Ch 12: A Deeper Look at Chemical Equilibrium Up to now we've ignored two points- 1. 2.

Ch 12: A Deeper Look at Chemical Equilibrium

Up to now we've ignored two points-

1.

2.

PbI2(s) = Pb2+(aq) + 2I-(aq)

Ksp = 7.9 x 10-9 (ignoring PbOH+, PbI3-, etc)

K'sp =

Activity Coefficients - concentrations are replaced by "activities"

definition old [B][A]

[D][C]K

ba

dc

aA + bB = cC + dD

activity using definitionnew [B]γ[A]γ

[D]γ[C]γK

bbaaA

ddccC'

B

D

We can calculate the activity coefficients if we know what the ionic strength of the solution is.

Charge Effects - an ion with a +2 charge affects activity of a given electrolyte more than an ion with a +1 charge

= ionic strength, a measure of the magnitude of the electrostatic environment

2iiZC

2

1μ Ci = concentration

Zi = charge

e.g. calculate the ionic strength of an aqueous soln of 0.50M NaCl and 0.75M MgCl2

The Extended Huckel-Debye Equation

305/1

51.0log

2

zA

A = activity coefficient

Z = ion charge

= ionic strength (M)

= hydrated radius (pm)

works well for 0.10M

Example (p. 247) - Find the activity coefficient in a solution of 3.3 mM Mg(NO3)2

Easy solution using Table 12-1:

Harder solution using Huckel-Debye:

Example - What is the concentration of Ba2+ in a solution saturated with BaSO4 in (a) pure H2O, (b) 0.050 M KCl

The Real Definition of pH

][HlogγlogΑpH HH

What is the concentration of H+ in (a) pure H2O and (b) 0.10M NaCl?

Systematic Treatment of Equilibria (Sec 12-3 and 12-4)

A procedure for solving any equilibrium problem no matter how complicated.

Charge Balance - the sum of the positive charges in solution must equal the sum of negative charges.

e.g. sulfate ion CSO42- = 0.0167 M

General charge balance equation -

n1[C1] + n2[C2] +…. = m1[A1] + m2[A2] + …

where C = cation concentration

n = cation charge

A = anion concentration

m = anion charge

e.g. write the charge balance equation for a soln of Na2SO4 and NaCl in water.

Mass Balance

The sum of all substances in solution containing a particular atom (or group of atoms) must equal the quantity added to solution.

e.g. solution of 0.050 M Hac

HAcHAcHAcHAcHAcHAcHAcHAcHAcHAc

e.g. solution of 0.025 M H3PO4

H3PO4

H3PO4

H3PO4

H3PO4

H3PO4

H3PO4

H3PO4

H3PO4

H3PO4

H3PO4

Mass balance for a sparingly soluble salt follows this approach:

e.g. PbCl2

initially: [I-] = 2 [Pb2+]

General Procedure

1. Write down all the relevant chemical equations2. Write the charge blance3. Write the mass balance4. Write down the equilibrium constant expressions

(only step where activities may be used)5. Make sure that the number of unknowns equals

the number of equations6. Solve the system of equations

-make approximations-use a computer

Coupled Equilibria: Solubility of CaF2

1. Relevant equations

CaF2(s) Ca2+ + 2 F-

F- + H2O HF + OH-

H2O H+ + OH-

2. Charge Balance

3. Mass Balance

4. Equilibrium Expressions

5.Number of equations = number of unknowns

[H+], [OH-], [Ca2+], [F-], [HF] = unknowns

6.Simplifying Assumptions and Solution

• fix the pH using a buffer { [H+] = CH+ } , removes one unknown

• adding a buffer and associated ions nullifies the charge balance equation

• so now we have 4 equations and 4 unknowns

After buffering to pH = 3.0, [H+] = 1.0 x 10-3 M

[OH-] = Kw/[H+] = 1.0 x 10-11 M and now subst into Kb

1.510 x 1.0

10 x 1.5

][OH

K

][F

[HF]11

11b

[HF] = 1.5[F-] and now subst 1.5[F-] for [HF] in the mass balance equation

[F-] + [HF] = 2[Ca2+]

[F-] + 1.5[F-] = 2[Ca2+]

[F-] = 0.80[Ca2+] and finally subst 0.80[Ca2+] for [F-] into Ksp

[Ca2+][F-]2 = Ksp

[Ca2+](0.80[Ca2+])2 = Ksp

[Ca2+] = (Ksp/0.802)1/3 = 3.9 x 10-4 M