High School Chemistry Rapid Learning Series - 21: Acids, Bases, Conjugates and pH

High School Chemistry Rapid Learning Series - 21
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A id d BAcids and Bases
HS Ch i t R id L i S i
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© Rapid Learning Inc. All rights reserved.
HS Chemistry Rapid Learning Series
Wayne Huang, PhD
Kelly Deters, PhD
Russell Dahl, PhD
Elizabeth James, PhD

Learning Objectives
Several definitions of “acid”
and “base”.
By completing this tutorial you will learn about…
Strong versus weak acids and
bases.
Conjugate acids and bases.
How to find pH of strong acids
and bases.
H lt b idi
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How salts can be acidic or
basic.
How buffers work.
Titrations.
Concept Map
Chemistry
Studies
Previous content
New content
and Bases
Acids
and Bases Can be
Matter
pHConjugates
Form
Form
Scale to measure
Can be
4/42
Strong
or Weak
Strong
or Weak
Buffer

Acid and Base
Definitions
5/42
Definition - Arrhenius
Arrhenius Acid – Produces the
hydronium ion in waterhydronium ion in water.
Arrhenius Base – Produces the
h d id i i
H3O+
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hydroxide ion in water.
OH-
Note: Neutralization is the combination of H3O+ and OH- to form
H2O, i.e. H3O+(aq) + OH-(aq) 2H2O(l)

O
HH Cl
Arrhenius Acids and Bases
Arrhenius Acid: HCl + H2O H3O+ + Cl-
O
HH Cl+1
-1
HH
O
HHHH
O
HH Acid
Water
Hydronium ion
Arrhenius Base: NH + H O NH + + OH-
7/42
Arrhenius Base: NH3 + H2O NH4
+ + OH-
HH
N
HH
HH
O
HHHH
HH
N
HH
HH
HH
O
HH
+1
-1
Definition - Brønsted-Lowry
Brønsted-Lowry Acid – Donates a proton (H+).
Brønsted-Lowry Base – Accepts a proton (H+).y p p ( )
Example: NH3 + HCl NH4
+ + Cl-
H+ donor – Bronsted-Lowry AcidH+ acceptor – Bronsted-Lowry Base
8/42

Definition – Lewis
Lewis Acid – Electron (lone pair) Acceptor.
Lewis Base – Electron (lone pair) Donor.
Example: BCl3 + :NH3 Cl3B-NH3
Electron pair acceptor – Lewis Acid Electron pair donor – Lewis Base
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Comparing the Definitions
How do the 3 definitions relate?
Acid Base Note
Produces
H3O+ in water.
Donates H+.
Accepts
electrons.
Produces
OH- in water.
Accepts H+.
Donates
electrons.
Requires water.
Does not need to
be in water.
Does not need to
use “H+”.
Arrhenius
Brønsted-
Lowry
Lewis
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Many Arrhenius acids/bases are also
Brønsted-Lowry and Lewis acids/bases.

Properties of Acids and Bases
What are some common properties?
Acids Bases
Taste sour (e.g. citrus). Taste bitter (e.g. soap).
React with metals to
form H2 gas.
Have pH levels below 7.
Turn Litmus red.
Feel slippery.
Have pH levels above 7.
Turn Litmus blue.
11/42
StrongStrong
versus
Weak
12/42

Definition - Strong and Weak
Strong – Most of the molecules
perform their “duty”.
e.g. Strong Acid: Most of the molecules donate
their H+ and completely ionized in solution.
(i.e. HCl H+ + Cl-, Ka = [H+][Cl-]/[HCl] … large).
Weak – Very few of the molecules
perform their “duty”.
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p y
e.g. Weak Acid: Only a very few of the
molecules donate their H+ and partially ionized
in solution.
(i.e. HC2H3O2 C2H3O2
- + H+,
Ka = [H+][C2H3O2
-]/[HC2H3O2] … small).
Acetic Acid can
be written as:
HC2H3O2 or
CH3COOH or
simply HAc.
Definition - Concentrated and Dilute
Concentrated – Many acid or
base molecules have beenbase molecules have been
added to the system with high
concentration, i.e. 16M HCl.
Dilute – Only a few acid or
base molecules have been
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added to the system with low
concentration, i.e. 0.16M HCl.

Possible Combinations
There are 4 combinations of strong, weak, concentrated
and dilute:
Concentrated Dilute
A lot added & almost
all do their “duty”.
A lot added, but very
f d th i “d t ”
Not many added, but
of what is there,
most will do their
“duty”.
Only a few added
and of those, only a
ll % d th i
Strong
Weak
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few do their “duty”. very small % do their
“duty”.
Weak
Common Strong Acids and Bases
There are only a few common strong acids and bases to
remember—the rest will most likely be weak!
HX(HCl, HBr, HI), HNO3, H2SO4, HClO4, HClO3
Sr(OH)2 , Ca(OH)2, Ba(OH)2, NaOH, KOH
Strong Acids
Strong Bases
16/42
Strong Acid Mnemonic: HX, HNO3, H2SO4, HClO4(3) = “Strong acids are eXtra
Nasty, Sour and Clear.”
Strong Base Mnemonic: Sr(OH)2, Ca(OH)2, Ba(OH)2, NaOH, KOH = “Strong bases
(for beach fun) are Surf, CaBaNa & Kayak.

Definition - Polyprotic Acids
Polyprotic Acid – Acid withyp
more than one proton to
donate.
They ionize to give more than
one H+ ions per molecule.
17/42
Examples:
Diprotic: H2CO3 (HCO3
-, CO3
2-)
Triprotic: H3PO4 (H2PO4
- ,HPO4
2- , PO4
3-)
Strength of Polyprotic acids
Each proton is “weaker” than the one before.
A negatively
charged ion is
An acid donates
a proton.
e.g: H2CO3
The results is a
negatively
charged ion.
Becomes HCO3
-
charged ion is
less likely to give
up another
proton and
become -2
charged.
Would be CO3
2-
18/42
H2CO3 is a stronger acid than HCO3
- .
Sulfuric acid (H2SO4) is the only common strong polyprotic acid.
The 1st hydrogen is “strong” and the 2nd one is “weak”.

Conjugates of
Acids and Bases
19/42
Definition - Conjugate Base
Conjugate Base – What’s left
after an acid has donated its
hydrogen.
HCl + H2O H3O+ + Cl-
Conjugate base
20/42
Acid
After the proton is donated, it can now act as a base…
it could accept a proton.

Definition - Conjugate Acid
Conjugate Acid – What’s left
after a base has accepted a
proton.
Base Conjugate acid
21/42
After the proton is accepted, it can now act as an
acid… it could donate the extra proton.
Labeling Species
Example: For each of the following, label the acid (A), the base
(B), the conjugate acid (CA) and conjugate base (CB).
NH3 + H2O NH4
+ + OH-
3 2 4
H2SO4 + H2O H3O+ + HSO4
-
AB CA CB
A B CA CB
22/42
NH3 + H2SO4 NH4
+ + HSO4
-
AB CA CB

Strength and Conjugates
How does the strength of a species relate to the
strength of its conjugate?
If it donated the
A strong acid
donates it’s
proton easily.
After donating
the proton, it
then becomes a
(conjugate) base.
If it donated the
proton easily, it
will not easily
grab and hold
onto another
one.
It will be a weak
base.
23/42
Strong Acid (e.g. HCl) Weak Conjugate Base (e.g. Cl-)
Weak Acid (HCO3
-)
Strong Base (OH-)
Weak Base (H2O)
Strong Conjugate Base (CO3
2-)
Weak Conjugate Acid (H2O)
Strong Conjugate Acid (H+)
Definition - Amphoteric
Amphoteric – A molecule that can act asp
an acid or base.
Base
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NH3 + H2O NH4
+ + OH-
Acid

pH
25/42
Definition - Logarithms
Logarithm – The number of
times a base must be
multiplied by itself to reach amultiplied by itself to reach a
given number.
# of multiples
x = logb(y) y = bx
26/42
Base
# you’re trying to reach

Definition - pH
pH scale – Logarithmic scale of the
acidity of a solution.
The pH scale uses base “10”, i.e. b = 10.
pH = - log[H3O+] …. x = logb(y)
log[H3O+] = 10-pH …. y = bx
27/42
pH has no units.
Note: pOH provides another way to express the basicity, which is defined
as pOH = -log[OH-] or [OH-] = 10-pOH. pOH + pH = 14.00 or the ionic
product of water Kw = [H3O+][OH-] = 10-14.
pH Scale
0 7 14
Some common items and their pH values:
2.0
Stomach
Contents
Acidic Neutral
6.5
~8.0
Tap Water
Basic
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7.4
Blood
Contents
3.0
Pop
4.5
Beer
5.5
Bread
Milk
Tap Water

pH of Strong Acids
For strong acids, assume there is a complete
dissociation.
Example: Find the pH of a 0.25 M solution of HCl.
[H3O+] = 0.25 M
pH = ?
HCl is a strong acid (complete dissociation):
If [HCl] = 0.25 M, then [H3O+] = 0.25 M
pH = -log[H3O+]
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pH ?
pH = 0.60
pH = -log(0.25M)
pH of Strong Bases
When working base problems, you can find [OH-] by the
base concentration…but you must find [H3O+] to find
pH.
Example: Find the pH of a 0.15 M solution of NaOH.
NaOH is a strong base:
NaOH Na+ + OH-
If [NaOH] = 0.15 M, then [OH-] = 0.15 M
Kw = [H3O+][OH-]
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[H3O+] = 6.7×10-14 M
pH = ?
pH = 13.18
pH = -log[H3O+]
pH = -log(6.7x10-14M)
1.0x10-14 = [H3O+](0.15M)

Acid-BaseAcid Base
Properties of
Salts
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How Salts Have Acid/Base Properties
Understanding conjugates is a key to understanding
acid/base properties of salts.
A weak acid has
a strong
conjugate base
that can form
salts.
That strong
conjugate base
will produce a
basic solution.
Salts made from
the conjugate of
a weak acid will
have a basic pH.
32/42
The opposite is also true: Salts made from the conjugate of a
weak base will be acidic.

Combinations Forming Salts
Cation from… Strong Acid Weak Acid
Anion from…
Strong Base
Weak Base
Neutral Salt
Acidic Salt
Basic Salt
Neutral Salt
Think of it as the “strong on wins” (e.g. strong acid & weak base = acidic).
33/42
Example: Determine if each salt will be acidic, basic or neutral:
NH4Cl
NaH2PO4
Weak base & strong acidCame from NH3 and HCl.
Acidic salt
Came from NaOH and H3PO4. Strong base & weak acid
Basic salt
Buffers
34/42

Definition - Buffer
Buffer – Solution containing a weak acid and its
conjugate or weak base and its conjugate that resists
change in pH levels (upon addition of small amount
of acid or base).)
e.g. H3PO4 (weak acid) and PO4
3-(conjugate base).
35/42
Conjugates are added in the form of a salt (soluble
ionic compound).
e.g. To add PO4
3- to a buffer, Na3PO4 salt is added.
How Does a Buffer Work?
Buffer: Weak Acid and
conjugate base.
How does a buffer resist pH changes?
Conjugate base “absorbs”
acid and produces more
weak acid already in buffer.
Strong Acid added
Weak acid “absorbs” base
and produces more
conjugate base already in
buffer.
Strong Base added
Eventually, so much strong acid or base could be added and all of
36/42
the buffer material would be used up.
Buffer capacity – Amount of strong acid or base
that can be absorbed without large pH change.

Titrations
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Definition - Titrations
Tit ti Additi fTitration – Addition of a
known volume of a known
concentration solution to a
known volume of unknown
concentration solution to
determine the concentration
38/42
determine the concentration.

Definition - End Point
End Point (or Stoichiometric
Point or Equivalent Point) –
When there is no reactant left
over they have all been reacted
End Point (indicator
changes color)
over - they have all been reacted
and the solution contains only
products (color change).
Indicators – Paper or liquid that
h l b d h H
The end point must be reached in order to use stoichiometry
to calculate the unknown solution concentration.
39/42
changes color based on the pH
level (range).
If the pH of the products is known, the
indicator (can be chosen to indicate
the end point (or stoichiometric point
if titrated correctly).
Before the
endpoint
At the
endpoint
There are several
t d fi id
There are several
t d fi id
Learning Summary
pH can bepH can be
ways to define acids
and bases.
ways to define acids
and bases.
40/42
pH can be
determined from the
concentration of
hydronium ions in a
solution.
pH can be
determined from the
concentration of
hydronium ions in a
solution.
Some acids and
bases are strong,
while
others are weak.
Some acids and
bases are strong,
while
others are weak.

Congratulations
You have successfully completed
the core tutorial
Acids and Bases
Wh t’ N t
Chemistry :: Biology :: Physics :: Math
What’s Next …
Step 1: Concepts – Core Tutorial (Just Completed)
Step 2: Practice – Interactive Problem Drill
Step 3: Recap – Super Review Cheat Sheet
42/42
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High School Chemistry Rapid Learning Series - 21: Acids, Bases, Conjugates and pH

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