10 Equilibrium

Question 1

The Equilibrium Expression

For the reaction

aA + bB ⇔ cC +dD

K_eq =

Answer 1

([C]c[D]d) / ([A]a[B]b)

1. [A], [B], [C], [D] are molar concentrations or partial pressures at equilibruim.
2. Products are in the numerator, and reactants are in the denominator.
3. Coefficients in the balanced equation become exponents in the equilibrium expression.
4. Solids and pure liquids are not included in the equilibrium expression - only aqueous reactants and products are included.
5. Units are not given for K_eq

Question 2

Kc =

Answer 2

the constant for molar concentrations

Question 3

Kp =

Answer 3

the constant for partial pressures

Question 4

Ksp =

Answer 4

the solubility product, which has no denominator because the reactants are solids.

Question 5

Ka =

Answer 5

the acid dissociation constant for weak acids.

Question 6

K_b =

Answer 6

the base dissociation constant for weak bases.

Question 7

K_w =

Answer 7

describes the ionization of water (K_w = 1 X 10^-14)

Question 8

A large value for Keq means that products

Answer 8

are favored over reactants at equilibrium, while a small value for K_eq means that reactants are favored over products at equilibrium.

Question 9

K_p =

Answer 9

K_c(RT)^{Delta}n

Kp = partial pressure constant (atm)

Kc = molar concentration constant (mol)

R = the ideal gas constant, 0.0821 (L-atm)/ (mol-K)

T = absolute temperature (K)

{Delta}n = (Moles of product - moles of reactant gas)

Question 10

The Reaction Quotient

For the reaction

aA + bB ⇔ cC +dD

Q =

Answer 10

([C]c[D]d) / ([A]a[B]b)

• If Q is less than the calculated K for the reaction, the reaction proceeds forward, generating products.
• If Q is greater than K, the reaction proceeds backward, generating reactants
• If Q = K, the reaction is already at equilibrium

Question 11

If two reactions can be added together to create a third reaction, then the Keq for the two reactions can be

Answer 11

multiplied together to get the K_eq for the third reaction.

Question 12

La Chatelier’s Law

Whenever a stress is placed on a situation at equilibrium,

Answer 12

the equilibrium will shift to relieve that stress.

Question 13

When the concentration of a reactant of product is increased,

Answer 13

the reaction will proceed in the direction that will use up the added substance.

Question 14

When the concentration of a reactant or product is decreased,

Answer 14

the reaction will proceed in the direction that will produce more of the substance that has been removed.

Question 15

When the volume in which a reaction takes place is increased,

Answer 15

the reaction will proceed in the direction that produces more moles of gas.

Question 16

When the volume in which a reaction takes place is decreased,

Answer 16

the reaction will proceed in the direction that produces fewer moles of gas.

Question 17

If there is no gas involved in the reaction, or if the reactants and products have the same number of moles of gas,

Answer 17

then the volume changes have no effect on the equilibrium.

Question 18

When temperature is increased,

Answer 18

the reaction will proceed in the endothermic direction.

Question 19

When temperature is decreased,

Answer 19

the reaction will proceed in the exothermic direction.

Question 20

When pressure is decreased,

Answer 20

the reaction will proceed toward the side with the greatest number of molecules.

Question 21

When Pressure is increased,

Answer 21

the reaction will proceed toward the side with the fewest molecules of gas.

Question 22

When you reverse an equation, the new K_eq is

Answer 22

the reciprocal of the original equation.

Question 23

When you double an equation, the new K_eq value is

Answer 23

the square
of the Keq for the original equation.

Question 24

When you halve an equation, the new Keq value is

Answer 24

the square root of the Keq for the original equation.

Question 25

When you add 2 equations to get a third one, the Keq value for the third one is

Answer 25

the product of the Keq’s for the original equations.