acids & bases slides
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High School Chemistry Rapid Learning Series - 21
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Rapid Learning CenterChemistry :: Biology :: Physics :: Math
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A id d BAcids and Bases
HS Ch i t R id L i S i
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HS Chemistry Rapid Learning Series
Wayne Huang, PhDKelly Deters, PhDRussell Dahl, PhD
Elizabeth James, PhD
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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 MapChemistry
Studies
Previous content
New contentand Bases
Acids and Bases Can be
Matter
pHConjugates
Form
Form
Scale to measure
Can be
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Strong or WeakStrong or Weak
Buffer
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Acid and Base Definitions
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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)
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OHH Cl
Arrhenius Acids and BasesArrhenius Acid: HCl + H2O H3O+ + Cl-
OHH Cl+1
-1
HHO
HHHHO
HH Acid
WaterHydronium ion
Arrhenius Base: NH + H O NH + + OH-
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Arrhenius Base: NH3 + H2O NH4+ + OH-
HH N HHHH
OHHHH
HH N HHHHHH
OHH
+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
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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 DefinitionsHow 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.
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Properties of Acids and BasesWhat 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.
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StrongStrong versus Weak
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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.
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Possible CombinationsThere 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 BasesThere 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
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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.
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Definition - Polyprotic Acids
Polyprotic Acid – Acid with ypmore than one proton to donate.
They ionize to give more than one H+ ions per molecule.
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Examples:Diprotic: H2CO3 (HCO3
-, CO32-)
Triprotic: H3PO4 (H2PO4- ,HPO4
2- , PO43-)
Strength of Polyprotic acidsEach 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 CO32-
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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”.
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Conjugates of Acids and Bases
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Definition - Conjugate Base
Conjugate Base – What’s left after an acid has donated its hydrogen.
HCl + H2O H3O+ + Cl-
Conjugate base
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Acid
After the proton is donated, it can now act as a base… it could accept a proton.
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Definition - Conjugate Acid
Conjugate Acid – What’s left after a base has accepted a proton.
HCl + H2O H3O+ + Cl-
Base Conjugate acid
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After the proton is accepted, it can now act as an acid… it could donate the extra proton.
Labeling SpeciesExample: 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
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NH3 + H2SO4 NH4+ + HSO4
-
AB CA CB
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Strength and ConjugatesHow 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.
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Strong Acid (e.g. HCl) Weak Conjugate Base (e.g. Cl-)Weak Acid (HCO3
-) Strong Base (OH-)Weak Base (H2O)
Strong Conjugate Base (CO32-)
Weak Conjugate Acid (H2O)Strong Conjugate Acid (H+)
Definition - Amphoteric
Amphoteric – A molecule that can act as pan acid or base.
HCl + H2O H3O+ + Cl-
Base
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NH3 + H2O NH4+ + OH-
Acid
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pH
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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
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Base
# you’re trying to reach
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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
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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:
2.0Stomach Contents
Acidic Neutral
6.5~8.0
Tap Water
Basic
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7.4Blood
Contents
3.0 Pop
4.5Beer
5.5Bread
MilkTap Water
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pH of Strong AcidsFor strong acids, assume there is a complete dissociation.
Example: Find the pH of a 0.25 M solution of HCl.
[H3O+] = 0.25 MpH = ?
HCl is a strong acid (complete dissociation):HCl + H2O H3O+ + Cl-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 BasesWhen 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 MpH = ?
pH = 13.18
pH = -log[H3O+]pH = -log(6.7x10-14M)
1.0x10-14 = [H3O+](0.15M)
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Acid-BaseAcid Base Properties of Salts
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How Salts Have Acid/Base PropertiesUnderstanding 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.
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The opposite is also true: Salts made from the conjugate of a weak base will be acidic.
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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).
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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 acidBasic salt
Buffers
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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 PO43-(conjugate base).
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Conjugates are added in the form of a salt (soluble ionic compound).
e.g. To add PO43- 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
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the buffer material would be used up.
Buffer capacity – Amount of strong acid or base that can be absorbed without large pH change.
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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
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determine the concentration.
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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.
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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.
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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.
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Acids and Bases
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