a the nature of acids and bases b acid strength c the ph scale d calculating the ph of strong acids l.
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Lecture #20. (a) The Nature of Acids and Bases; (b) Acid Strength; (c) The pH Scale; (d) Calculating the pH of Strong Acids. Chemistry 142 B Autumn Quarter 2004 J. B. Callis, Instructor. Definition

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a the nature of acids and bases b acid strength c the ph scale d calculating the ph of strong acids

Lecture #20

(a) The Nature of Acids and Bases; (b) Acid Strength; (c) The pH Scale; (d) Calculating the pH of Strong Acids

Chemistry 142 B

Autumn Quarter 2004

J. B. Callis, Instructor

slide2

Definition

Brønsted-Lowery Acid - A substance that can donate a hydrogen ion, e.g. when chloric acid is dissolved in water:

HClO3(aq) + H2O(l) = H3O+(aq) + ClO3-(aq)

acid base acid base

note: the hydrogen ion is symbolized as the hydronium ion to emphasize that it is hydrated.

note: acids and bases occur as conjugate acid-base pairs, e.g. HClO3 and ClO3- form such a pair.

slide3

Definition

Brønsted-Lowry Base - A substance that can accept a hydrogen ion, e.g. when ammonia is dissolved in water:

H2O(l) + NH3(aq) = NH4+(aq) + OH-(aq)

acid base conjugate conjugate

acid base note: The Brønsted-Lowry scheme is not limited to aqueous solutions:

HCl(in NH3) + NH3(l) = NH4+(in NH3) + Cl-(in NH3)

acid base conjugate conjugate

acid base

problem 20 1 identifying acids and conjugate bases
Problem 20-1: Identifying Acids and Conjugate Bases
  • Which of the following can act as Bronsted-Lowry acids? Give the formula of the conjugate Bronsted-Lowry base for each of them.
  • Cl-
  • HSO4-
  • NH4+
  • NH3
  • H2O
slide5

Amphoteric Molecules - Can Function as either acids or bases, depending on reaction conditions.

Water - (a) acts as an acid in donating a hydrogen ion to NH3, (b) acts as a base in accepting a hydrogen ion from HClO3

Hydrogen phosphate ion can act as an acid:

HPO42-(aq) + H2O(l) = H3O+(aq) + PO43-(aq)

or as a base:

HPO42-(aq) + H2O(l) = H2PO4-(aq) + OH-(aq)

slide6

Water Dissociates Slightly:

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

acid base acid base

We can express this process of autoionization in terms of an equilibrium:

Kw = [H3O+][OH-]

Where Kw = 1.0 x 10-14 at 25 oC. Note that H2O(l) does not enter into the equilibrium.

It is easy to show that the concentration of H+ and OH- are

1 x 10-7 M in pure water.

slide7

Aqueous Acid Solution: contains an excess of H3O+ ions over OH- ions.

Strong Acid: Ionizes completely in aqueous solution. Example:

HCl(aq) + H2O(l) -> H3O+(aq) + Cl-(aq)

The single arrow indicates that the reaction goes to completion. Other strong acids include HBr, HI, H2SO4, HNO3 and HClO4

Calculation: What is the concentration of OH- ions when a 0.1 M solution of HCl is made up?

[OH-] = Kw/[H3O+] = 1.0 x 10-14/0.10 = 1.0 x 10-13 M

slide8

Graphical representation of the behavior of acids of different strengths in aqueous solution.

  • Strong acid.
  • Weak Acid
slide9

Aqueous Basic Solution: contains an excess of OH- ions over H3O+ ions.

Strong Base: Ionizes completely in aqueous solution. Example:

H2O(l) + NH2-(aq) -> NH3(aq) + OH-(aq)

The single arrow indicates that the reaction goes to completion.

Calculation: What is the concentration of H3O+ ions when a 0.1 M solution of NaOH is made up?

[H3O+] = Kw/[OH-] = 1.0 x 10-14/0.10 = 1.0 x 10-13 M

slide10

The relationship of acid strength and conjugate base strength for the dissociation reaction

HA(aq) + H2O(l) = H3O+(aq) + A-(aq)

Acid Conjugate

base

slide11

The Definition of pH

pH = -log10[H3O+]

Problem 20-2 : (a) For pure water, [H+] = 1.0 x 10-7 M, so pH =

(b) The pH of 0.1 M HCl is

pH =

(c) The pH of 0.1 M NaOH is

pH =

slide12

Classification of pH Values

pH < 7 Acidic Solution

pH = 7 Neutral Solution

pH > 7 Basic Solution

slide13

The pH scale and pH values of some common substances.

The pH scale is a compact way to represent solution acidity. It involves base 10 logs (log), not natural logs (ln)

The definition of pH in terms of [H+] neglects any correction for non-ideality of the solutions.

significant figures for logarithms
Significant Figures for Logarithms

The number of decimal places in the log is equal to the number of significant figures in the original number.

Problem 20-3:

[H+] = 3.5 x 10-5 M

pH =

problem 20 4 calculating ph
Problem 20-4: Calculating pH

The concentration of H3O+ in a sample of wine is 2.0 x 10-4 M. Calculate the pH of the wine.

Ans:

answers to problems in lecture 20
Answers to Problems in Lecture 20
  • (a) This ion cannot act as a Bronsted-Lowry acid because it has no hydrogen. (b) An acid. Conjugate base: NH3 (c) An acid. Conjugate base: NH2- (d) An acid. Conjugate base: OH-
  • (a) pH = 7.00 (b) pH 1.00 (c) pH = 13.00
  • pH = 4.56
  • pH = 3.70