Transcript Acids and Bases

Acids and Bases
PGCC CHM 101 Sinex
General properties
ACIDS
• Taste sour
• Turn litmus
• React with active
metals – Fe, Zn
•
•
•
•
BASES
Taste bitter
Turn litmus
Feel soapy or
slippery (react with
fats to make soap)
React with acids
• React with bases
blue to red
red to blue
Definitions
Arrehenius
only in water
• Acids – produce H+
• Bases - produce OH-
• Acids – donate H+
Bronsted-Lowry
• Bases – accept H+
any solvent
Lewis
• Acids – accept e- pair
• Bases – donate e- pair
used in organic chemistry,
wider range of substances
The hydrogen ion in aqueous solution
H+ + H2O  H3O+ (hydronium ion)
Examples
Arrhenius
HCl
Bronsted-Lowry
HCl
Lewis
BF3
NaOH
HCN
:NH3
NH3
The Bronsted-Lowry Concept
Conjugate pairs
HCl
Cl-
NH4+ NH3
CH3COOH
CH3COO-
HNO3 NO3-
How does a conjugate pair differ?
H+ transfer
Neutralization
In general: Acid + Base  Salt + Water
All neutralization reactions are double displacement reactions.
HCl + NaOH

NaCl + HOH
HCl + Mg(OH)2 
H2SO4 + NaHCO3 
Does pure water conduct electrical current?
Water is a very, very, very weak electrolyte.
H2O  H+ + OH-
How are (H+) and (OH-) related?
(H+)(OH-) = 10-14
For pure water: (H+) = (OH-) = 10-7M
This is neutrality and
at 25oC is a pH = 7.
water
Let’s examine the behavior of
an acid, HA, in aqueous solution.
HA
What happens to the HA molecules in solution?
100% dissociation of HA
HA
H+
Strong Acid
AWould the
solution be
conductive?
Partial dissociation of HA
HA
H+
Weak Acid
AWould the
solution be
conductive?
HA  H+ + A-
HA
H+
A-
Weak Acid
At any one
time, only a
fraction of
the molecules
are
dissociated.
Strong and Weak Acids/Bases
Strong acids/bases – 100% dissociation into ions
HCl
HNO3
H2SO4
NaOH
KOH
Weak acids/bases – partial dissociation,
both ions and molecules
CH3COOH
NH3
acid rain (NOx, SOx)
pH of 4.2 - 4.4 in
Washington DC area
pH
0-14 scale for the chemists
2
3
4
5
acidic
(H+) > (OH-)
normal rain (CO2)
pH = 5.3 – 5.7
6
7
8
neutral @ 25oC
(H+) = (OH-)
distilled water
fish populations
drop off pH < 6
and to zero pH < 5
9
10
11
basic or alkaline
(H+) < (OH-)
natural
waters pH =
6.5 - 8.5
12
pH of Rainwater
across United States in 2001
You
are
here!
Why is the eastern US more acidic?
http://nadp.sws.uiuc.edu/isopleths
What is acid rain?
Dissolved carbon dioxide lowers the pH
CO2 (g) + H2O  H2CO3  H+ + HCO3-
Atmospheric pollutants from combustion
NO, NO2 + H2O …  HNO3
SO2, SO3 + H2O …  H2SO4
pH < 5.3
both
strong
acids
Behavior of oxides in water– Group A
basic
1A
amphoteric
acidic
3A 4A 5A 6A 7A
2A
Group B
105
Db
107
Bh
basic: Na2O + H2O  2NaOH
(O-2 + H2O  2OH-)
acidic: CO2 + H2O  H2CO3
8A
When life goes either way
amphoteric (amphiprotic) substances
Acting like
a base
+
H+
H2CO3
accepts H+
HCO3-
Acting like
an acid
- H+
CO3-2
donates H+
pH
The biological view in the human body
acidic
1
2
3
4
5
6
basic/alkaline
7
8
9
10
Tortora & Grabowski, Prin. of Anatomy & Physiology, 10th ed., Wiley (2003)
11
Does the pH influence the
activity of an enzyme?
Trypsin is a digestive enzyme. Where?
Intestinal pH range 7.0-8.5
The amino acid glycine - amphoteric
It’s an acid and a base!
Gain of H+
Loss of H+
H3N+-CH2-COOH
H2N-CH2-COOH
H2N-CH2-COOChime structure
The amino acid glycine - Zwitterion formation
Transfer of H+ from carboxylic
acid group to amine group.
+
A dipolar
ion forms.
H2N-CH2-COOH
Chime structure
H3N+-CH2-COOintramolecular acid-base reaction
Show how water can be amphoteric.
H2O
+
H+
- H+
Dilution
water (solvent)
solute
moles of solute remain constant
Vfinal
diluted, Mfinal
molesinitial = molesfinal
Vinitial
concentrated, Minitial
adding water lowers the solute concentration
Mfinal x Vfinal = Minitial x Vinitial
Titration Calculation
HCl + NaOH
indicator

NaCl + HOH
A way to analyze solutions!
at equivalence point: moleHCl = moleNaOH
moles = M x VL
Macid x Vinitial acid = Mbase x Vburet