Chemical Equilibrium Q, K, and

Calculations Chapter 16

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Equilibrium• the forward and reverse reactions proceed at

equal rates• the concentrations of reactants and products

are constant =• Can be reached from either direction

(starting with either “reactants” or “products” has the same result)

CO + 3H2CH4 + H2O

Example Reaction

CO + 3 H2 CH4 + H2O

Q and K

• Reaction Quotient: Q– Describes a rxn at _____________

• Equilibrium Constant: K– Describes a rxn at _____________

• Both values are calculated from the same expression

K =[C]c [D]d

[A]a [B]b

conc. of products

conc. of reactantsequilibrium constant

c

a A(aq) + b B(aq) c C (aq) + d D(aq)

For a general reaction occurring __________:

[ ] = molarity

Equilibrium Constant Expression

Equilibrium Constant Expressions

• The exponents are based on the __________!!

• ________ OVER _______

• There are no _____ for Q or K

When writing the expressions

• Include only these phases: ___ + ____

• Omit these phases: ____+ ____

• For reactions of __________:

Kp = PC PD

PA PB

c d

a b

P = partail pressure of the gas

a A(g) + b B(g) c C (g) + d D(g)

Q and K

• If concentrations or partial pressures represent the reaction at equilibrium, you can calculate ______.

• If concentrations or partial pressures represent the reaction at any time, you can calculate ______.

Write the equilibrium expression

1) N2(g) + 3H2 (g) <----> 2NH3 (l) (Kp)

2) CO(g) + 3H2(g) <----> CH4(g) + H2O(g) (Kc & Kp)

2 Red(g) <----> Blue (g)

• Write the expression for Kc and Kp

• Applet

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PCl3(g) + Cl2(g) <--> PCl5(g) Kc = [PCl5] [PCl3][Cl2]

Different amounts of R & P are mixed at 546 K to determine K

Calculating K

Comment on the equilibrium position.

Calculating K

H2(g) + I2(g) <---> 2HI

• At equilibrium, the concentration for both H2 and I2 = 0.0037 M and HI = 0.0276. Write the expression for K and calculate its value? What this value mean?

• K > 1 the reaction is ________ favored• K< 1 the reaction is ________ favored

Calculating Q

• H2(g) + I2(g) <---> 2HI

• 2 moles each of H2 and I2 and 3 moles of HI are placed in a 0.5 L flask. Determine if the rxn is at equilbrium? If not will it move forward or reverse?

• Q = K the reaction _______________

• Q < K the reaction ________________

• Q > K the reaction ________________

Kc and Kp are related

• Kp = Kc(RT)∆n

• R= 0.08206 Liter atm mol K

• ∆n = # product gases - # reactant gases

• H2(g) + I2(g) <---> 2HI

• If Kc = 56, calculate Kp at 25 °C.

Kp = Kc(RT)∆n

A real one! Kp = Kc(RT)∆n

• N2(g) + 3 H2(g) <---> 2NH3(g)

• The Kp = 5.8 x105, calculate Kc at 25 °C.

MORE ABOUT K!!!!

• K values for various forms of a balanced equation relate to the coefficients

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The K value for the reverse rxn is the ______________

Multiplying a reaction by a # _______________

MORE ABOUT K!!!!

at 298 K

C(s) + 1/2 O2(g) <---> CO(g) K1 = 4.6 X 1023

2 C (s) + O2 (g) <--> 2CO(g) K2 = ?

MORE ABOUT K!!!!

• When adding reactions to produce a net equation, ________ K values.

• For a 2 reactions, with K1 and K2, Knet = K1K2