Transcript chem 102
Chemistry 102(001) Fall 2014
Instructor: Dr. Upali Siriwardane
Office
: CTH 311
Phone
257-4941
Office Hours
: M,W 8:00-9:30 & 11:00-12:30 am; Tu,Th, F 8:00 10:00 am. or by appointment.;
Test Dates
:
September 23 , 2014 (Test 1): Chapter 13 October 16, 2014 (Test 2): Chapter 14 &15 November 11, 2014 (Test 3): Chapter 16 &7 November 13 , 2014 (Make-up test) comprehensive: Chapters 13-17
CHEM 102 Fall 2014, LA TECH 15-1
Chapter 15
.
Acids and Bases
15.1 Heartburn 659 15.2 The Nature of Acids and Bases 660 15.3 Definitions of Acids and Bases 662 15.4 Acid Strength and the Acid Dissociation Constant (K a ) 665 15.5 Autoionization of Water and pH 668 15.6 Finding the and pH of Strong and Weak Acid Solutions 673 15.7 Base Solutions 682 15.8 The Acid –Base Properties of Ions and Salts 685 15.9 Polyprotic Acids 693 15.1 0 Acid Strength and Molecular Structure 698 15.1 1 Lewis Acids and Bases 700 15.1 2 Acid Rain 701
CHEM 102 Fall 2014, LA TECH 15-2
Types of Reactions
a)
Precipitation Reactions . Reactions of ionic compounds or salts b) Acid/base Reactions.
Reactions of acids and bases c) Redox Reactions. reactions of oxidizing & reducing agents
CHEM 102 Fall 2014, LA TECH 15-3
What are Acids &Bases?
Definition?
a) Arrhenius b) Bronsted-Lowry c) Lewis
CHEM 102 Fall 2014, LA TECH 15-4
Arrhenius Definitions
Arrhenius, Svante August (1859-1927), Swedish chemist, 1903 Nobel Prize in chemistry
•
Acid solution. Anything that produces hydrogen ions in a water HCl (aq) H + ( aq) + Cl ( aq)
•
Base solution.
Anything that producs hydroxide ions in a water
•
NaOH (aq) Na + ( aq) + OH ( aq) Arrhenius definitions are limited proton acids and hydroxide bases to aqueous solutions.
CHEM 102 Fall 2014, LA TECH 15-5
Brønsted-Lowry definitions
Expands the Arrhenius definitions to include many bases other than hydroxides and gas phase reactions Acid Proton donor Base Proton acceptor This definition explains how substances like ammonia can act as bases.
Eg. HCl(g) + NH 3 (g) ------> NH 4 Cl(s) HCl (acid), NH 3 (base).
NH 3 (g) + H 2 O(l) NH 4 + + OH -
CHEM 102 Fall 2014, LA TECH 15-6
Lewis Definition
G.N. Lewis was successful in including acid and bases without proton or hydroxyl ions. Lewis Acid: A substance that accepts an electron pair. Lewis base: A substance that donates an electron pair.
E.g. BF 3 (g) + :NH 3 (g) F 3 B:NH 3 (s) the base donates a pair of electrons to the acid forming a coordinate covalent bond common to coordination compounds. Lewis acids/bases will be discussed later in detail
CHEM 102 Fall 2014, LA TECH 15-7
1) Acids and bases can be defined in several ways. Which definitions of the bases that fits the description below?
a) a compound that produces more OH- ions in water: b) a proton acceptor: c) an electron pair donor:
CHEM 102 Fall 2014, LA TECH 15-8
Types of Acids and Bases
Binary acids Oxyacid Organic acids Acidic oxides Basic oxides Amine Polyprotic acids
CHEM 102 Fall 2014, LA TECH 15-9
Types of Acids and Bases
Binary acids : HCl, HBr, HI, H 2 S More than two elements : HCN Oxyacid : HNO 3 , H 2 SO 4 , H 3 PO 4 Polyprotic acids : H 2 SO 4 , H 3 PO 4 Organic acids : R-COOH, R= CH 3 -, CH 3 CH 2 Acidic oxides : SO 3 , NO 2 , CO 2 , Basic oxides : Na 2 O, CaO Amine :
NH
3
.
R-NH 2 , R= CH 3 -, CH 3 CH 2 - : primary R 2 -NH : secondary, R 3 -N: tertiary Lewis acids & bases: BF 3 and NH 3
CHEM 102 Fall 2014, LA TECH 15-10
Acidic Oxides
These are usually oxides of non-metallic elements such as P, S and N.
E.g. NO 2 , SO 2 , SO 3 , CO 2 They produce oxyacids when dissolved in water SO 3 CO 2 + H + H 2 2 O ---> H O ---> H 2 2 SO CO 4 3 NO2 + H 2 O ---> HNO 3
CHEM 102 Fall 2014, LA TECH 15-11
Basic Oxides
Oxides oxides of metallic elements such as Na, K, Ca.
They produce hydroxyl bases when dissolved in water.
e.g
.
CaO + H 2 O ---> Ca(OH) 2 Na 2 O + H 2 O ---> 2 NaOH
CHEM 102 Fall 2014, LA TECH 15-12
Protic Acids
Monoprotic Acids : The form protic refers to acidity due to protons.
Monoprotic acids have only one acidic proton. e.g.
HCl.
Polyprotic Acids : They have more than one acidic proton.
e.g. H 2 SO 4 H 3 PO 4 - diprotic acid - triprotic acid.
CHEM 102 Fall 2014, LA TECH 15-13
Polyprotic Acids
acids where more than one hydrogen per molecule is released
CHEM 102 Fall 2014, LA TECH 15-14
Amines
Class of organic bases derived from ammonia NH 3 by replacing hydrogen by organic groups. They are defined as bases similar to NH 3 Lowery or Lewis by acid/base Bronsted definitions.
CHEM 102 Fall 2014, LA TECH 15-15
Amines
CHEM 102 Fall 2014, LA TECH 15-16
2) Identify types of acids/bases as: binary acids, oxy acids, organic acids, acidic oxides, basic oxides, amine and polyprotic acids.
a) HF d) H 2 SO 4 g) NO 2 k) R-NH 2
CHEM 102 Fall 2014, LA TECH
b) HBr e) HNO h) SO 3 3 c) H 3 PO 4 f) R-COOH j)CaO
15-17
Influence of Molecular Structure on Acid Strength
Binary Hydrides
•
hydrogen & one other element Bond Strengths
•
weaker the bond, the stronger the acid Stability of Anion
•
higher the electronegativity, stronger the acid
CHEM 102 Fall 2014, LA TECH 15-18
Binary Acids
Compounds containing acidic protons bonded to a more electronegative atom.
e.g. HF, HCl, HBr, HI, H 2 S The acidity of the haloacid (HX; X = Cl, Br, I, F) Series increase in the following order: HF < HCl < HBr < HI
CHEM 102 Fall 2014, LA TECH 15-19
Oxyacids
Compounds containing acidic - OH groups in the molecule.
Acidity of H 2 SO 4 is greater than H 2 SO 3 the extra O (oxygens) because of The order of acidity of oxyacids from the a halogen (Cl, Br, or I) shows a similar trend.
HClO 4 > HClO 3 > HClO 2 > HClO perchloric chloric chlorus hyphochlorus
CHEM 102 Fall 2014, LA TECH 15-20
Influence of Molecular Structure on Acid Strength
Oxyacids
•
hydrogen, oxygen, & one other element
•
H-O-E higher the electronegativity on E, stronger the acid as this weakens the bond between the O and H
CHEM 102 Fall 2014, LA TECH 15-21
Oxo Acid
< < < <
CHEM 102 Fall 2014, LA TECH 15-22
3) Which of the following is stronger acid or base: a) H 2 SO 4 or H 2 SO 3 : b) HCl or HI: c) HClO or HClO 3 : d) H 2 S or HF: e) CF 3 COOH or CH 3 COOH: f) CH 3 COOH or CH 3 CH 2 COOH
CHEM 102 Fall 2014, LA TECH 15-23
3) Which of the following is stronger acid or base: a) H 2 SO 4 or H 2 SO 3 : K a H 2 SO 4 >> 1 ; H 2 SO 3 = 1.4 x 10 -2 b) HCl or HI: c) HClO or HClO 3 : d) H 2 S or HF: H 2 S: K a :H 2 S=6.3 x 10 -8 ; HF= 6.3 x 10 -4 e) CF 3 COOH or CH 3 COOH: 2.5 x 10 -3 and 1.8 x 10 -5 f) CH 3 COOH or CH 3 CH 2 COOH: 1.8 x 10 -5 and 1.2 x 10 -5
CHEM 102 Fall 2014, LA TECH 15-24
Dissociation
Strong Acids : HCl(aq) + H 2 O(l) H 3 + O(aq) + Cl (aq) H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 (aq) Dissociation Equilibrium Weak Acid/base : H 2 O(l) + H 2 O(l) H 3 + O(aq) + OH (aq) This dissociation is called autoionization of water. HC 2 H 3 O 2 (aq) + H 2 O(l) H 3 + O(aq) + C 2 H 3 O 2 (aq) NH 3 (aq) + H 2 O(l) NH 4 + + OH (aq) Equilibrium constants: K a , K b and K w
CHEM 102 Fall 2014, LA TECH 15-25
4) Write equations for the dissociation equilibrium reactions for the following acids and bases in water. Which of these are acid or dissociations?
a) HCl: b) H 2 SO 4 :
CHEM 102 Fall 2014, LA TECH 15-26
4) Write equations for the dissociation equilibrium reactions for the following acids and bases in water. Which of these are acid or dissociations?
c) H 2 O (autoionization): What is auto ionization? d) HC 2 H 3 O(acetic acid): e) NH 3 :
CHEM 102 Fall 2014, LA TECH 15-27
Bronsted acid/conjugate base and base/conjugate acid pairs in acid/base equilibria HCl(aq) + H 2 O(l) H 3 + O(aq) + Cl (aq) HCl(aq): acid H 2 O(l): H 3 + O(aq): Cl (aq): H 2 O/ H 3 + O: HCl/Cl : base conjugate acid conjugate base base/conjugate acid pair acid/conjugate base pair
CHEM 102 Fall 2014, LA TECH 15-28
Brønsted-Lowry Definitions
Conjugate acid-base pairs.
Acids and bases that are related by loss or gain of H+ as H 3 O + and H 2 O.
Examples.
Acid Base H 3 O + HC 2 H 3 O 2 NH 4 + H 2 SO 4 HSO 4 H 2 O C 2 H 3 O 2 NH 3 HSO 4 SO 4 2-
CHEM 102 Fall 2014, LA TECH 15-29
Select acid, base, acid/conjugate base pair, base/conjugate acid pair
H 2 SO 4 (aq) + H 2 O(l) acid base conjugate acid conjugate base base/conjugate acid pair acid/conjugate base pair H 3 + O(aq) + HSO 4 (aq)
CHEM 102 Fall 2014, LA TECH 15-30
5) For HOCl write: a) Dissociation equilibrium reaction for the HOCl: b) Identify the acid/conjugate base pair: c) Identify the base/conjugate acid pair: d) The equilibrium constant expression:
CHEM 102 Fall 2014, LA TECH 15-31
Strong Acid vs. Weak Acids
Strong acid completely ionized Hydrioidic HI Hydrobromic HBr Perchloric HClO 4 Hyrdrochloric HCl Chloric Sulfuric Nitric HClO 3 H 2 SO 4 HNO 3 Weak acid partially ionized Hydrofluoric acid HF Formic acid HCOOH Acetic acid CH 3 COOH Nitrous acid HNO 2 Acetyl Salicylic acid C 9 H 8 O 4 Hydrocyanic acid HCN K a K a K a K a K a K a K a ~ 10 ~ 10 ~ 10 ~ 10 ~ 10 ~ 10 ~ 20 11 9 7 7 3 2 K a K a K a K a K a K a = 6.6x10
-4 = 1.77x10
-4 = 1.76x10
-5 = 4.6x10
-4 = 3x10 -4 = 6.17x10
-10 pK a pK a pK a pK a pK a pK a pK a = -11 = -9 = -7 = -7 = -3 = -2 = -1.3
pK a pK a pK a pK a pK a pK a = 3.18
= 3.75
= 4.75
= 3.34
= 3.52
= 9.21
CHEM 102 Fall 2014, LA TECH 15-32
Strong Base vs. Weak Base
Strong Base completely ionized Lithium hydroxide Sodium hydroxide Potassium hydroxide Rubidium hydroxide Cesium hydroxide Boarder-line Bases Magnesium hydroxide Calcium hydroxide Strotium hydroxide Barium hydroxide Weak Base partially ionized Ammonia Ethyl amine
CHEM 102 Fall 2014, LA TECH
LiOH NaOH KOH RbOH CsOH Mg(OH) 2 Ca(OH) 2 Sr(OH) 2 K b ~ 0.01 to0.1 Ba(OH) 2 NH 3 CH 3 CH 2 NH 2 K b ~ 10 2 -10 3 K b =1.79x10
-5 K b =5.6x10
-4 pK b pK b = 4.74
= 3.25
15-33
•
Acid and Base Strength
Strong acids Ionize completely in water. HCl, HBr, HI, HClO 3 , HNO 3 , HClO 4 , H 2 SO 4 .
•
Weak acids Partially ionize in water.
Most acids are weak.
•
Strong bases
•
Weak bases Ionize completely in water. Strong bases are metal hydroxides - NaOH, KOH Partially ionize in water.
CHEM 102 Fall 2014, LA TECH 15-34
Common Acids and Bases
Acids nitric hydrochloric sulfuric acetic Formula Molarity* HNO HCl 3 16 12 H 2 SO 4 HC 2 H 3 O 2 18 18 Bases ammonia sodium hydroxide *undiluted.
CHEM 102 Fall 2014, LA TECH
NH 3 (aq) NaOH 15 solid
15-35
6. Identify stronger and weaker acids:
CHEM 102 Fall 2014, LA TECH 15-36
Autoionization of Water
Autoionization When water molecules react with one another to form ions.
H 2 O(l) + H 2 O(l) H 3 O+(aq) + OH (aq) (10 -7 M) (10 7M) Acids and bases alter the dissociation equilibrium of water based on Le Chaterlier’s principle Kw = [ H 3 O + ] [ OH ] = 1.0 x 10 -14 at 25 o C ion product of water Note: [H 2 O] is constant and is included in K w .
CHEM 102 Fall 2014, LA TECH 15-37
What is pH?
K w = [H 3 + O][OH ] = 1 x 10 -14 [H 3 + O][OH ] = 10 -7 x 10 -7 Extreme cases: Basic medium [H 3 + O][OH ] = 10 -14 x 10 0 Acidic medium [H 3 + O][OH ] = 10 0 x 10 -14 pH value is -log[H + ] spans only 0-14 in water.
CHEM 102 Fall 2014, LA TECH 15-38
pH and other “p” scales
Substance pH
We need to measure and use acids and bases over a very
1 M HCl 0.0
pOH
Coffee
pH + pOH
Pure Water
= 14 = -log[H 3O + ] = -log[OH ]
Blood Milk of Magnesia Household ammonia 1M NaOH
CHEM 102 Fall 2014, LA TECH
2.2 - 2.4
2.5
5.0
7.0
7.35 - 7.45
10.5
12.0
14.0
15-39
pH scale
A logarithmic scale used to keep track of the large changes in [H+].
0 7 14 10 0 M 10 -7 Very M Neutral acidic 10 -14 M Very Basic When you add an acid to, the pH gets smaller.
When you add a base to, the pH gets larger.
CHEM 102 Fall 2014, LA TECH 15-40
pH of Aqueous Solutions
CHEM 102 Fall 2014, LA TECH 15-41
pH, pK
w
and pOH
The relation of pH, K w and pOH K w = [H + ][OH ] log K w = log [H + ] + log [OH ] -log K w = -log [H + ] -log [OH ] ; previous equation multiplied by -1 pK w = pH + pOH; pK w since K w =1 x 10 -14 = 14 14 = pH + pOH pH = 14 - pOH pOH = 14 - pH
CHEM 102 Fall 2014, LA TECH 15-42
Measuring pH Arnold Beckman inventor of the pH meter father of electronic instrumentation
CHEM 102 Fall 2014, LA TECH 15-43
7.
Identify the following as acidic/basic/neutral and estimate/calculate pH.
Solution Acidic/basic/neutral pH of the solution a) [ H + ] > [OH ] and [H + ] > 1.0 x 10 -7 M : : b) [H + ] < [OH ] and [H + ] < 1.0 x 10 -7 M : : c) [H + ] = [OH ] = 1.0 x 10 -7 M : : d) [H + ] > [OH ] = 1.0 × 10 14 M : : e) [H + ]< [OH ] = 1.0 x 10 +7
M : : CHEM 102 Fall 2014, LA TECH 15-44
pH and pOH calculations of acid and base solutions
a) Strong acids/bases dissociation is complete for strong acid such as HNO 3 or base NaOH [H + ] is calculated from molarity (M) of the solution b) weak acids/bases needs K a , K b or percent(%)dissociation
CHEM 102 Fall 2014, LA TECH 15-45
pH of 0.5 M H
2
SO
4
Solution
H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 (aq
)
HSO 4 (aq
)
+ H 2 O(l) H 3 + O(aq) + SO 4 2 (aq
)
[H 3 + O][HSO 4 ] H 2 SO 4 ; K a1 = ------------------ [H 2 SO 4 ] [H 3 + O][ SO 4 2 ] H 2 SO 4 ; K a2
CHEM 102 Fall 2014, LA TECH
= ------------------- ; K a2 ignored [HSO 4 ]
15-46
pH of 0.5 M H
2
SO
4
Solution
H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 (aq ) the moles of H + of H 2 SO 4 ions in the solution is equal to moles at the beginning. [H 2 SO 4 ] = [H + ] = 0.5 mole/L pH = -log [H + ] pH = -log(0.5) pH = 0.30
CHEM 102 Fall 2014, LA TECH 15-47
1.5 x 10
-2
M NaOH.
1.5 x 10 -2 M NaOH.
NaOH is also a strong base dissociates completely in water. [NaOH] = [HO ] = 1.5 x 10 -2 mole/L pOH = -log[HO ]= -log(1.5 x 10 -2 ) pOH = 1.82 As defined and derived previously: pK w = pH + pOH; pK w = 14 pH = pK w + pOH pH = 14 - pOH pH = 14 - 1.82 ; pH = 12.18
CHEM 102 Fall 2014, LA TECH 15-48
8) For a 0.10 M solution of Ba(OH) 2 .Is it a strong base?
a) How many OH- are in the compound?
b) Calculate the [OH ] and [H+]: c) pH of the solution: CHEM 102 Fall 2014, LA TECH 15-49
9) Calculate the pH of the strong acid 0.2 M H 2 SO 4 .
a) Is it a strong acid?
b) Is it polyprotic acid?
c) Dissociation equilibria: d) Why second dissociation equilibria is not considered for [H + ] concentration?
e) Calculate the [H + ] f) pH of the solution: CHEM 102 Fall 2014, LA TECH 15-50
pH of Mixtures of Strong and Weak Acids
the presence of the strong acid retards the dissociation of the weak acid The pH of the solution is mainly based on the strong acid Eg. 1.0 M HCl and 1.0 HC 2 H 3 O 2 HCl(aq) + H 2 O(l)
H 3 + O(aq) + Cl (aq) HC 2 H 3 O2(aq) + H 2 O(l) H 3 + O(aq) + C 2 H 3 O 2 -(aq)
CHEM 102 Fall 2014, LA TECH 15-51
pH of Mixtures of Acids and Bases
The pH of the solution is mainly based on the excess acid or base present Eg. 10 mL of 1.0 M HCl and 20 mL 1.0 NaOH Moles of excess NaOH ( M x L) = 1.0 x 0.001= 0.001
Mixed together volume = 30 mL = 0.030 L Molarity of excess NaOH = 0.001/0.030= 0.030
Calculate pOH and then pH
CHEM 102 Fall 2014, LA TECH 15-52
Equilibrium, Constant, K
a
& K
b K a : Acid dissociation constant for a equilibrium reaction. K b : Base dissociation constant for a equilibrium reaction. Acid: HA + H 2 O H 3 + O + A Base: BOH + H 2 O B + + OH [H 3 + O][ A ] [B + ][OH ] K a = --------------- ; K b = ---------------- [HA] [BOH]
CHEM 102 Fall 2014, LA TECH 15-53
Acid Dissociation Constant
HCl(aq) + H 2 O(l) H 3 + O(aq) + Cl (aq)
[H
3 +
O][Cl-] K
a
= ---------------- [HCl] [H
+
][Cl-] K
a
= ---------------- [HCl]
CHEM 102 Fall 2014, LA TECH 15-54
Base Dissociation Constant
NH 3 + H 2 O NH 4 + + OH K = [NH 4 + ][OH ] [NH 3 ]
CHEM 102 Fall 2014, LA TECH 15-55
Comparing K
w
and K
a
& K
b Any compound with a K a value greater than K w water will be a an acid in water. of Any compound with a K b value greater than K w water will be a base in water. of
CHEM 102 Fall 2014, LA TECH 15-56
Ionization Constants for Acids
CHEM 102 Fall 2014, LA TECH 15-57
WEAKER/STRONGER Acids and Bases & K
a
and K
b
values
A larger value of K a or K b indicates an equilibrium favoring product side.
Acidity and basicity increase with increasing K a K b . or pK a = - log K a and pK b = - log K b Acidity and basicity decrease with increasing pK a or pK b .
CHEM 102 Fall 2014, LA TECH 15-58
Which is weaker?
a. HNO 2 b. HOCl 2 c. HOCl d. HCN ; K a = 4.0 x 10 -4 .
; K a = 1.2 x 10 -2 .
; K a = 3.5 x 10 -8 .
; K a = 4.9 x 10 -10 .
CHEM 102 Fall 2014, LA TECH 15-59
What is K
a1
and K
a2
?
H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 (aq) HSO 4 (aq) + H 2 O(l) H 3 + O(aq) + SO 4 2 (aq)
CHEM 102 Fall 2014, LA TECH 15-60
K
a
Examples
H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 (aq
)
HSO 4 (aq
)
+ H 2 O(l) H 3 + O(aq) + SO 4 2 (aq
)
[H 3 + O][HSO 4 ] H 2 SO 4 ; K a1 = ------------------ [H 2 SO 4 ] [H 3 + O][SO 4 2 ] H 2 SO 4 ; K a2 = ------------------ [HSO 4 ]
CHEM 102 Fall 2014, LA TECH 15-61
K
a
Examples
HC 2 H 3 O 2 (aq) + H 2 O(l) H 3 + O(aq) + C 2 H 3 O 2 (aq) [H + ][C 2 H 3 O 2 ] H C 2 H 3 O 2 ; K a = ----------------- [H C 2 H 3 O 2 ] NH 3 (aq) + H 2 O(l) NH 4 + [NH 4 + ][OH ] + OH (aq) NH 3 ; K b = ------------- [ NH 3 ]
CHEM 102 Fall 2014, LA TECH 15-62
% Dissociation gives x (amount dissociated) need for pH calculation
Amount dissociated % Dissoc. = ------------------------- x 100 Initial amount/con.
x % Dissoc. = --------------------------- x 100 concentration
CHEM 102 Fall 2014, LA TECH 15-63
How do you calculate pH of weak acids/bases?
a) From % dissociation b) From K a or K b What is % dissociation Amount dissociated % Dissoc. = ------------------------- x 100 Initial amount
CHEM 102 Fall 2014, LA TECH 15-64
How do you calculate % dissociation from K
a
or K
b 1.00 M solution of HCN; K a = 4.9 x 10 -10 What is the % dissociation for the acid?
CHEM 102 Fall 2014, LA TECH 15-65
1.00 M solution of HCN; K
a
= 4.9 x 10
-10 1.00 M solution of HCN; K a = 4.9 x 10 -10 First write the dissociation equilibrium equation: HCN(aq) + H 2 O(l) <===> H [HCN] [H + ] 3 + O(aq) + CN (aq) [CN ] Ini. Con.
1.00 M 0.0 M 0.00 M Cha. Con -x Eq. Con. 1.0 - x x x x x K a = [H 3 + O ][CN ] ------------------ [HCN]
CHEM 102 Fall 2014, LA TECH
x 2 = --------------- 1.0 - x
15-66
1.00 M solution of HCN; K
a 1.0 - x ~ 1.00 since x is small
= 4.9 x 10
-10 x 2 K a = -----------; K a = 1.0
x = 4.9 x 10 -10 4.9 x 10 -10 = = 2.21 x 10 -5 x 2 Amount disso.
---------------- x 100 Ini. amount 2.21 x 10 -5 =- ------------- x 100 1.00
% Diss.
=2.21 x 10 -5 x 100 = 0.00221 %
CHEM 102 Fall 2014, LA TECH 15-67
10) Calculate the % dissociation of 2.00 M solutions of HCN (K a = 4.9 x 10 -10 ) a) Dissociation equilibria: b) ICE setup: c) Amount dissociated: d) % dissociation : : CHEM 102 Fall 2014, LA TECH 15-68
Calculate the pH of a weak acid from % dissociation 1 M HF, 2.7% dissociated Notice the conversion of % dissociation to a fraction (x): 2.7/100=0.027) x=0.027
CHEM 102 Fall 2014, LA TECH 15-69
Calculate the pH of a weak acid from % dissociation HF(aq) + H 2 O(l) <===> H [H + ][F ] 3 + O(aq) + F (aq) K a = ---------- [HF] [HF] Ini. Con.
[H 1.00 M + ] 0.0 M [F ] Chg. Con -x x Eq.Con.
1.0-0.027 0.027
pH = -log [H + ] pH = -log(0.027) pH = 1.57 0.00 M x 0.027
CHEM 102 Fall 2014, LA TECH 15-70
11) Calculate the K a of if 5.0 M HF, 2.7% dissociated: a) Dissociation equilibrium: b) ICE setup: c) Amount dissociated: d) K a : CHEM 102 Fall 2014, LA TECH 15-71
Weak acid Equilibria
Example Determine the pH of a 0.10 M benzoic acid solution at 25 o C if
K
a = 6.28 x 10 -5 HBz (aq) + H 2 O (l) H 3 O + (aq) + Bz -(aq) The first step is to write the equilibrium expression
K
a = [H 3 O + ][Bz ] [HBz]
CHEM 102 Fall 2014, LA TECH 15-72
Weak acid Equilibria
HBz Initial conc., M 0.10
Change,
D
M -
x
Eq. Conc., M 0.10 -
x
H 3 O + 0.00
x x x x
Bz 0.00
[H 3 O + ] = [Bz ] =
x
We’ll assume that [Bz ] is negligible compared to [HBz]. The contribution of H 3 O + also negligible.
from water is
CHEM 102 Fall 2014, LA TECH 15-73
Weak Acid Equilibria
Solve the equilibrium equation in terms of
x K x
a = 6.28 x 10 -5 = x 2 0.10
= (6.28 x 10 -5 )(0.10) H 3 O + = 0.0025 M pH = 2.60
CHEM 102 Fall 2014, LA TECH 15-74
pH from K
a
or K
b 1.00 M solution of HCN; K a = 4.9 x 10 -10 First write the dissociation equilibrium equation: HCN(aq) + H 2 O(l) H 3 + O(aq) + CN (aq) [HCN] [H + ] [CN ] Ini. Con.
1.00 M 0.0 M 0.00 M Chg. Con -x x Eq. Con. 1.0 - x x x x
CHEM 102 Fall 2014, LA TECH 15-75
Weak Acid Equilibria
[H 3 + O ][CN ] K a = -------------- [HCN] pH = 4.65
CHEM 102 Fall 2014, LA TECH
= x 2 --------------- 1.0 - x K a 1.0 - x ~ 1.00 since x is small x 2 = -----------; K 1.0
a = 4.9 x 10 -10 = x 2 x = 4.9 x 10 -10 pH = -log [H + ] = 2.21 x 10 -5 pH = -log(2.21 x 10 -5 )
15-76
The Conjugate Partners of Strong Acids and Bases
The conjugate acid/base of a strong base/acid has no net effect on the pH of a solution The conjugate base of a weak acid hydrolyze in water and basic or pH of a solution > 7.00 E.g. Na + C 2 H 3 O 2 acetate sodium The conjugate acid of a weak base hydrolyze in water and acidic or pH of a solution < 7.00 E.g NH 4 Cl
CHEM 102 Fall 2014, LA TECH 15-77
12) Calculate the [H + ], [OH ] and pH of 0.90 M HC 2 H 3 O 2 ; Ka= 1.8 x 10 -5
.
a) Dissociation equilibria: b) ICE setup: c) [H + ] and [OH ]: d) pH: CHEM 102 Fall 2014, LA TECH 15-78
13) Calculate the [H + ], [OH ] pOH and pH 5.0 M NH 3 ; K b = 1.8 x 10 -5 a) Dissociation equilibria: b) ICE setup: c) [H+] and [OH-]: d) pOH and pH: CHEM 102 Fall 2014, LA TECH 15-79
14) Calculate the pH of a 0.015 M solution of lactic acid. The K a for lactic acid is 1.4 x 10 -4 .
CHEM 102 Fall 2014, LA TECH 15-80
15) Calculate the pH of a 0.14 M solution of an acid with K a = 6.2 x 10 -8 (pH = 4.03) CHEM 102 Fall 2014, LA TECH 15-81
Acid-Base Properties of Typical Ions
CHEM 102 Fall 2014, LA TECH 15-82
Hydrolysis
Reaction of a basic anion or acidic cation with water is an ordinary Brønsted-Lowry acid-base reaction.
CH 3 COO (aq) NH4 + (aq) + H 2 O (l) + H 2 O (l) CH 3 COOH (aq) NH 3 (aq) + OH (aq) + H 3 O + (aq) This type of reaction is given a special name.
Hydrolysis The reaction of an anion with water to produce the conjugate acid and OH .
The reaction of a cation with water to produce the conjugate base and H 3 O + .
CHEM 102 Fall 2014, LA TECH 15-83
What salt solutions would be acidic, basic and neutral?
1) strong acid + strong base = neutral 2) weak acid + strong base = basic 3) strong acid + weak base = acidic 4) weak acid + weak base = neutral, basic or an acidic solution depending on the relative strengths of the acid and the base.
CHEM 102 Fall 2014, LA TECH 15-84
What pH? Neutral, basic or acidic?
• •
a)NaCl neutral
• •
b) NaC 2 H 3 O 2
• •
c) NaHSO 4
•
d) NH 4 Cl
• CHEM 102 Fall 2014, LA TECH
basic
acidic acidic
15-85
1) If the following substance is dissolved in pure water, will the solution be acidic, neutral, or basic?
a) Solid sodium carbonate-(Na 2 CO 3 ): b) Sodium chloride- (NaCl): c) Sodium acetate- (NaC 2 H 3 O 2 ): d) Ammonium sulfate-((NH 4 ) 2 SO 4 ): CHEM 102 Fall 2014, LA TECH 15-86
How do you calculate pH of a salt solution?
Find out the pH, acidic or basic?
If acidic it should be a salt of weak base If basic it should be a salt of weak acid if acidic calculate K a from K a = K w /K b if basic calculate K b from K b = K w /K a Do a calculation similar to pH of a weak acid or base
CHEM 102 Fall 2014, LA TECH 15-87
What is the pH of 0.5 M NH
4
Cl salt solution? (NH
3
; K
b
= 1.8 x 10
-5
)
Find out the pH, acidic if acidic calculate K a from K a = K w /K b K a = K w /K b = 1 x 10 -14 /1.8 x 10 -5 ) K a = 5.56. X 10 -10 Do a calculation similar to pH of a weak acid
CHEM 102 Fall 2014, LA TECH 15-88
Continued
NH 4 + + H 2 O H 3 + O + NH 3 [NH 4 + ] [H 3 + O ] [NH 3 ] Ini. Con.
0.5 M 0.0 M 0.00 M Change -x x x Eq. Con. 0.5 - x K a (NH 4 + ) = x [H 3 + O ] [NH 3 ------------------- ] [NH 4 + ] x 2 ---------------- ; (0.5 - x) appro.:0.5 - x .
= x 0.5
CHEM 102 Fall 2014, LA TECH 15-89
Continued
x 2 K a (NH 4 + ) = ----------- = 5.56 x 10 -10 0. 5 x 2 = 5.56 x 10 -10 x 0.5 = 2.78 x 10 -10 x= 2.78 x 10 -10 = 1.66 x 10 -5 [H + ] = x = 1.66 x 10 -5 M pH = -log [H + ] = - log 1.66 x 10 -5 pH = 4.77
pH of 0.5 M NH 4 Cl solution is 4.77 (acidic )
CHEM 102 Fall 2014, LA TECH 15-90