Transcript Unit Twelve Acids and Bases - Chippewa Falls High School

Strengths
 What is an electrolyte?
 A solution that contains ions and will conduct electrical current
 Acids and bases ionize (molecular) or dissociate (ionic) into ions
 Dissociation – when ionic compounds break apart to form ions

Examples: NaCl, NaOH, Na2SO4
 Ionization – when molecular compounds break apart to form ions
(acids)

Examples: HCl, H2SO4
 Dissolving – when solute molecules are surrounded by solvent
molecules and go into solution
Strengths
 Strengths depend on how much ionization/dissociation
takes place
 Strong acids and bases completely ionize/dissociate in
solution


100% dissociated (lots of H+ and OH-)
Strong electrolytes
 Weak acids and bases do not completely ionize/dissociate
in solution


< 100% dissociated (only a few H+ and OH-)
Weak electrolytes
Strong versus weak
 For strong acids and bases we use a single arrow to
indicate the forward reaction is favored
 For weak acids and bases we use a double arrow to
indicate the partial ionization (forward and reverse
reactions take place)
Strong Acids and Bases
 There are six strong acids
 HClO4
 H2SO4
 HNO3
 HCl
 HBr
 HI
 There are eight strong bases
 LiOH
 NaOH
 KOH
 RbOH
 CsOH
 Ca(OH)2
 Sr(OH)2
 Ba(OH)2
Water
 Amphoteric – water can act as an acid or a base
 Self-ionizations – water can also act as an acid and a
base with itself! (Even neutral, pure, distilled water)
Kw
 In pure water at 25°C, the preceding reaction occurs only to
a very small extent, resulting in equal, small concentrations
of H+ and OH–.
 [H+] = [OH–] = 1.0 x 10-7 M
 Ion product constant for water (Kw) – the product of the
concentration of H+ and OH– in aqueous solutions
 Kw = [H+][OH–]
 Kw = (1.0 x 10-7 M)(1.0 x 10-7 M)
 Kw = 1.0 x 10-14 M
 Can be used for aqueous solutions at 25°C (↑temp, ↑
movement, ↑ dissociation)
 Kw will not change when the concentrations change
because strengths are based on the amount of ionization.
Kw
 Since Kw is constant, and Kw = [H+] [OH‾], it follows
that:
 If [H+] increases, then [OH‾] decreases, and
 If [H+] decreases, then [OH‾] increases.
 In a neutral solution: [H+] = [OH-]
 In an acidic solution: [H+] > [OH-]
 In a basic solution: [H+] < [OH-]
pH and pOH Scale
 The pH scale relates to the strengths
 pH scale measures the hydrogen ion concentration and the
pOH measures the hydroxide ion concentration
 Logarithmic scale – a change in 1 pH unit corresponds to a
tenfold change in [H+] (lime (pH=2) versus plum(pH=3))
 pH < 7 : acidic solution
 pH = 7 : neutral
 pH > 7 : basic solution
pOH
13
12
11
10
9
8
7
6
5
4
3
2
1
Calculations
 Kw = [H+] [OH‾]
 14 = pH + pOH
 pH = -log[H+]
 [H+] = 10-pH
 pOH = -log[OH‾]
 [OH‾] = 10-pOH
Practice Problems
 What are the [H+] and [OH‾] concentrations in a 0.01 M
HCl solution?
 What are the [H+] and [OH‾] concentrations in a 0.0001
M NaOH solution?
 What are the [H+] and [OH‾] concentrations in a 0.00001
M HNO3 solution?
 What is the pH of a 0.0001 M HNO3 solution?
 What is the pOH of a 0.001 M KOH solution?
Practice Problems
 Calculate the [H+] of a solution with a pH of 8.37.
 What is the concentration of OH- in a solution with a
pOH of 4.80?
 Calculate the pH of a solution with [OH-] = 1.3 x 10-2 M.
Is the solution acidic or basic? Hint: Start with KW first
to find [H+].
 Calculate the OH- concentration for a solution with a
pH of 3.66.
 Worksheet Two will be due Thursday