AcidsBases&Buffers

Acids, Bases & Buffers

Introduction • Sourness in foods is caused by acids, molecules that release protons. • The chemical opposite, bases, are all around us.

Some Substances that are Acids Foods Vinega Vin egarr Mus Mustar tard d Oranges Lemons

acid HC2H3O2) Pickle Pic kle Ju Juice ice (all contain acetic acid H Grapefruit itss Gelatin inss Most sodas (all contain citric acid  H3C6H5O7)

Sour Milk Buttermilk Yogurt (all contain lactic acid  HC3H5O3) acid HC7H5O2) Cranberry Juice (contains benzoic acid H Apples Watermelons H2C4H4O5) Spinach Grapejuice

Grapejuice Wine (all contain malic acid 

Tomatoes (both contain oxalic acid  H2C2O4) acid H2C4H4O6) Wine (both contain tartaric acid H

  acids with lots of C atoms. All acids above are ORGANIC ACIDS -- acids

Some Substances that are Acids Other Common Acids STRONG

WEAK

Hydrocloric acid

(HCl)

Carbonic Acid

(H2CO3)

Nitric Acid

(HNO3)

Phosphoric Acid (H3PO4)

Sulfuric acid

(H2SO4)

Hydrofluoric Acid

(HF)

Perchloric Acid

(HClO4)

Hydrocyanic Acid

(HCN

Hydrobromic acid

(HBr)

Sulfurous acid

Hydroiodic Acid

(HI)

(H2SO3)

How do we recognize an acid by its chemical c hemical formula? Ans: There is an H or H’s at the fore of the formula

Some Substances that are Bases Household Products Household Ammonia (contains ammonia (NH3) in water) Drain and Oven Cleaners (contains sodium hydroxide, NaOH NaOH) Antacids (Rolaids, Tums, Alka Seltzer, Maalox (contains Al(OH Al(OH))3 ) Milk of Magnesia (contains Mg(OH Mg(OH))2 ) Baking Soda (contains NaHCO3 ) Toothpaste Soap

Most cleaning products are basic (a few glass cleaning  products contain acetic acid or HCl)

Some Substances that are Bases Other Common Bases STRONG Lithium hydroxide Sodium hydroxide

WEAK LiOH) (LiOH NaOH) (NaOH

Ammonia

(NH3) Amines (e.g CH3CH2NH2, (CH3)2NH, and C5H5N)

Potassium hydroxide (KOH) Cesium hydroxide (CsOH CsOH) Calcium hydroxide (Ca(OH Ca(OH))2) Barium hydroxide

Ba(OH))2 ) (Ba(OH

Amines are ORGANIC BASES How can we generally recognize a base by its chemical formula? Ans: OH or OH’s at the end of the formula & a metal at the beginning of the formula.

General Properties of Aqueous Acids and Bases Aqueous means dissolved in water

Acids have a sour taste

Bases have a bitter taste

Acids are electrolytes

Bases are electrolytes

(Electrolytes conduct electricity)

Definitions of Acids & Bases Arrhenius Acid: H+ or H3O+ producer in water  (ex: HCl, HNO3, H2SO4) Base: OH- producer in water  (ex: NaOH, LiOH, Ca(OH) 2) Bronsted-Lowry Acid: proton (H+) donor  (ex: NH4+, HSO4-, all Arrhenius acids) Base: proton acceptor  (ex: NH3, H2O, CO32-) Lewis Acid: electron-pair acceptor  (ex: H+, positive ions --Al3+, Fe2+) Base: electron pair donor  (ex: OH-, negative ions– F -, O2-, N3-)

An Acid-Base Reaction Neutralization is the reaction of an H + (H3O+) ion from the acid and the OH - ion from the base to form water, H2O. The neutralization reaction is exothermic and releases approx. 56 kJ per mole of acid and base. H+(aq) + OH-(aq)

H2O(l)

H0rxn = -55.9 kJ

Strong and Weak Acids & Bases

The Extent of Dissociation for Strong Acids

 g or l ) + H2O(l ) Strong acid: HA( g

 aq) + A-( aq  aq) H2O+( aq

Complete dissociation

The Extent of Dissociation for Weak Acids

 aq) + H2O(l ) Weak acid: HA( aq

 aq) + A-( aq  aq) H2O+( aq

Partial Dissociation

Strong & Weak Acids Strong acids dissociate completely into ions in water: HA(g or l) + H2O(l)

H3O+(aq) + A-(aq)

Nitric acid acid is an example: HNO3 (l) + H2O(l)

H3O+(aq) + NO3-(aq)

Weak acids dissociate dissociate very slightly into ions in water: HA(aq) + H2O(aq)

H3O+(aq) + A-(aq)

In a dilute solution of a weak acid, acid, the great great majority of HA molecules are undissociated:

In the same manner, bases are considered either strong or weak.

Classifying the Relative Strengths of Acids and Bases

Qualitative Classifications (memorize these) these) Strong acids. There are two types of strong acids: 1. HCl, HBr, and HI 2. Acids containing O in which the # of O atoms exceeds the number of ionizable H atoms by two or more, such as HNO3, H2SO4, HClO4 Weak acids. Many more weak acids than strong ones. Four types are: 1. HF 2. Those acids in which H is not not bounded to O or to a halogen, such as HCN and H2S

3.

4.

Acids containing O in which the # of O atoms exceeds by one the number of ionizable H atoms, such as HClO, HNO2, and H3PO4 Organic acids (general formula RCOOH), such as CH3COOH

Classifying the Relative Strengths of Acids and Bases

Qualitative Classifications (memorize these) these) Strong bases. Soluble compounds containing O 2- or OH- ions are strong bases. The cations are usually those of the most active metals: 1) M2O or MOH, where M= Group 1A(1) metals (Li, Na, K, Rb, Cs)

2) MO or M(OH)2, where M = Group 2A(2) metals (Ca, Sr, Ba) [MgO and Mg(OH)2 are only slightly soluble, but the soluble portion dissociates completely.] Weak bases. Many compounds with an electron-rich nitrogen are weak  bases (none are Arrhenius bases). The common structural feature is an N atom that has a lone electron pair in its Lewis structure. 1) Ammonia (:NH3)

2) Amines (general formula RNH2, R2NH, R3N), such as CH3CH2NH2, (CH3)2NH, (C3H7)3N, and C5H5N

The Hydronium Ion, + H3O

H+ comes from acid and combines with H2O to yield H3O+, the hydronium ion

H3O+ Concentration, [H3O+] • The acidity of a solution is normally specified by the concentration of H 3O+ in moles per liter of solution, M + Strong acids: acid concentration = [H O ] – 3

– Weak acids: acid concentration > [H 3O+] + -7 Pure water has a [H O ] = 1 x 10 M. • 3

Autoionization of Water  H3O+(aq) + OH-(aq)

H2O(l) + H2O(l)

K c =

[H3O+][OH-] [H2O]2

The ion-product for water, K w: K c[H2O]2 = K w = [H3O+][OH-] = 1.0 x 10-14 (at 25°C) For pure water the concentration of hydroxyl and hydronium ions must be equal: [H3O+] = [OH-] =

1.0 x 10-14 = 1.0 x 10 -7 M  (at 25°C)

The molarity of pure water is:

1000g/L = 55.5 M  18.02 g/mol

Acid and Base Character and the pH Scale A scale called the pH scale is used express the H + ion conc’n in water: H

+ (aq)

+ H2O(l)

+ (aq)

H3O

[H+] = [H3O+]

pH = - log[H3O+] What is the pH of a solution that is 10 -12 M in hydronium ion ? pH = -log[H3O+] = (-1)log 10-12 = (-1)(-12) = 12 What is the pH of a solution that is 7.3 x 10 -9 M in H3O+ ? pH = -log(7.3 x 10-9) = 8.14

 pH of a neutral solution = 7.00

[H3O+] = [OH-]

 pH of an acidic solution < 7.00

[H3O+] > [OH-]

  pH of a basic solution

[H3O+] < [OH-]

> 7.00

Relationship b/w [H3O+] and [OH-] and the relative acidity of solutions

The pH Values of  Some Familiar  Aqueous Solutions pH + pOH = 14 For every change of 1 unit on the pH scale, [H3O+ ] changes by a factor of 10.

Calculating [H3O+], pH, [OH-], and pOH

Problem

ydi

A chemist dilutes concentrated hydrochloric acid to make two solutions: (a) 3.0 M and (b) 0.0024 M . Calculate the [H3O+], pH, [OH-], and pOH of the two solutions at 25°C.

Calculating [H3O+], pH, [OH-], and pOH Plan: We know that hydrochloric acid is a strong acid, so it dissociates completely in water; therefore [H 3O+] = [HCl]init..

We use the [H3O+] and Kw to calculate the [OH -] and pH as well as pOH. Solution:  M  3.0 M  (a) [H3O+] = 3.0

pH = -log[H3O+] = -log(3.0) = -0.477

1 x 10-14 [OH ] = = + 3.0 [H3O ] -

K w

= 3.333 x 10-15 M 

pOH = - log(3.333 x 10 -15) = 14.477  M  (b) [H3O+] = 0.0024 0.0024 M 

pH = -log[H3O+] = -log(0.0024) = 2.62

-14 1 x 10 [OH-] = = 0.0024 [H3O+]

K w

= 4.167 x 10-12 M  pOH = -log(4.167 x 10 -12) = 11.38

Methods for Measuring the pH of an Aqueous Solution

pH (indicator) paper 

pH meter 

Acid Rain: Fossil Fuel Combustion • SO2 and NO2 formed during fossil fuel combustion combine with atmospheric water to form acid rain.

• Unpolluted rain is slightly acidic due to atmospheric carbon dioxide.

• Most acidic rainfall occurs in the northeastern U.S.

Acid Rain: The Effects • The environment into which acid rain falls determines its fate. • In some cases naturally occurring geography can serve to neutralize the acid. • Rapid acidification occurs when neutralization is not possible.

• Lakes and Streams – Approx. 2000 lakes and streams in the eastern U.S. have elevated pH. – Some aquatic species cannot survive. – U.S. emissions have contaminated Canadian lakes.

• Building Materials – Acids dissolve stone, marble, paint – Rusting of steel is accelerated

• Forests and Reduced Visibility – Trees cannot grow and fend off disease. – Sulfate aerosols account for 50% of visibility problems in the eastern U.S.

Clean Air Act Amendments • Cut SO2 emissions to half of 1980 levels by 2010 – Use low-sulfur coal, rremove sulfur before burning – Use flue gas scrubbers – Conservation and efficiency of customers

• SO2 emission allowances – Can be traded among utilities but congress reduces number of allowances as per regulations

What is a Buffer ? A solution that changes pH only slightly small amounts of strong acid or base are added to it .

The Effect of Addition of Acid or Base to Unbuffered or Buffered Solutions

pH of unbuffered of unbuffered (top) changes by a large amount; amount; pH of buffered of buffered (bottom) changes only slightly

3 Kinds of Buffer Solutions Buffer: a solution of weak acid & its salt • Acid Buffer: (conjugate base); its pH < 7. Examples HNO2 /NO2- ; HClO2 /ClO2- ; CH3COOH/CH 3CO2-

Buffer: a solution of weak base & its salt • Base Buffer: (conjugate acid); its pH > 7. Examples NH4+ /NH3 ; H2PO4- / HPO42 - ; (CH3)3NH+ / (CH  / (CH3)3N •

 Neutral Buffer

Why are Buffers Important ? • Human blood & other cell fluids buffered at pH =  alkalosis) or a 7.40 ± 0.05. A rise in pH above 7.45 ( alkalosis  acidosis) can be lifedrop in pH below 7.35 ( acidosis threatening. (Intravenous solutions are buffered) • Some chemical reactions can only be done within a narrow pH range; • pH meters have to be calibrated; • Culturing (growing in prepared medium) of bacteria done in specific small pH range; • Largest buffered system-the OCEAN-relies on presence of bicarbonates (HCO3) and silicates.

How does a buffer resist pH changes?

How a Buffer Works • Small quantities of H 3O+ (acid) or OH- (base) added to buffer cause a cause a small amount of one buffer component to  convert into the other. • As long as amounts of H 3O+ and OH- are small compared to conc’ns of acid and base in buffer, added ions will have little effect on the pH since they are consumed by buffer components.