Acids and Bases

PGCC CHM 101 Sinex

General propertiesACIDS

• Taste sour• Turn litmus• React with active

metals – Fe, Zn

• React with bases

BASES• Taste bitter• Turn litmus• Feel soapy or

slippery (react with fats to make soap)

• React with acids

blue to red red to blue

Definitions• Acids – produce H+

• Bases - produce OH-

• Acids – donate H+

• Bases – accept H+

• Acids – accept e- pair• Bases – donate e- pair

Arrehenius

Bronsted-Lowry

Lewis

only in water

any solvent

used in organic chemistry,wider range of substances

ExamplesArrhenius

Bronsted-Lowry

Lewis

HCl NaOH

HCl NH3

:NH3BF3

HCN

The hydrogen ion in aqueous solutionH+ + H2O H3O+ (hydronium ion)

The Bronsted-Lowry Concept

Conjugate pairs

HCl Cl- CH3COOH CH3COO-

NH4+ NH3 HNO3 NO3

-

How does a conjugate pair differ?H+ transfer

NeutralizationIn general: Acid + Base Salt + Water

All neutralization reactions are double displacement reactions.

HCl + NaOH NaCl + HOH

HCl + Mg(OH)2

H2SO4 + NaHCO3

H2O H+ + OH-

Does pure water conduct electrical current?

(H+)(OH-) = 10-14

For pure water: (H+) = (OH-) = 10-7MThis is neutrality and at 25oC is a pH = 7.

Water is a very, very, very weak electrolyte.

How are (H+) and (OH-) related?

water

HA

Let’s examine the behavior of an acid, HA, in aqueous solution.

What happens to the HA molecules in solution?

HAH+

A-

Strong Acid

100% dissociation of HA

Would the solution be conductive?

HAH+

A-

Weak Acid

Partial dissociation of HA

Would the solution be conductive?

HAH+

A-

Weak Acid

HA H+ + A-

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 NaOHHNO3 KOHH2SO4

Weak acids/bases – partial dissociation, both ions and molecules

CH3COOH NH3

pH

2 3 4 5 6 7 8 9 10 11 12

neutral @ 25oC(H+) = (OH-)

distilled water

acidic(H+) > (OH-)

basic or alkaline(H+) < (OH-)

natural waters pH = 6.5 - 8.5

normal rain (CO2)pH = 5.3 – 5.7

acid rain (NOx, SOx)pH of 4.2 - 4.4 in Washington DC area

0-14 scale for the chemists

fish populationsdrop off pH < 6 and to zero pH < 5

You are

here!

http://nadp.sws.uiuc.edu/isopleths

pH of Rainwateracross United States in 2001

Increasing acidity

Why is the eastern US more acidic?

air masses

What is acid rain?

CO2 (g) + H2O H2CO3 H+ + HCO3-

Dissolved carbon dioxide lowers the pH

Atmospheric pollutants from combustionNO, NO2 + H2O … HNO3

SO2, SO3 + H2O … H2SO4

bothstrong acids

pH < 5.3

105Db

107Bh

Behavior of oxides in water– Group Abasic amphoteric acidic

3A 4A 5A 6A 7A1A

2A

8A

Group B

basic: Na2O + H2O 2NaOH(O-2 + H2O 2OH-)

acidic: CO2 + H2O H2CO3

When life goes either wayamphoteric (amphiprotic) substances

HCO3-

H2CO3 CO3-2

+ H+ - H+

Acting like a base

Acting like an acid

accepts H+ donates H+

pH

1 2 3 4 5 6 8 9 10 11

The biological view in the human body

gastric juice

vaginal fluid

urinesalivacerebrospinal fluid

bloodpancreatic juice

bileacidic basic/alkaline

7

Tortora & Grabowski, Prin. of Anatomy & Physiology, 10th ed., Wiley (2003)

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 - amphotericIt’s an acid and a base!

Loss of H+Gain of H+

H2N-CH2-COOHH3N+-CH2-COOH H2N-CH2-COO-

Chime structure

The amino acid glycine - Zwitterion formation

Transfer of H+ from carboxylic acid group to amine group.

H2N-CH2-COOHH3N+-CH2-COO-

+ -

A dipolar ion forms.

intramolecular acid-base reaction

Chime structure

Show how water can be amphoteric.

H2O

+ H+ - H+

Dilutionwater (solvent) solute

concentrated, Minitial

diluted, Mfinal

adding water lowers the solute concentration

moles of solute remain constant

Vinitial

Vfinal

molesinitial = molesfinal

Mfinal x Vfinal = Minitial x Vinitial

Titration CalculationHCl + NaOH NaCl + HOH

at equivalence point: moleHCl = moleNaOH

moles = M x VL

Macid x Vinitial acid = Mbase x Vburet

A way to analyze solutions!

indicator