Objectives:
Connect pH numbers to what they represent.
Practice converting pH values.
Evaluate strengths of acids & bases.
What is the chemical equation for
this beaker of distilled water?
H2O + H2 ↔ H3O+ + OHor
+
H2O ↔ H + OH
DEMONSTRATION
 Form water molecules
 Move around room
 Dissociate & ressociate randomly
 Equilibrium is a dynamic state
 It is rare for a hydrogen atom to leave the stability of
the water molecule
 More likely to happen if water is
 heated or
 Influenced by other chemicals
Chemical Equilibrium
 Is the concentration of water the same as the total
concentration of H+ and OH-?
 No
 What does it mean to be at equilibrium?
 The reaction is still flowing back and forth, however,
there is no net gain in either products or reactants
H2O ↔ H+ + OH-
Chemical Equilibrium
 Can an equation tend to one direction or the other?
 Yes (usually in the direction of the products)
 Which is in higher concentration in the beaker?
H2O or [H+ + OH- ]?
 H2O
pH Concentration
 How do you write the concentration of pH value 3?
 H+ = 1 x 10-3
 How do you write pH 3 as a decimal?
 0.001
 How do you write pH 3 as a fraction?
 1/1,000
 What does this mean?
 If you reach into the beaker you would find only one
dissociated H+ ion for every intact H2O molecules.
Calculate the ratio of dissociated to
intact water molecules in a beaker
of distilled water of pH 7 at 25C
 pH 7, concentration of hydrogen [H+] = 1 x 10-7
 Which is [H+] = 1/10,000,000
 Which is one dissociated molecule of H2O for every 10
million intact H2O molecules
 What is the difference in hydrogen concentration
between pH 5 and pH 2?
 How are the concentrations of [H+] and [OH-] related?
 What is the concentration of [OH-] if the pH is 3?
 What is the concentration of [OH-] if the pH is 13?
 Which of these two examples is more basic/alkaline?
Electronegativity & pH
 Electronegativity can reveal the strengths of acids and
bases
 If a hydrogen atom is in a molecule with other atoms
that have a very strong ability to dominate the electrons,
the the molecule will be polarized with hydrogen being
stripped of its electron (think HCl)
 If the hydrogen does not have much association with its
electron, then it leaves the molecule to become an ion
and the solution is said to be more acidic
QUESTIONS
 What is the difference between a strong acid/base and
a weak acid/base?
 A strong acid/base dissociates readily

Difference in electronegativity greater
 A weak acid/base dissociates less readily

Minimal difference in electronegativity
 Which acids/bases are weaker and which are stronger
– HCl, NH3, H2CO3, NaOH?
 Strong – HCl and NaOH
 Weak –NH3 and H2CO3