Transcript Document

Aqueous Equilibrium
Common Ion Effect
Learning Outcomes and Prereqs
Pre-reqs:
Identify how changing concentrations of reactants or products shift
an equilibrium (Le Chatelier’s Principle)
Learning Outcomes
Identify the effect of adding a conjugate base of an acid on pH and
ionization.
Identify the effect of adding a conjugate acid of a base on pH and
ionization.
Reminder
Le Chatelier’s Principle
If you apply stress to a system it shifts to relieve the
stress.
Reaction shifts left or right to relieve the stress.
Analogy-
The common ion effect
If you have a solution of an acid and its conjugate
base the ionization is lessened by the presence of the ion.
Same is true for a base and its conjugate acid
Conjugate acid/base added through it’s salt.
So….what does that mean? Examples?
The common ion effect
Adding a conjugate lessons the effect of ionization.
Examples?
Formic acid and sodium formate
H
Ammonia and Ammonium Chloride
Why do we care?
If ionization is decreased how does this affect the OH- and
H+ concentrations?
H
Question: If HCOO- is added, what happens to the
concentration of H+? What does that do to pH?
[H+]:
It decreases
pH:
It increases
But does it really matter?
An example to illustrate:
Determine the pH of a 0.20M NH3 solution. Then determine the
pH of a solution that is 0.20M NH3 and also 0.30M NH4Cl
Great so we can do the calculation problems
just like every other problem we’ve done! It’s
just a bit more algebra!
Or maybe there is an easier way
Review
If you add the conjugate base of an acid:
the ionization will be lessened
This means less [H+] so a higher pH
If you add the conjugate acid of a base:
The ionization will be lessened
This means less [OH-] so a lower pH.
Henderson Hasselbalch
Equation
Learning outcomes
Derive the Henderson Hasselbalch Equation
Use the Henderson Hasselbalch equation to solve a
problem.
Identify when we are allowed to use this, and when we have
to use the previous methods of solving acid/base problems.
Incredibly related quote:
Lets do something similar to the previous problem
we did: but now for the most general of cases.
If we do it once in general we can use the results.
Henderson-Hasslebalch:
Creating the generic equation
Henderson-Hasslebalch:
Problem solving tips
Use this for buffer problems only.
Be sure you are using the Ka NOT the Kb
As with all equations, get good at manipulating them.
Only use if weak acid approximation is valid
Example: Using the HendersonHasslebalch equation
0.20M CH3COOH and 0.30M CH3COONa are
mixed. Find the pH.
Review
The Henderson Hasselbalch equation comes from
solving a generic acid problem where both an acid and
its conjugate base are present.
You can use this equation to solve problems whenever
the 5% approximation is valid (note: you don’t have to
show me, but you do need to check in case it doesn’t
work)
Introduction to Buffers
Motivation and
Learning Outcomes
Identify why our body needs buffers, and what happens if
something causes that system to fail.
Define buffers.
Identify how buffers work to keep pH changes small
Identify if two components form a buffer
Real life importance check:
pH of blood=7.4
Acidosis <7.35
Some products of your metabolism:
Ammonia
Urea (a base)
Uric acid
CO2 (acidic when reacting with
water)
Alkalosis> 7.50
Urea
Alkalosis and
Acidosis
Metabolic
Alkalosis and
Acidosis
Respiratory
Hypoventilation
increased CO2
acidosis
Hyperventilation
decreased CO2
alkalosis
Real life importance check:
pH of blood=7.4
Acidosis <7.35
Alkalosis> 7.50
Products of your metabolism:
Ammonia
Urea (a base)
Uric acid
CO2 (acidic when reacting with water)
So how can your body
control your pH so well?
Urea
Buffers
Acid and its conjugate base, or a base and its
conjugate acid
Essentially the systems we’ve talked about in the
common ion effect questions
It works by converting a strong acid into a weak acid,
or a strong base into a weak base.
A strong base can’t exist in solution with a weak acid
A strong acid can’t exist in solution with a weak base
Activity Part 1:
(I’ll need some volunteers)
Buffers
Acid and its conjugate base, or a base and its conjugate
acid
Basically the systems we’ve talked about in the
common ion effect questions
It works by converting a strong acid into a weak acid,
or a strong base into a weak base.
A strong base can’t exist in solution with a weak acid
A strong acid can’t exist in solution with a weak base
Activity Part 2:
(I’ll need some volunteers again)
Acids react with bases
Bases react with acids
Real life importance check:
pH of blood=7.4
Acidosis <7.35
Alkalosis> 4.5
Products of your metabolism:
Ammonia
Urea (a base)
Uric acid
CO2 (acidic when reacting with water)
Urea
So how can your body control your pH so well?
It uses buffers!!!!
Buffers in the Body:
We discussed how CO2 can become acid in the body
This equilibrium is as follows, if both are present it’s a buffer
Buffers in the Body:
How do we regulate CO2 levels?
Breathing levels
What happens to the CO2 if we breath
shallowly?
Respiratory acidosis
Why is this bad?
Many reasons: here is one:
Hemoglobin in red blood cells binds either
O2 or H+
𝐻𝑏𝐻 + + 𝑂2 ⇌ 𝐻𝑏𝑂2 + 𝐻 +
Hypoventilation
increased
CO2
acidosis
Hyperventilation
decreased
CO2
alkalosis
Which of the following are buffers:
KNO2/HNO2
yes
KCl/HCl
no
KHSO4/ H2SO4
no
Na2HPO4/NaH2PO4
Yes
KCN/HCN
Yes
Na2SO4/NaHSO4
Yes
NH3/NH4NO3
Yes
Review
Buffers prevent large changes in pH caused by the addition of
acids or bases.
This is needed in many systems. Our blood buffer system uses
buffers to keep the pH in a very narrow range.
Buffers work by having both an acid and its conjugate base, or a
base and its conjugate acid present.
Therefore if an acid is added, it reacts with the base
If a base is added, it reacts with the acid
This keeps the pH change small.
Buffer Calculations.
Learning outcomes
Use the Henderson-Hasslebalch equation to solve for
the pH of buffers.
Use the Henderson-Hasslebalch equation to solve for
the pH of buffers after adding an acid or base.
Test your knowledge of buffers by answering a couple
of conceptual questions.
Buffers
Calculate the pH of the solution that results from mixing 0.200L
of 0.050M (CH3)2NH with 0.320L of 0.040M (CH3)2NH2Cl.
(Kb=5.88x10-4)
You try:
Calculate the pH of the solution that results from mixing 72.0mL
of 0.015M (CH3)2NH with 20mL of 0.200M (CH3)2NH2Cl
The pH of blood plasma is 7.40, Assuming the principal buffer
system is HCO3- and H2CO3 find the ratio of HCO3- and
H2CO3. (Ka=4.2x10-7)
Adding acids and bases
Acids react with bases
Bases react with acids
Buffers when adding a base or acid
It works by converting a strong acid into a weak acid, or a strong
base into a weak base.
A strong base can’t exist in solution with a weak acid
A strong acid can’t exist in solution with a weak base
Notice NOT equilibrium arrow!!!
Use “new” concentration of acid or base to do equilibrium calculations in
the same manner as before.
Steps for buffer problem with addition of
acid or base.
Step 1: Decide if it is a buffer
Step 2: Write reaction of buffer with strong acid or base
Step 3: (if needed) do ICF chart (my take on ICE, its not an
equilibrium its “final”) to find amount of weak acid or base
Suggested, do this in mols
Step 4: fill into the HH equation if assumptions are valid (Ka/Kb
is low, concentrations high) or use ICE chart and Ka or Kb ect…as
we did with other equilibrium problems.
If using HH equation use moles
If using Ka or Kb equation use molarity
Buffers when adding a base or acid Examples:
Calculate the pH of 1.00L of the buffer 1.00M
CH3COONa and 1.00M CH3COOH. Ka=4.2x10-7
A) before
and after the addition of *
B) 0.080 mol NaOH
or
C) 0.120 mol HCl
*assume no change in volume
***Note: You’ll notice your book always uses an ICE chart, filling into K, solving for X, finding
pH protocol, I’m not sure why they use this after having taught you the easier way to do it.
Choose whichever you like most.
Example Questions:
We said the normal ratio of HCO3- :H2CO3 is 20:1, Is
your blood better at buffering added acid or added base?
Review
For most cases you can use the Henderson-Hasslebalch
equation for solving buffer problems.
(When the 5% rule is valid, you need to test this for
buffers, but you don’t need to show me, otherwise use
an ice chart).
If you add an acid or a base to a buffer, first react the
species away ( usually using moles) then solve a HH
equation.
Chemistry and life segment
The biology here will not be tested (however you better
know the difference between an alcohol and a carboxylic
acid since we talked about that earlier in the class).
Little side note while we are on
biological pH topics:
Hangovers: Dispelling
another myth
“Acetic acid causes acidosis
and this is why you get a hangover”
Hangovers:


Metabolic pathwayAcetaldehyde can cause migraine strength headaches and
vomiting.
Acetic acid is harmless
Alcohol decreases the Anti-diuretic hormone (adh)
Increases urine output, Increases dehydration
Depletes electrolytes leading to metabolic acidosis!