Ch 16 aqueous ionic equilibrium
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Ch. 16: Aqueous Ionic Equilibrium. Dr. Namphol Sinkaset Chem 201: General Chemistry II. I. Chapter Outline. Introduction Buffers Titrations and pH Curves Solubility Equilibria and K sp Complex Ion Equilibria. I. Last Aspects of Equilibria.

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Ch. 16: Aqueous Ionic Equilibrium

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Ch 16 aqueous ionic equilibrium

Ch. 16: Aqueous Ionic Equilibrium

Dr. Namphol Sinkaset

Chem 201: General Chemistry II


I chapter outline

I. Chapter Outline

  • Introduction

  • Buffers

  • Titrations and pH Curves

  • Solubility Equilibria and Ksp

  • Complex Ion Equilibria


I last aspects of equilibria

I. Last Aspects of Equilibria

  • In this chapter, we cover some final topics concerning equilibria.

  • Buffers are designed to take advantage of Le Châtelier’s Principle.

  • Solubility can be reexamined from an equilibrium point of view.

  • Complex ions are introduced and their formation explained using equilibrium ideas.


Ii ph resistive solutions

II. pH Resistive Solutions

  • A solution that resists changes in pH by neutralizing added acid or base is called a buffer.

  • Many biological processes can only occur within a narrow pH range.

  • In humans, the pH of blood is tightly regulated between 7.36 and 7.42.


Ii creating a buffer

II. Creating a Buffer

  • To resist changes in pH, any added acid or base needs to be neutralized.

  • This can be achieved by using a conjugate acid/base pair.


Ii how the buffer works

II. How the Buffer Works

  • For a buffer comprised of the conjugate acid/base pair of acetic acid/acetate:

    • OH-(aq) + CH3COOH(aq) H2O(l) + CH3COO-(aq)

    • H+(aq) + CH3COO-(aq)  CH3COOH(aq)

  • As long as we don’t add too much OH- or H+, the buffer solution will not change pH drastically.


Ii calculating the ph of buffers

II. Calculating the pH of Buffers

  • The pH of a buffer can be calculated by approaching the problem as an equilibrium in which there are two initial concentrations.

  • Since an acid and its conjugate base are both in solution, problem can be solved from a Ka or Kb point of view.


Ii sample problem

II. Sample Problem

  • A solution was prepared in which [CH3COONa] = 0.11 M and [CH3COOH] = 0.090 M. What is the pH of the solution? Note that the Ka for acetic acid is 1.8 x 10-5.


Ii sample problem1

II. Sample Problem

  • A student dissolves 0.12 mole NH3 and 0.095 mole NH4Cl in 250 mL of water. What’s the pH of this buffer solution? Note that the Kb for ammonia is 1.8 x 10-5.


Ii a special buffer equation

II. A Special Buffer Equation

  • Buffers are used so widely that an equation has been developed for it.

  • This equation can be used to perform pH calculations of buffers.

  • More importantly, it can be used to calculate how to make solutions buffered around a specific pH.


Ii henderson hasselbalch eqn

II. Henderson-Hasselbalch Eqn.


Ii the h h equation

II. The H-H Equation

  • The Henderson-Hasselbalch equation works for a buffer comprised of conjugate acid/base pairs.

  • It works provided the “x is small” approximation is valid.


Ii sample problem2

II. Sample Problem

  • A student wants to make a solution buffered at a pH of 3.90 using formic acid and sodium formate. If the Ka for formic acid is 1.8 x 10-4, what ratio of HCOOH to HCOONa is needed for the buffer?


Ii sample problem3

II. Sample Problem

  • A researcher is preparing an acetate buffer. She begins by making 100.0 mL of a 0.010 M CH3COOH solution. How many grams of CH3COONa does she need to add to make the pH of the buffer 5.10 if the Ka for acetic acid is 1.8 x 10-5?


Ii upsetting the buffer

II. Upsetting the Buffer

  • A buffer resists changes to pH, but it is not immune to change.

  • Adding strong acid or strong base will result in small changes of pH.

  • We calculate changes in pH of a buffer by first finding the stoichiometric change and then performing an equilibrium calculation.


Ii illustrative problem

II. Illustrative Problem

  • A 1.00 L buffer containing 1.00 M CH3COOH and 1.00 M CH3COONa has a pH of 4.74. It is known that a reaction carried out in this buffer will generate 0.15 mole H+. If the pH must not change by more than 0.2 pH units, will this buffer be adequate?


Ii stoichiometric calculation

II. Stoichiometric Calculation

  • When the H+ is formed, it will react stoichiometrically with the base.

  • We set up a different type of table to find new equilibrium concentrations.

  • We use ≈0.00 mol because [H+] is negligible.


Ii equilibrium calculation

II. Equilibrium Calculation

  • We use the new buffer concentrations in an equilibrium calculation to find the new pH.

  • Again, use ≈0.00 M because [H3O+] is negligible.

Solve to get [H3O+] = 2.44 x 10-5 M, and pH = 4.613.


Ii how the buffer works1

II. How the Buffer Works


Ii a simplification for buffers

II. A Simplification for Buffers

  • In buffer problems, # of moles can be used in place of concentration.

  • This can be done because all components are in the same solution, and hence have the same volume.


Ii sample problem4

II. Sample Problem

  • Calculate the pH when 10.0 mL of 1.00 M NaOH is added to a 1.0-L buffer containing 0.100 mole CH3COOH and 0.100 mole CH3COONa. Note that the Ka for acetic acid is 1.8 x 10-5.


Ii buffers using a weak base

II. Buffers Using a Weak Base

  • Up until now, we’ve been creating buffers with a weak acid and it’s conjugate base.

  • Can also make a buffer from a weak base and its conjugate acid.

  • Henderson-Hasselbalch still applies, but we need to get Ka of the conjugate acid.


Ii pk a pk b relationship

II. pKa/pKb Relationship


Ii sample problem5

II. Sample Problem

  • Calculate the pH of a 1.0-L buffer that is 0.50 M in NH3 and 0.20 M in NH4Cl after 30.0 mL of 1.0 M HCl is added. Note that the Kb for NH3 is 1.8 x 10-5.


Ii making effective buffers

II. Making Effective Buffers

  • There are a few parameters to keep in mind when making a buffer:

    • The relative [ ]’s of acid and conjugate base should not differ by more than factor of 10.

    • The higher the actual [ ]’s of acid and conjugate base, the more effective the buffer.

    • The effective range for a buffer system it +/- 1 pH unit on either side of pKa.


Ii buffer capacity

II. Buffer Capacity

  • Buffer capacity is defined as the amount of acid or base that can be added to a buffer without destroying its effectiveness.

  • A buffer is destroyed when either the acid or conjugate base is used up.

  • Buffer capacity increases w/ higher concentrations of buffer components.


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