KEQ FOR GASES

Kp

Equilibrium and Pressure

2SO2(g) + O2(g) 2SO3(g)

If reactants and products are gases, partial pressures of each species can be used to determine the equilibrium

The total pressure = PSO2 + PO2+ PSO3

Concentration of Gases

For reactions that contain gases:PV = nRTP = (n/V)RTP = CRT “C” is concentration in mol/LC = P/RT

The concentration of a gas is directly proportional to the pressure exerted by the gas.

Problems Involving Pressure

2 NO2 (g) N2O4 (g)

KP = 7.63atm at 300K.

What are the equilibrium pressures of NO2 and N2O4 if a flask initially contains 1.000 atm of NO2?

What is the value of the Kc at 300K.

Relating Kp and Kc

Kp = Kc (RT)Δn

Kp = partial pressure constant using atm

Kc = molar concentration constant using M

R = ideal gas constant, 0.0821 L-atm/mol-KT = absolute temperature in KΔn= moles of product – moles of reactants

DIFFERENT TYPE OF K

Ksp - Solubility Product of a Salt

KCl (s) K+ (aq) + Cl- (aq)

CaCl2 (s) Ca+2 (aq) + 2 Cl- (aq)

Ksp values

Acid Formula Ka value

Potassium nitrate

KNO3 Very large

Calcium sulfate CaSO4 3.0 x 10 -5

Lead (II) chloride

PbCl2 1.7 x 10 -5

Lead (II) iodide PbI2 1.0 x 10 -8

Silver iodide AgI 1.0 x 10 -16

Silver sulfide Ag2S 1.0 x 10 -49

Why are some substances more soluble than others?

Is any substance truly insoluble?

Problem Involving Ksp

The Ksp of lead (II) chloride at 298K is 1.7 x 10-5.

Write the dissociation of lead (II) chloride in water.

Write the expression for the solubility product, Ksp, of lead (II) chloride.

Determine the concentration of the chloride ions and the lead ions of a saturated solution at 298K.

Kw - Autoionization of WaterKw is 1.00 x 10-14 at room temperature.

What would be the Keq expression?

How does the [H3O+] compare to the [OH-]?

What would be the [H3O+]? What would be the pH?

Like any other equilibrium constant, the value of Kw varies with temperature.

Ka – Acid Dissociation ConstantArrhenius acids dissociate to produce hydrogen ions in solution.

What is the difference between a strong and weak acid?

Ka values

Acid Formula Ka value

Perchloric acid HClO4 Very large

Hydrochloric acid

HCl Very large

Nitric acid HNO3 Very large

Sulfuric acid H2SO4 Very large

Phosphoric acid H3PO4 7.5 x 10 -3

Citric acid H3C6H5O7 7.1 x 10 -4

Hydrofluoric acid

HF 3.5 x 10 -4

Problem Involving Ka

Acetic acid, HC2H3O2, has a Ka of 1.7 x 10-5 at 298K. Write the expression for the acid dissociation of acetic

acid. Determine the concentration of the hydrogen ions and

the acetate ions at 298K of a 1.0 M acetic acid.

Kb – Base Dissociation ConstantArrhenius bases dissociate to produce hydroxide ions in solution.

Some bases dissociate completely; other dissociate only partially. This determines if a base is strong or weak.

Kb values

Based Formula Ka value

Potassium hydroxide

KOH Very large

Sodium hydroxide

NaOH Very large

Zinc hydroxide Zn(OH)2 9.6 x 10 -4

Methylamine CH3NH2 3.7 x 10 -4

Ammonia NH3 1.8 x 10 -5

Problem Involving Kb

1.0 moles of pure ammonia gas is dissolved to make a 1 Liter solution at 298K. Ammonia partially dissociates to produce the ions shown below. Ammonia, NH3, has a Kb of 1.8 x 10-5 at 298K.

Write the expression for the Kb of ammonia. What is the hydroxide concentration at equilibrium? What is the pH of a 1.0 M NH3(aq) solution?

DIFFERENT TYPE OF KMULTISTEP REACTIONS

Overall Keq

A + B C Keq = K1

C D + E Keq = K2

A + B D + E Keq = K1K2

Practice Problem

Mg(OH)2 (s) Mg+2(aq) + 2 OH- (aq) Ksp = 5.6 x 10-11

MgF2 (s) Mg+2(aq) + 2 F- (aq) Ksp = 5.8 x 10-10

NET REACTION: 2 F- (aq) + Mg(OH)2 (s) 2 OH- (aq) + MgF2 (s)

Manipulate the equations for the dissolving of Mg(OH)2 and MgF2 to obtain the net ionic reaction.

Calculate the Keq of the net ionic reaction.