14.1 The Nature of Acids & Bases 14.2 Acid Strength

Why study acid-base chemistry? It is a very important class of chemical reactions o Acid-Base reactions o Precipitation reaction o Redox reactions Acid-base reactions are essential to living systems There are MANY industrial applications. (H 2 SO 4 is one of the most important industrial chemicals) Copyright © Cengage Learning. All rights reserved 2

Acid-Base Definitions Arrhenius ( ) was the first to define acids & bases. Arrhenius Acid-Base definitions: Acids produce H + ions in solution HCl   H + + Cl - Bases produce OH - ions. NaOH   Na + + OH -

Acid-Base Definitions Brønsted–Lowry( ) was the first to define acids & bases. Brønsted–Lowry Acid-Base definitions: Acids are proton (H + ) donors Bases are proton acceptors HCl + H 2 O   H 3 O + + Cl - This is an improved definition –because it expands Arrhenius’s definition NH 3 + H 2 O   NH OH - It also implies that acids and bases always are paired The hydronium ion ammonia Ammonium ion

HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) Conjugate base the acid molecule after a proton is lost. Conjugate acid forms when the proton is transferred to the base. Copyright © Cengage Learning. All rights reserved 5 Acid in Water

Strong acid: – Ionization equilibrium lies far to the right. – Yields a weak conjugate base. Weak acid: – Ionization equilibrium lies far to the left. – Weaker the acid, stronger its conjugate base. Copyright © Cengage Learning. All rights reserved Acid Strength

Copyright © Cengage Learning. All rights reserved Acid Strength How is a strong acid different than a week acid?

Various Ways to Describe Acid Strength Copyright © Cengage Learning. All rights reserved 8

Water as an Acid and a Base Water is amphoteric: – Behaves either as an acid or as a base. H 2 O (l) + H 2 O (l)   H 3 O + (aq) + OH - (aq) K = ? At 25°C: K w = [H + ][OH – ] = 1.0 × 10 –14 No matter what the solution contains, the product of [H + ] and [OH – ] must always equal 1.0 × 10 –14 at 25°C. Copyright © Cengage Learning. All rights reserved 9

Three Possible Situations [H + ] = [OH – ]; neutral solution [H + ] > [OH – ]; acidic solution [OH – ] > [H + ]; basic solution Copyright © Cengage Learning. All rights reserved 10

Self-Ionization of Water Copyright © Cengage Learning. All rights reserved 11 To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) acid base conjugate conjugate acid base What is the equilibrium constant expression for an acid acting in water? Copyright © Cengage Learning. All rights reserved 12 CONCEPT CHECK! Review Questions

If the equilibrium lies to the right, the value for K a is __________. large (or >1) If the equilibrium lies to the left, the value for K a is ___________. small (or <1) Copyright © Cengage Learning. All rights reserved 13 CONCEPT CHECK! Review Questions

HA(aq) + H 2 O(l) H 3 O + (aq) + A – (aq) If water is a better base than A –, do products or reactants dominate at equilibrium? Does this mean HA is a strong or weak acid? Is the value for K a greater or less than 1? Copyright © Cengage Learning. All rights reserved 14 CONCEPT CHECK! Review Questions

Consider a 1.0 M solution of HCl. Order the following from strongest to weakest base and explain: H 2 O(l) A – (aq) (from weak acid HA) Cl – (aq) Copyright © Cengage Learning. All rights reserved 15 CONCEPT CHECK! Review Questions

Let’s Think About It… How good is Cl – (aq) as a base? Is A – (aq) a good base? The bases from strongest to weakest are: A –, H 2 O, Cl – Copyright © Cengage Learning. All rights reserved 16

Consider a solution of NaA where A – is the anion from weak acid HA: A – (aq) + H 2 O(l) HA(aq) + OH – (aq) base acid conjugate conjugate acid base a)Which way will equilibrium lie? left Copyright © Cengage Learning. All rights reserved 17 CONCEPT CHECK! Review Questions

Consider a solution of NaA where A – is the anion from weak acid HA: A – (aq) + H 2 O(l) HA(aq) + OH – (aq) base acid conjugate conjugate acid base b) Is the value for K b greater than or less than 1? less than 1 Copyright © Cengage Learning. All rights reserved 18 CONCEPT CHECK! Review Questions

Consider a solution of NaA where A – is the anion from weak acid HA: A – (aq) + H 2 O(l) HA(aq) + OH – (aq) base acid conjugate conjugate acid base c) Does this mean A – is a strong or weak base? weak base Copyright © Cengage Learning. All rights reserved 19 CONCEPT CHECK!

Acetic acid (HC 2 H 3 O 2 ) and HCN are both weak acids. Acetic acid is a stronger acid than HCN. Arrange these bases from weakest to strongest and explain your answer: H 2 O Cl – CN – C 2 H 3 O 2 – Copyright © Cengage Learning. All rights reserved 20 CONCEPT CHECK!

Let’s Think About It… H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH – (aq) acid base conjugate conjugate acid base At 25°C, K w = 1.0 × 10 –14 The bases from weakest to strongest are: Cl –, H 2 O, C 2 H 3 O 2 –, CN – Copyright © Cengage Learning. All rights reserved 21

Discuss whether the value of K for the reaction: HCN(aq) + F – (aq) CN – (aq) + HF(aq) is >1<1 =1 (K a for HCN is 6.2×10 –10 ; K a for HF is 7.2×10 –4.) Explain your answer. Copyright © Cengage Learning. All rights reserved 22 CONCEPT CHECK!

Calculate the value for K for the reaction: HCN(aq) + F – (aq) CN – (aq) + HF(aq) (K a for HCN is 6.2×10 –10 ; K a for HF is 7.2×10 –4.) K = 8.6 × 10 –7 Copyright © Cengage Learning. All rights reserved 23 CONCEPT CHECK!