Unit 2 Acids and Bases http://www.cdli.ca/courses/ http://www.cbhs.k12.nf.ca/adri anyoung/ 1
Jan 06, 2016
Unit 2Acids and Bases
http://www.cdli.ca/courses/http://www.cbhs.k12.nf.ca/adrianyoung/
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Topics Properties / Operational Definitions Acid-Base Theories pH & pOH calculations Equilibria (Kw, Ka, Kb) Indicators Titrations STSE: Acids Around Us
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An Operational Definition is a list of properties, or operations that can be performed, to identify a substance.
See p. 550 for operational definitions of acids and bases
Operational Definitions
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Operational Definitions (Properties – see p. 550)
Acids pH < 7 taste sour react with
active metals (Mg, Zn) to produce hydrogen gas
Bases pH > 7 taste bitter no reaction with
active metals feel slippery
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Operational Definitions
Acids blue litmus turns
red react with
carbonates to produce CO2 gas
Bases red litmus turns
blue no reaction with
carbonates
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Operational Definitions
Acids conduct electric
current neutralize bases to
produce water and a “salt”
Bases conduct electric
current neutralize acids to
produce water and a “salt”
any ionic compound
any ionic compound6
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Acid-Base Theories
1. Arrhenius Theory (p. 549 )acid – any substance that dissociates in water to produce H+ ions
ie. an acid must contain H+ ions
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Arrhenius Theory
eg.
HCl(aq) →
H2SO4(aq) →
HSO4-(aq) →
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Arrhenius Theory
base – any substance that dissociates in water to produce OH- ions
ie. a base must contain OH- ions
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Arrhenius Theory
eg. NaOH(aq)
Ca(OH)2(aq)
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Arrhenius Theory
Which is an Arrhenius acid?
a) KOH c) CH4
b) HCN d) CH3OH
Which is a Arrhenius base?
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Limitations of Arrhenius theory (p.551)
1. H+ cannot exist as an ion in water.
The positive H+ ions are attracted to the polar water molecules forming
HYDRONIUM ions or H3O+(aq)
H+(aq) + H2O(l)
→ H3O+(aq) 14
Limitations of Arrhenius theory2. CO2 dissolves in water to produce
an acid.
NH3 dissolves in water to produce a base.
Neither of these observations can be explained by Arrhenius theory
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Limitations of Arrhenius theory3. Some acid-base reactions can occur
in solvents other than water.
Arrhenius theory can explain only aqueous acids or bases.
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Limitations of Arrhenius theory
4. Arrhenius theory is not able to predict whether certain species are acids or bases.
eg. NaHSO4 H2PO4- HCO3
-
Arrhenius theory needs some work
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Acid-Base Theories
2. Modified Arrhenius Theory (p. 552)
acid – any substance that reacts with water to produce H3O+ ions
eg.
HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)
To be used when Arrhenius is inadequate
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Modified Arrhenius Theory
base – any substance that reacts with water to produce OH- ions
eg.
NH3(aq) + H2O(l) )→ NH4+
(aq) + OH-(aq)
Acids & Bases #1: #’s 1 - 6
pp. 558, 559 #’s 1, 3, 8, & 919
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Acid-Base Theories
3. Brønsted-Lowry Theory (p. 553)
acid – any substance from which a proton (H+) may be removed
ie. an acid is a substance that loses a proton (H+)
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Brønsted-Lowry Theory
base – any substance that can remove a proton (H+) from an acid.
ie. a base is a substance that gains a proton (H+)
In BLT , an acid-base reaction requires the transfer of a proton
(H+) from an acid to a base.23
Brønsted-Lowry Theory
eg.
HCN(aq) + NH3(aq) →
acid
base
conjugate base
conjugate acid
← CN-(aq) + NH4
+(aq)
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Brønsted-Lowry Theory
What is a conjugate acid-base pair?? (p. 554)
Two particles (molecules or ions) that differ by one proton are called a conjugate acid-base pair.
The conjugate base forms when an acid loses a proton.
The conjugate acid forms when a base gains a proton (H+).
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Brønsted-Lowry Theory
acid
base
conjugate acid
conjugate base 26
Brønsted-Lowry Theory
eg.
NH3(aq) + H2O(l) → NH4+
(aq) + OH-(aq)
H2O(l) + H2O(l) →
acid
base
conjugate base
conjugate acid
←
←
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Brønsted-Lowry Theory
- an amphoteric substance can be either an acid or a base
- usually these are negative ions that contain at least one hydrogen atom
eg. H2O, HCO3-(aq), H2PO4
-(aq)
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Brønsted-Lowry Theory
p.557 #’s 1 – 7
p. 558 #’s 8, 9
p. 559 #’s 2, 4-7, 10,11
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Strength of Acids and Bases A strong acid is an acid that ionizes
or dissociates 100% in water
eg. HCl(aq)→
Strong acids react 100% with water (BLT)
eg. HCl(aq) + H2O(l) →
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Strength of Acids and Bases
NOTE: The equilibrium symbol, , is NOT used for strong acids because there is NO REVERSE REACTION.
Strong acids produce more H+ ionsOR more H3O+ ions than weak acids
with the same molar concentration
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Strength of Acids and BasesA weak acid is an acid that ionizes
or dissociates LESS THAN 100%
eg. HF(aq)
Weak acids react less than 100% with water
eg. HF(aq) + H2O(l)
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Strength of Acids and Bases
For weak acids, an equilibrium is established between the original acid molecule and the ions formed.
DO NOT confuse the terms strong and weak with concentrated and dilute.
Weak acids produce fewer H+ ionsOR fewer H3O+ ions than strong acids
with the same molar concentration
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Strength of Acids and Bases
eg. Classify the following acids: 0.00100 mol/L HCl(aq)
strong and dilute 12.4 mol/L HCl(aq)
strong and concentrated 10.5 mol/L CH3COOH(aq)
weak and concentrated34
Strength of Acids and Basesmonoprotic – acids that contain or lose
one proton
diprotic – acids that contain or lose two protons
polyprotic –
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Strength of Acids and Bases A strong base is a base that dissociates
100% in water, or reacts 100% with water, to produce OH- ion.
The only strong bases are hydroxide compounds of most Group 1 and Group 2 elements
eg. NaOH(s) → Na+(aq) + OH-
(aq)
Ca(OH)2(s) → Ca2+(aq) + 2 OH-
(aq)36
Strength of Acids and Bases A weak base is a base that reacts less
than 100% in water to produce OH- ion.
eg. S2-(aq) + H2O(l) HS-
(aq) + OH-(aq)
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Writing Acid-Base Equations (BLT)
Step 1: List all the molecules/ions present in the solution
ionic compounds form cations and anions strong acids exist as hydronium ion and
the anion (conjugate base) for weak acids use full formula of the
compound (i.e. un-ionized molecule) always include water in the list.
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Writing Acid-Base Equations (BLT)
Step 2: Identify the strongEST acid and the strongEST base from Step 1.
Step 3: Write the equation for the reaction by transferring a proton from the strongest acid to the strongest base.
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Writing Acid-Base Equations (BLT)
Step 4: Determine the type of reaction arrow to use in the equation.
Stoichiometric (100%) reactions occur between: Hydronium (H3O+) and bases stronger
than nitrite (NO2-)
hydroxide (OH-)and acids stronger than hypochlorous acid (HOCl)
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Writing Acid-Base Equations (BLT)
Step 5: Determine the position of the equilibrium by comparing the strengths of both acids in the equation.
The favoured side is the side with the weaker acid!
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Writing Acid-Base Equations (BLT)
Sample problems: Write the net ionic equation for the acid-
base reaction between:
- aqueous sodium hydroxide (NaOH(aq)) and hydrochloric acid (HCl(aq)).
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species present
Na+(aq) OH-
(aq) H3O+(aq) Cl-(aq) H2O(l)
strongest acidstrongest base
H3O+(aq) + OH-
(aq) H2O(l) + H2O(l)
H3O+(aq) + OH-
(aq) → 2 H2O(l)
OR
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Writing Acid-Base Equations (BLT)
Sample problems: Write an equation for the acid-base
reaction between nitrous acid (HNO2(aq)) and aqueous sodium sulfite (Na2SO3(aq)).
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species present
Na+(aq) SO3
2-(aq)HNO2(aq) H2O(l)
SA SB
HNO2(aq) + SO32-
(aq) NO2-(aq) + HSO3
- (aq)
Weaker AcidStronger Acid Products favored
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Write the Net Ionic Equation for each aqueous reaction below:
1. Na2CO3(aq) and CH3COOH(aq)
2. NH3(aq) and HNO2(aq)
3. HNO3(aq) and RbOH
4. H2SO4(aq) and K3PO4(aq)
5. HF(aq) and NH4CH3COO(aq)
6. CaCl2(aq) and PbSO4(aq)
p. 564 #’s 10 &11 46
1.a) species present
Na+(aq) CO3
2-(aq)
CH3COOH(aq) H2O(l)
SASB
CH3COOH(aq) + CO32-
(aq) CH3COO-(aq) + HCO3
-(aq)
Weaker Acid
Products Favoured
Stronger Acid
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species present
HNO2(aq)NH3(aq) H2O(l)
SASB
HNO2(aq) + NH3(aq) NO2-(aq) + NH4
+ (aq)
Weaker AcidProducts favoredStronger Acid
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species present
Rb+(aq) OH-
(aq)H3O+(aq) NO3
-(aq) H2O(l)
strongest acid strongest base
H3O+(aq) + OH-
(aq) H2O(l) + H2O(l)
H3O+(aq) + OH-
(aq) → 2 H2O(l)
OR
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species presentK+
(aq)HSO4-(aq) PO4
3-(aq) H2O(l)
SA SB
H3O+(aq) + PO4
3-(aq) H2O(l) + HPO4
2-(aq)
H3O+(aq)
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species present
HF(aq) H2O(l)
SA SB
HF(aq) + CH3COO-(aq) F-
(aq) + CH3COOH(aq)
Weaker AcidProducts favoredStronger Acid
NH4+
(aq) CH3COO-(aq)
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species present
Pb2+(aq) SO4
2-(aq)Ca2+
(aq) Cl-(aq) H2O(l)
strongest acid
strongest base
H2O(l) + SO42-
(aq) HSO4-(aq) + OH-
(l)
Weaker AcidReactants favored
Stronger Acid52
NO!! Products are NOT always favoured
Try these:
CH3COOH(aq) & NH4F(aq)
HCN(aq) + NaHS(aq)
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Acid-Base Calculations
Kw
Ka
Kb
[H3O+] [OH-]
pH pOH54
Kw (Ionization Constant for water)
With very sensitive conductivity testers, pure water shows slight electrical conductivity.
PURE WATER MUST HAVE
A SMALL CONCENTRATION OF
DISSOLVED IONS
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Kw
H2O(l) + H2O(l) H3O+(aq) + OH-
(aq)
K = [H3O+] [OH-]
[H2O] [H2O]Kw =
Auto-Ionization of water
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Kw
In pure water at 25 °C;
[H3O+] = 1.00 x 10-7 mol/L
[OH-] = 1.00 x 10-7 mol/L
Calculate Kw at 25 °C.
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LCP: What happens if we add OH- ions (NaOH(aq)) to water?
shift to the left [H3O+] ? [OH-] ?
Does Kw change?
GET REAL!!
H2O(l) + H2O(l) H3O+(aq) + OH-
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[H3O+] [OH-]
0.00357 M
4.89 x 10-3 mol/L
12.5 M
1.50 mol/L
Kw = [H3O+] [OH-]
1.00 x 10-14 = [H3O+] [OH-]
2.80 x 10-12
2.04 x 10-12
8.00 x 10-16
6.67 x 10-15
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Calculations with Kw (p. 564 – 566)
For strong acids and strong bases, the [H3O+] and [OH-] may be calculated using the solute concentration.
eg. What is the [H3O+] in a 2.00 mol/L solution of HNO3(aq)?
Ans: 2.00 mol/L
[OH-] = ???60
Calculations with Kw
eg. What is the [OH-] in a 2.00 mol/L solution of NaOH(aq)?
Ans: 2.00 mol/L
eg. What is the [OH-] in a 2.00 mol/L solution of Ca(OH)2(aq)?
Ans: 4.00 mol/L[H3O+] = ???
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Calculations with Kw
eg. What molar concentration of Al(OH)3(aq) is needed to obtain a [OH-] = 0.450 mol/L?
Ans: 0.150 mol/L
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What is the [H3O+] and [OH-] in:[H3O+] [OH-]
1.0 x 10-8 1.0 x 10-6
5.00 x 10-14 0.200
1.50 6.67 x 10-15
1.0 x 10-2 1.0 x 10-12
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solute [H3O+] [OH-]
0.680 mol/L HCl(aq)
1.50 M NaOH
0.0500 M Ca(OH)2(aq)
_____ mol/L HClO4(aq) 0.450 M
____ mol/L Mg(OH)2(aq) 0.500 mol/L
p. 566 #’s 12 - 15
1.47 x 10-14
6.67 x 10-15
1.00 x 10-13
0.450
0.250
2.22 x 10-14
2.00 x 10-14
0.680
1.50
0.100
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pH and pOH (See p. 568)By what factor does the [H3O+] change when the pH value changes by 1?
by 2?
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pH and pOH (See p. 568)The [H3O+] changes by a
factor of 10 (10X) for each pH changes of 1.
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pH = -log [H3O+]
[H3O+] = 10-pH
pOH = -log [OH-]
[OH-] = 10-pOH
pH and pOH FORMULAS
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pH and pOH
eg. What is the pH of a 0.0250 mol/L solution of HCl(aq)?
What is the pOH of a 0.00087 mol/L solution of NaOH(aq)?
What is the pH of a 1.25 mol/L solution of KOH(aq)? 68
Significant digits in pH values?
The number of significant digits in a concentration should be the same as the number of digits to the right of the decimal point in the pH value.eg. In a sample of OJ the
[H3O+] = 2.5 × 10−4 mol/L
pH = 3.60 (See p. 568)69
[H3O+] [OH-] pH pOH
0.0035
1.2 x 10-5
4.68
9.15
8.33 x 10-15
-1.10
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[H3O+] [OH-] pH pOH
0.0035
1.2 x 10-5
4.68
9.15
8.33 x 10-15
-1.10
2.9 x 10-12 2.46 11.54
8.3 x 10-10 9.08 4.92
2.1 x 10-5 4.8 x 10-10 9.32
4.851.4 x 10-5 7.1 x 10-10
1.20 -0.079 14.079
15.107.9 x 10-1613
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pH, pOH and Kw
p. 569 #’s 16 – 19
p. 572 #’s 20 – 25
Examine #23. Where is the energy term in this equation?
H2O(l) + H2O(l) H3O+(aq) + OH-
(aq)
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Dilutions
When a solution is diluted the number of moles does not change.
OR ninitial = nfinal
CiVi = CfVf
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eg. 400.0 mL of water was added to 25.0 mL of HCl(aq) that had a pH of 3.563. Calculate the pH of the resulting solution.calculate [H3O+]dilution formulacalculate pH
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Before dilution:[H3O+] = 10-3.563
= 2.735 x 10-4 After dilution:
(2.735 x 10-4) (25.0 mL) = (Cf)(425.0 mL)
[H3O+] = 1.609 x 10-5
pH = -log (1.609 x 10-5)
= 4.793p.574 #’s 26 - 29
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