chapter 14 - aqueous why study acids & bases? equilibria...
TRANSCRIPT
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Chapter 14 - Aqueous
Equilibria: Acids & Bases
CHM152 GCC
Why study acids & bases?
Acids and bases are important in
numerous reactions, from industrial
processes to biological ones.
Many household substances, including
cleaning solutions and food/beverages
that we consume, are acids or bases.
In the environment, the pH of rain,
water and soil can also have significant
effects.
Taste sour
Corrode most metals
Turn litmus paper red
React with bases to form
salt and water
examples: vinegar, lemon juice, gastric juice,
soft drinks
Acids Bases
Taste bitter, chalky
Feel soapy, slippery
Turn litmus paper blue
React with acids to form
salt and water
examples: milk of magnesia,
bleach, ammonia, detergents
Acid-Base Concepts
• CHM 151 taught Arrhenius theory
Acids produce hydrogen ions in water
HA (aq) H+ (aq) + A- (aq)
Bases produce hydroxide ions in water
MOH (aq) M+ (aq) + OH- (aq)
• 152 uses Brønsted-Lowry theory
Acid: Substance that can donate H+
Acid loses H+
Base: Substance that can accept H+
Base gains H+
Bronsted Lowry reactions involve the
transfer of 1 H+ ion
• Identify the acid, the base, the conjugate acid
and conjugate base.
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Acid-Base Pairs
• Identify the acid-base pairs above.
• Note: H2O is amphoteric – it can act as an acid or a base depending on what its reacting with.
Identify Conjugate Acid or Base
a) What is the conjugate acid of CO32-
b) What is the conjugate base of H2PO4-?
Strength of Acids/Bases Conjugate Acid-Base Pair Trends
1. A stronger acid loses its proton more readily
than a weaker acid and a stronger base gains
a proton more readily than a weaker base.
2. The stronger the acid, the weaker its
conjugate base. Likewise, the stronger the
base, the weaker its conjugate acid.
3. Proton transfer reactions proceed from the
stronger acid to the stronger base. Thus, the
equilibria lies on the side of the weaker acid-
base pair.
Strengths of Acids/Bases
• Competition for protons (H+)
e.g. HS-(aq) + HF(aq) H2S(aq) + F-(aq)
From Table 15.1
• Which acid is stronger?
• Which base is stronger?
• Which side does equilibria lie on?
Strong Acids
a substance that ionizes completely in water to
form H3O+ ions (H+). SA’s are strong electrolytes.
HNO3(aq) + H2O(l) → H3O+(aq) + NO3
- (aq)
A one way arrow is used since this reaction is
complete, all of the HNO3 molecules break
apart to form H3O+ and NO3
- ions.
These acids have very weak conjugate bases
For SA, eq lies very far to the right side!
KNOW 7 Strong acids! Memorize these!
HCl, HBr, HI, HClO4, HNO3, H2SO4 , HClO3
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Weak Acids
an acid that ionizes only to a small extent in H2O.
WA’s are weak electrolytes!
E.g. HCN(aq) + H2O(l) H3O+(aq) + CN-(aq)
• A two-way arrow is used since this reaction is not
complete but instead consists of an equilibrium
mixture of HCN molecules, H3O+ and CN- ions
• Equilibrium lies to the left since most of the acid
molecules have not ionized.
• Common WA’s: HF, HNO2, HCN, H3PO4, H2CO3,
carboxylic acids like CH3COOH
Strength of Acids
• HCl, H2SO4
• HNO2, HF
• H2O
Strong Bases
A substance that ionizes completely in water to
form OH- ions. SB’s are strong electrolytes!
SB dissociates in water; it doesn’t react with it:
NaOH(aq) Na+ (aq) + OH- (aq) (100 % ions)
Strong Bases: These Group 1A & 2A hydroxides
LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2,
Ba(OH)2, Sr(OH)2 Memorize the 8 SB’s!
•The reaction goes to completion – SB’s completely
dissociate into ions.
Weak Base
A base that ionizes only to a small extent in H2O.
WB’s are weak electrolytes!
NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)
Equilibrium lies to the left since most of the
ammonia molecules have not accepted protons
to form NH4+ and OH- ions.
Common examples of WB’s are ammonia and
amines (e.g. CH3NH2)
SA, WA, SB, WB solutions
Draw the following in water: HNO3, HF, Ca(OH)2, and NH3. Assume you have 3 formula units or molecules for each substance.
Hydronium Ions
• HA (aq) H+ (aq) + A- (aq)
• H+ is very reactive and will bond with O in
H2O to form H3O+
• H+ and H3O+ mean the same thing but H3O
+
is more accurate
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Dissociation of Water
• H2O (l) + H2O (l) H3O+ (aq) + OH- (aq)
• Identify the acid-base pairs
• This is the dissociation of water, an
amphoteric substance.
• Write an equilibrium expression for this eqn.
• Kw is ion-product constant for water.
• Calculate Kw if [H3O+] = 1.0 x 10-7
(experimentally determined for pure H2O)
Dissociation of Water
• Kw = [H3O+][OH-] = 1x10-14
• We can compare [H3O+] & [OH-] for solutions.
• They’re equal in neutral solutions.
• Which one is greater in acidic solutions?
• Basic solutions?
• [H3O+] = 5.6 x 10-4 M. What is [OH-]?
• Is solution acidic or basic?
Acidic, Basic, Neutral Solns pH Scale
• Logarithmic scale (like Richter scale for
earthquakes).
• [H3O+] = 1x10-2 M has a pH of 2
• [H3O+] = 1x10-12 M has a pH of 12
• pH = -log [H3O+] [H3O
+] = 10-pH
• pOH = -log [OH-] [OH-] = 10-pOH
• Kw = [H3O+][OH-] = 1x10-14
• pH + pOH = 14
pH Sig Figs
[H3O+] = 5.6 x 10-4 M
• pH = -log (5.6 x 10-4) = 3.25 (3 related to
exponent; tells us acidity)
2 sig figs
Find [H3O+], pH, and pOH if [OH-] = 9.8x10-9 M
2 decimal places
Group Quiz #9
Calculate [H3O+], [OH-] and pH for a solution if
the pOH is 10.325.
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pH of Strong Acids/Bases
HNO3(aq) + H2O(l) H3O+(aq) + NO3
-(aq)
Calculate pH of 0.103 M HNO3 solution.
Sr(OH)2 Sr2+(aq) + 2OH-(aq)
Calculate pH of 0.020 M Sr(OH)2 soln.
Calculate [H3O+], [OH-], pH, and pOH of a solution
made by adding 2.250 g of LiOH to 250.0 mL of water.
Equilbria of Weak Acids
• HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Write equilibrium expression.
• pKa = -log Ka: stronger acid = lower pKa
• % Ionization = [H3O+] / [HA]i x 100%
• As acid strength increases, indicate if the following will be higher or lower: [H3O
+]
pH
% ionization
Ka
pKa
pKa’s of Weak Acids Weak Acid Problems
• Set up ICE tables & Ka
• With small K values, assume x will be small to
simplify math.
WA problem types:
• Given [HA]i and Ka, find all equilibrium
concentrations and pH
• Given [HA]i and pH, find all equilibrium
concentrations and Ka
• Given [HA]i and %dissociation, find eq
concentrations, pH and Ka
Find pH of Weak Acid
Find the equilibrium concentrations for all
substances and the pH of a 0.50 M solution of
HOCl (hypochlorous acid). Ka = 3.5 x 10-8
ICE Table
HOCl + H2O H3O+ + OCl-
I
C
E
Example 14.10, Problems 14.14, 14.15
Find Ka for weak acid
• Find the Ka of a 1.25 M solution of nitrous
acid, HNO2. The pH of this solution is
measured to be 1.62.
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Percent Ionization
• % dissociation is the same calculation as
checking assumption of a small x.
% dissociation = ([H3O+]eq / [HA]i) x 100
• Will the percent dissociated increase or
decrease as an acid is diluted?
• Calculate the percent dissociation from the
previous problem.
Dilution and % Dissociation
• As we add
water, we can
dissociate
more. More
dilute = more
dissociation.
Find Ka for weak acid
• A 0.200 M solution of a weak acid is 9.4
percent dissociated. Using this information,
calculate [H3O+], [A-], [HA]eq, and Ka.
Group Quiz #10
• The acid-dissociation constant for benzoic
acid, HC7H5O2, is 6.3 x 10-5. Calculate the
equilibrium concentrations of H3O+,
C7H5O2-, and HC7H5O2 in the solution if the
initial concentration of the acid is 0.50 M.
Polyprotic Acids
• H2C2O4(aq) + H2O(l) HC2O4-(aq) + H3O
+(aq)
Ka1 = [HC2O4-][H3O
+] / [H2C2O4] = 5.9 x 10-2
• HC2O4-(aq) + H2O(l) C2O4
2-(aq) + H3O+(aq)
Ka2 = [C2O42-][H3O
+] / [HC2O4-] = 6.4 x 10-5
• In general, Ka1 > Ka2 > Ka3 … Why?
• pH of a polyprotic acid solution primarily arises from
[H+] formed in the 1st step.
Weak Bases
• Similar to WA problems but base hydrolysis reaction forms OH- (this is x!) and use Kb
• NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)
Kb = [NH4+][OH-] / [NH3]
• Table 14.4: Kb Values for Weak Bases
• Why are amines (derivatives of NH3) basic?
• Lone e- pair (d-) on N attracts H (d+) from water. N wins the competition and pulls H+ off of water.
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Weak Bases
• Calculate the pH of a 0.50 M dimethylamine ((CH3)2NH) solution. Kb = 5.4 x 10-4
Example 14.12, Problems 14.19, 14.20
Group Quiz #11
A 0.065 M solution of methylamine, CH3NH2,
has a pH of 11.70. What is Kb for CH3NH2?
Relation Between Ka and Kb
• Weak acid: Ka = [H+][A-] / [HA]
• Weak base: Kb = [HA][OH-] / [A-]
• Multiply: Ka * Kb
• Ka·Kb = [H+] [OH-] = Kw Ka·Kb = 10-14
• True only if the acid and base are conjugate pairs!
• What is the Kb for hydrazine (N2H4) if Ka for N2H5+ is
1.1 x 10-8?
• Example 14.13, Problem 14.21
Properties of Salts
• Acid + Base Salt + Water
• Salts are ionic products of acid-base reaction.
• Cation in salt: comes from the base
• Anion in salt: comes from the acid
• Salts can be neutral, acidic, or basic depending
on the strength of the acid or base from which
they’re made
Hydrolysis Reactions
• Many salts cause pH shifts when dissolved in
water. These salts contain the conjugate acid (or
base) of a weak base (or acid).
NH4Cl: NH4+ + H2O NH3 + H3O
+
• pH < 7
NaCH3CO2: CH3COO- + H2O CH3COOH + OH-
• pH > 7
• These reactions are called hydrolysis.
General Ion Categories
1) Acidic ions: Most cations (except 1A & 2A)
E.g. NH4+, Al3+, Cu2+
2) Neutral Ions: cations from strong bases and
anions from strong acids
Cations: Group 1A & 2A (except Be)
Anions: NO3-, Cl-, Br -, I-, ClO4
-, ClO3-(except HSO4
-)
3) Basic Ions: Most anions derived from weak
acids.
E.g. F-, CN-, CH3COO- , NO2-
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Types of Salts
• Neutral Salts
NaOH + HCl NaCl + H2O
Acid and base are strong, but resulting salt is neutral.
Neutral ions don’t hydrolyze!
• Acidic Salts
HCl + NH3 NH4Cl
strong acid + weak base
Ammonium salts are acidic (cation from weak base, anion from strong acid)
Types of Salts
Basic Salts
• Salts made from weak acids and strong bases give basic salts
• Anion from weak acid; cation from strong base
• Give an example.
Acidic cations and basic anions
(NH4)2CO3: NH4+ acts as weak acid; CO3
2- acts as weak base. Have to compare Ka and Kb values in order to determine if its acidic or basic.
Example Salt Problems
• Is KF an acidic, basic or neutral salt?
Write the hydrolysis reaction and calculate the pH of a 0.10 M KF solution. Ka (HF) = 3.5x10-4
• Is NH4Cl acidic, basic, or neutral?
Write the hydrolysis reaction and calculate the pH of a 0.10 M NH4Cl solution.
Kb (NH3) = 1.8 x 10-5
Classify Salts
• Classify the following salts as acidic, basic,
or neutral. For acidic or basic salts, write
the reaction of hydrolysis.
KBr
NaF
LiCN
NH4Cl
Group Quiz #12
• Classify the following salts as acidic, basic, or neutral. For acidic or basic salts, write the equation of hydrolysis.
NH4ClO3
Ba(NO2)2
CaI2
Lewis Acids and Bases
• Broadest acid base definition since Lewis acids don’t have to contain H+.
• Looks at electron transfer instead of H+ transfer
• Acids are electron-pair acceptors, bases are electron-pair donors
H+ + :NH3 NH4+
Ammonia donates the electron pair to H+ to make the bond; H+ accepts the electron pair
This definition is used in CHM235/236 where you learn to push e- pairs!
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Lewis Acids and Bases
Metal cations= Lewis acids
Small, highly charged metals also make
acidic salts; Al(H2O)63+ (Ka = 1.4 x 10-5)
High charge on metal electrons on O are
attracted to +metal cation.
Metal acids
• Al3+ forms acid solutions! Easier to break
O-H bonds and form hydronium!
Acid Strength Factors
• Why is HF weak but HCl, HBr, and HI strong?
• Determined by strength and polarity of H-X bond.
• H-F is strongest in series (H and F are small and
close together); other bonds are weaker and break
easily in water
Acid Strength Factors
• In the same row, bond strengths are relatively
similar, but electronegativity changes are greater
Acid Strength Factors