? K c = N2 (g) + 3 H 2(g)  2 NH3 (g) Calculate ‘Q’ – the reaction quotient. Calculate ‘Q’ – the reaction quotient.

Calculated the same way as Kc – but just the ‘question’, ‘Q’. Calculated the same way as Kc – but just the ‘question’, ‘Q’. Compare to Kc. Compare to Kc. Q = [NH 3 ] 2 [H 2 ] 3 [N 2 ] = (1.20) 2 Q =2.05 ((1.00) 3 (0.704))

Compare and decide… Three possibilities… Three possibilities… 1. If Q > K 2. If Q = K 3. If Q < K Remember… K = Products Reactants K = Q =2.05 Q > KThis means there are too many ________ and the reaction must move to the _______.

The Q and K relationship

For the Haber process, N 2(g) + 3 H 2(g)  2 NH 3(g) K p = 1.45 E-5 at 500 o C. In an equilibrium mixture of the three gases at 500 o C, the partial pressure of H 2 is atm and that of N 2 is atm. What is the partial pressure of NH 3 in this equilibrium mixture?