Chapter 16: Acids and Bases. Students will learn… 3 definitions of Acids and Bases Acid Strength – pH scale Water as acid and base Calculating pH of strong.

Chapter 16 Acids and Bases

Students will learnhellip

bull 3 definitions of Acids and Basesbull Acid Strength ndash pH scalebull Water as acid and basebull Calculating pH of strong acids and

basesbull Buffered solutionsReview naming acids Pg 132 amp 133

Three Definitions of Acids-Bases

bull Arrheniusbull BrOslashnsted-Lowrybull Lewis

3 Definitions of Acids and Bases

H+ ion always combines with water molecule forming hydronium ion H3O+

Acids Bases

Arrhenius Produce H+ ion (= proton)

Produce OHndash ion

BrOslashnsted-Lowry Donate H+ ions Accept H+ ions

Lewis Accept an electron-pair

Donate an electron-pair

bull Arrhenius Acids HCl HC2H3O2 HCN

bull Arrhenius Bases NaOH Ca(OH)2

bull BrOslashnsted-Lowry Acids and Bases can only be determined from chemical equations

H2O + HCl rarr H3O+ + Cl- base acid conjugate conjugate

acid baseconjugate pairconjugate pair

Conjugate Acid-Base Pair

bull An acid and a base that only differ by one H+ ion

bull Always made of one reactant and one productbull Often in reversable reactions(Ex) HSO4

- + H2O harr SO42- + H3O+

acid base base acid

HSO4- + H2O harr H2SO4 + OH-

base acid acid base

Examples

1 Which of the following represent conjugate acidndashbase pairs a) HCl HNO3

b) H3O+ OHndash

c) H2SO4 SO42ndash

d) HCN CNndash

2 Write the conjugate base for each of the following

(a) HClO4

(b) H3PO4

(c) CH3NH3+

Strong vs Weakbull Strong 100 dissociation of H+ or OHndash

ndash Strong electrolytes (conduct electricity very well)

bull Weak Less than 100 dissociation of H+ or OHndash

ndash Weak electrolytes (conduct electricity somewhat)

Common Strong and Weak AcidsBases

bull Strong acids H+7A H2SO4 HClO4 HNO3

bull Weak acids HCN HC2H3O2

bull Strong bases 1A+OH Sr(OH)2 Ba(OH)2

bull Weak bases NH3 (ammonia) in water = NH4OH (ammonium hydroxide)

Relative Strength of Acids and Bases

ExampleAcetic acid (HC2H3O2) and HCN are both weak acids Acetic acid is a stronger acid than HCN

Arrange these bases from weakest to strongest

Clndash CNndash C2H3O2ndash

Terms related Acidsbull Amphoteric Ions or molecules that can become an

acid and a base(Ex) HSO4

- + H2O rarr SO42- + H3O+

acid baseHSO4

- + H2O rarr H2SO4 + OH-

base acid(Ex) H2O + H2O rarr H3O+ + OH-

bull Oxyacids acids containing oxygen(Ex) HNO3 H2SO4 HClO

bull Monoprotic Acidsndash Have one proton (H+) to donate

(Ex) HCl HNO3 HI HClO3

bull Diprotic Acidsndash Have more than two protons to donate

(Ex) H2SO4

bull Triprotic Acidsndash Have three protons to donate

(Ex) H3PO4

Self-ionization of Water

bull In neutral water (1)H2O + H2O harr H3O+ + OHndash

(2) [H3O+] = [OHndash] = 1times10-7 molL

(3) [H3O+] [OHndash] = (1times10-7)(1times10-

7)=1times10-14 25 deg= Kw

Kw = ion-product constant for water

The constant changes to the temperature

Acidic or Basic Solutions

bull Kw =1times10-14 always maintained

[H3O+] [OH-]

pure water [H3O+]=1times10-7 molL [OH-]= 1times10-7 molL

acidic solution [H3O+]gt1times10-7 molL [OH-]lt 1times10-7 molL

basic solution [H3O+]lt1times10-7 molL [OH-]gt 1times10-7 molL

Common Misunderstanding

bull An acidic solution has only H+ ions

bull A basic solution has only OH‒ ions

Example

(1) In an acidic aqueous solution which statement below is correct

a) [H+] lt 10 times 10ndash7 Mb) [H+] gt 10 times 10ndash7 Mc) [OHndash] gt 10 times 10ndash7 Md) [H+] lt [OHndash]

(2) In an aqueous solution in which [OHndash] = 20 x 10ndash10 M the [H+] = ________ M and the solution is ________

a) 20 times 10ndash10 M basicb) 10 times 10ndash14 M acidicc) 50 times 10ndash5 M acidicd) 50 times 10ndash5 M basic

pH and pOH Scale

bull pH = ndashlog[H+]ndash A compact way to represent solution acidityndash pH decreases as [H+] increases

ndash [H+] = 10-pH

bull pOH = ‒log [OH‒]ndash pOH decreases as [OH‒] increases

ndash [OH‒] = 10-pOH

bull pH + pOH = 14

bull pH = 7 neutralbull pH gt 7 basic

Higher the pH more basicbull pH lt 7 acidic

Lower the pH more acidic

Acidity and pH Scale

high pH = weak acidhigh pOH = weak base

Examples(1) Calculate the pH for each of the following

solutionsa) 10 times 10ndash4 M H+

b) 0040 M OHndash

(2) The pH of a solution is 585 What is the [H+] for this solution

(3) Consider an aqueous solution of 20 times 10ndash3 M HCl

What is the pH

If the solution was 20 times 10ndash3 M HNO2 a weak acid you canrsquot take the same approach Why

Buffered Solutions

bull resists a change in its pH when either an acid or a base has been added Presence of a weak acid and its

conjugate base buffers the solution

Characteristics of Buffered Solutions

1 The solution contains a weak acid HA and its conjugate base Andash

2 The buffer resists changes in pH by reacting with any added H+ or OHndash so that these ions do not accumulate

3 Any added H+ reacts with the base Andash H+(aq) + Andash(aq) rarr HA(aq)

4 Any added OHndash reacts with the weak acid HA

OHndash(aq) + HA(aq) rarr H2O(l) + Andash(aq)

Example

If a solution is buffered with NH3 to which has been added NH4Cl what reaction will occur if a strong base such as NaOH is added

a) NaOH + NH3 rarr NaNH4Ob) NaOH + NH4

+ rarr Na+ + NH3 + H2Oc) NaOH + Clndash rarr NaCl + OHndash

d) NaOH + H2O rarr NaH3O2