Acids and Bases
Characteristics of:
Acids
Bases



pH < 7
Sour tasting
Indicators
o Litmus: red
o Phenolphthalein: clear
o BTB: yellow





Not slippery to touch
Metals react by releasing H2 gas


pH > 7
Bitter tasting
Indicators
o Litmus: blue
o Phenolphthalein:
pink
o BTB: blue
Slippery
No reaction with metals
Aside:
H+1 is interchangeable with H3O+1
In an Organic acid, the hydrogen is at end of the formula.
In an Inorganic acid, the hydrogen is in front of the formula.
Theoretical Definitions of Acids and Bases
Svante Arrhenius definition of an acid and a base:

Acid releases H+1 in water
Example: HCl
Example: CH3COOH

Base releases OH-1 in water
Example:
Example:
Bronsted-Lowry definition of an acid and a base




Acid an acid is a proton donor (and it does not have to be in water)
Base is a proton acceptor.
An acid always has a conjugate base and a base always has a conjugate acid. They
always occur in pairs. A conjugate acid-base pair only differs by a proton.
According to the Bronsted-Lowry concept, acid-base reactions are universally
reversible and results in an acid-base equilibrium.
Examples:
CH3COOH + H2O
NH3 + H2O
H3O+1 + CH3COO-1
NH4+1 + OH-1
Important information:
Substances like H2O that can act as an acid or a base are called AMPHIPROTIC or
AMPHOTERIC.
Example: H2O + H2O
Example: HCO3-1 + H2O
Example: HCO3-1 + H2O
A substance that can transfer more than one proton is POLY PROTIC
Example:
Hydrochloric acid:
Sulfuric acid:
Phosphoric acid:
Before we proceed:
A strong acid has a weak attraction for protons while a strong base has a
very strong attraction for protons.
The stronger the acid the weaker its conjugate base and the weaker the
acid the stronger its conjugate base.
Strong Acids:
Strong acid is an acid that ionizes quantitatively (completely) in water to form hydrogen
ions. The percent ionization of a strong acid is approximately 99% or greater.
 HCl
 HBr
 HI
 HNO3
 H2SO4
 HClO4
What about the oxyacids with variable numbers of oxygens?
HClO
HClO2
HClO3
HClO4
Strong Bases


All oxides and hydroxides of alkali metals.
Alkaline earth metal oxides and hydroxides below beryllium.
Water Equilibrium
Experiments have shown that pure, distilled water conducts slightly. This indicates that a
small number of ions are present.
Auto ionization of water:
H2O
+
H2O
At SATP it was found that in one billion water molecules approximately TWO water
molecules ionize. The reason why so few ions are present has to do with the molecules
having the right orientation and energy.
The equilibrium law expression:
It has been calculated that the concentration of water molecules in pure water or in a
dilute aqueous solution is essentially constant:
Calculate the concentration of water by taking the density of water in g/L and dividing it
by the molar mass of water.
Revisiting the equilibrium law expression, the water concentration value combines with
the equilibrium constant of water to produce the ion product for water, Kw.
Equilibrium equation for auto ionization shows that there is a 1:1 ration of hydroxide ions
to hydrogen ions in pure water. At 25C the concentration of both hydroxide and
hydrogen ions is the same. From this the equilibrium constant for the ionization of water
can be determined.
Note the water value changes with temperature:
Auto ionization takes place in all the aqueous solutions but when acids or bases dissolved
in water then is not equal.