Learning objective: Using the Brønsted-Lowry theory of acids and bases. Chemistry, 2nd Canadian...

Learning objective:

Using the Brønsted-Lowry theory of acids and bases.

Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Proton Transfers in WaterBrønsted-Lowry definition of acid-base reactions

Acid – any substance that can donate a proton (H+) to another substance. Can be neutral, cations or anions

e.g. HNO3, HCl, NH4+, H2PO4

-


Proton Transfers in WaterBrønsted-Lowry definition of acid-base reactions

Base – any substance that can accept a proton (H+) from another substance. Can be neutral or anions

e.g. NH3, CO32-, PO4

3-


Conjugate Acid-Base Pairs


conjugate acid-base pair

conjugate acid-base pairThe acid has an extra proton in each case!

Types of Acids and Bases Monoprotic acids: only

capable of donating one proton HCl, HI, HS-, HPO4

-, HSO4-

Polyprotic acids: capable of donating two or more protons H2CO3, H3PO4,

H2PO4-, H2SO4, H2S

Monoprotic bases: only capable of accepting one proton Cl-, HPO4

-, HSO4-

Polyprotic bases: capable of accepting two or more protons SO4

2-, CO32-, PO4

3-


Amphiprotic: molecules or ions which can behave either way (either an acid or a base)

Autohydrolysis of Water

Water is the solvent for most acid/base reactions

Hydronium ion: hydrated hydrogen ion


2 H2O (l) H3O+ (aq) + OH-(aq)

Referred to as autohydrolysis – self-hydrolysis.Occurs in any aqueous solution.

Water Autohydrolysis Constant


14eq w 3K = K [H O ][OH ] 1 10

2 H2O (l) H3O+ (aq) + OH-(aq)

Strong Acids (Strong Electrolytes)

Strong Bases (Strong Electrolytes)

HCl Hydrochloric acid LiOH Lithium hydroxideHBr Hydrobromic acid NaOH Sodium hydroxideHI Hydroiodic acid KOH Potassium

hydroxideHNO3 Nitric acid

HClO4 Perchloric acid

H2SO4 Sulfur acid

Common Strong Acids & Strong Bases

Strong acids will donate a proton to water to form hydronium ion. The hydronium ion concentration will be equal to the acid concentration.

Strong bases will dissociate in solution to form hydroxide ion, the concentration of which can be calculated from the base’s molarity.


The pH ScaleLearning objective:

Relating pH to concentrations of ions in solution.


The pH ScaleUsed to express the hydronium ion

concentration


10 3pH log [H O (aq)]

pH3[H O (aq)] 10

Features of the pH Scale1. pH = 7.00 defines a neutral solution. Acid

solutions have pH < 7 and basic solutions have pH > 7.

2. The more acidic the solution, the lower its pH.

3. A change in pH of one unit reflects a tenfold change in the hydronium ion concentration.

4. The immense range of concentrations from > 1M to < 10-14 M is compressed into a more convenient range, from ~ -1 to ~ 15



pH and Ion ConcentrationsWhat are the concentrations of hydronium and

hydroxide ions in a beverage with a pH of 3.05?


pH and pOHRecall that Kw = [H3O+][OH-]

log10 Kw = pKw = log([H3O+][OH-])

= log[H3O+] + log[OH-]

= pH + pOH

= 14

…true in any aqueous solution


pH pOH 14

pH and pOHWhat is the pH of a 0.25M solution of NaOH?


Overview of pH and pOH

Given any one of these following: [H3O+] ,[OH-],

pH or pOH, the other three can be determined.


pH = - log [H3O+] [H3O+] = 10-pH

pOH = - log [OH-] [OH-] = 10-pOH

[H3O+][OH-] = Kw pH + pOH = pKw

At 25.0 °C: [H3O+ ][OH-] = 1.0 x 10-14

At 25.0 °C:pH + pOH = 14.00

15.3 Weak Acids and Bases

Learning objective:

Calculating concentrations and pH in weak acid and base solutions.


15.3 Weak Acids and Bases

Think back to the table of strong acids and bases. There aren’t too many listed.

What about the other acids and bases? They are referred to as weak acids and bases because they do not totally dissociate in solution.

Weak acids contain the acid and water as major species.

Weak bases contain the base and water as major species


Weak AcidsProton transfer to water is not quantitative, so

there exits an equilibrium where only a small fraction of the acid molecules have transferred their protons to water.

In a solution, the major species are water molecules and the acid, HA.

HA + H2O Ý H3O+ + A-


3a

[H O ][A ]K

[HA]

Example 15 – 6 Calculating Ka

The pH of a 0.25 M aqueous HF solution is 1.92. Calculate the Ka for this weak acid.


Percent Hydrolysis

The hydronium ion concentration is at equilibrium.

The weak acid concentration is its initial concentration .


3 eq

initial

[H O ]% HA hydrolyzed 100%

[HA]

Example 15 – 7 Percent Hydrolysis

Determine the percent hydrolysis for an aqueous solution of HF that is 25 mM.


Weak BasesProton transfer from water is not quantitative.

In solution, the major species are thus water molecules and the base, A-.

A- + H2O Ý HA + OH-


b

[HA][OH ]K

[A ]

Example 15 – 8 pH of a Weak Base

Ammonia has Kb = 1.8 x 10-5. What is the pH of 0.25 M aqueous ammonia?


15.4 Recognizing Acids and Bases

Learning objective:

Recognizing and naming some common acids and bases.


15.4 Recognizing Acids and Bases

Oxoacids – an acid that contains an inner atom bonded to a variable number of oxygen atoms and acidic OH groups.

General formula HxEOy


Some Weak Oxoacids


Carboxylic AcidsRCO2H

All carboxylic acids are weak acids.


Some Weak Acids


Example 15 – 9 Identifying Acids

Examine the following formulas. Decide if each represents a strong acid, a weak acid or neither. Justify your conclusions.

(a) Cl3CCO2H

(b) CH3CH2CH2OH

(c) HCN

(d) HClO4


Weak BasesWater is a weak base

Another common weak base is NH3. Many other weak bases are derivatives of ammonia called amines, where some of the N – H bonds have been replaced with C – H bonds.


Representative Organic Bases


Example 15 – 10 Acidic and Basic

Properties of Amino Acids

Lysine and aspartic acids are two of the amino acids found in proteins. Describe the acid-base characteristics of these compounds.


15.5 Acidic and Basic SaltsLearning objective:

Calculating the pH of solutions of salts of weak acids or bases.


15.5 Acidic and Basic Salts

An aqueous solution of a soluble salt contains cations and anions that can often have acid-base properties.

Anions that are conjugate bases of weak acids make a solution basic.

Cations that are conjugate acids of weak bases make a solution acidic.


A Basic Salt

For instance, dissolve some NaF in water:

NaF (s) → Na+ (aq) + F- (aq)

The solution is basic because:

H2O (l) + F- (aq) Ý HF (aq) + OH- (aq)


b

[HF][OH ]K

[F ]

Relating Ka, Kb and Kw

Where Ka is the ionization constant for a weak acid (e.g. HF) and Kb is the ionization constant for its conjugate base (F-).

Also, pKa + pKb = 15.00


a b wK K K

Relative Magnitudes of Ka and Kb


Acid Strength

KaConjugate Base

StrengthKb

Strong >1 Very Weak < 10-16

Weak 10-16 to 1 Weak 10-16 to 1

Very Weak < 10-16 Strong >1

Strong acids have weak conjugate bases and vice-versa

Example 15 – 11 Salt of a Weak Acid

Sodium hypochlorite (NaOCl) is the active ingredient in laundry bleach. Typically, bleach contains 5.0% of this salt by mass, which is a 0.67 M solution. Determine the concentrations of all species and compute the pH of laundry bleach.


Example 15 – 12 Salt of a Weak Base

What are the important acid-base equilibria in an aqueous solution of pyridinium chloride (C5H5NHCl)? What are the values of their equilibrium constants?


15.6 Factors Affecting Acid Strength

Learning objective:

Explaining the factors that contribute to the strength of an acid.


Factors Affecting Acid Strength

Effect of charge – affects the ability to donate and accept protons (remember opposites attract)








Structural Factors

In order to donate a proton, a molecule must break a H – X bond. This becomes easier as bond strengths decrease, and therefore the acids become stronger.


Example 15 – 13 Acidities of Simple Carboxylic Acids

Oxalic acid, HO2C – CO2H, has Ka1 = 1.3 x 10-2 and Ka2 = 1.4 x 10-4. Formic acid HCO2H, has Ka = 1.8 x 10-4. Explain why the first proton of oxalic acid is substantially more acidic than the proton of formic acid, but the second proton is less acidic.


15.7 Multiple EquilibriaLearning objective:

Calculating concentrations in solutions involving multiple equilibria.


15.7 Multiple EquilibriaThere are always at least two equilibria involving

acids and bases. The expression for the reaction of the acid (or base)

with water The expression for the autoionization of water

Usually, you only need to deal with the equilibrium that contains the major species.

Polyprotic acids provide the most common example of multiple equilibria that play a role in determining concentrations of minor species.


Polyprotic Acidse.g. Sulphurous Acid, H2SO3

In water, three equilibria exist:

H2O + H2O Ý H3O+ + OH- Kw = 1.0 x 10-14

(usually negligible)

H2SO3 + H2O Ý H3O+ + HSO3- Ka1 = 1.4 x 10-2

HSO3- + H2O Ý H3O+ + SO3

2- Ka2 = 6.3 x 10-8

Ka2 < Ka1 due to charge effects


Representative Polyprotic Acids


Example 15 – 14 Ion Concentrations in a

Polyprotic Acid Solution

Carbonated water contains carbonic acid, a diprotic acid that forms when carbon dioxide dissolves in water.

CO2 (g) + H2O (l) Ý H2CO3 (aq)

A typical carbonate beverage contains 0.050 M H2CO3. Determine the concentrations of the ions present in this solution.


Example 15 – 15 Molecular View

of a Polyprotic AcidThe drawing shows a molecular view of a very small

region of an aqueous solution of oxalic acid. For clarity, water molecules are not shown. Redraw this molecular picture to show the solution (a) after two hydroxide ions react with these molecules, and (b) after four hydroxide ions react with these molecules. Include in your drawings the water molecules that form as products.


Example 15 – 16 Ion Concentrations in a

Polyprotic Anion Solution

Potassium sulphite is commonly used as a food preservative, because the sulphite anion undergoes reactions that release sulphur dioxide, an effective preservative. Determine the concentrations of the ionic species present in a solution of potassium sulphite that is 0.075 M.


Chapter 15 Visual Summary














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Learning objective: Using the Brønsted-Lowry theory of acids and bases. Chemistry, 2nd Canadian...

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