cmh 121 luca preziati chapter 8: acids and bases acid = produces h + an acid is a compound that: 1....

CMH 121 Luca Preziati

Chapter 8: Acids and Bases

Acid = produces H+

An acid is a compound that:

1. Has H somewhere

2. Has the tendency (is capable) of giving that H (as H+) toanother compound: Proton Donor

Base = produces OH- in water

A base is a compound that:

1. Not all bases have an OH group.

2. Has the tendency (is capable) of accepting H+ from anothercompound: Proton Acceptor



Acid = produces H+

Base = produces OH-

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

H O

:

+ H Cl

:: : H O H

:

+H

+Cl -

::: ::

H

NaOH(s) H2O Na+(aq) +OH-(aq)

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

H O H

::+ H N H

H

H+ + O

:::

-H N

H

H: H



Strong acid:Strong acid: One that reacts completely with water to form H3O+ ions.

Strong base:Strong base: One that reacts completely with water to form OH- ions.

Here are the six most common strong acids and the four most common strong bases:

HClHBrHIHNO3

H2SO4

HClO4

LiOHNaOHKOH

Ba(OH)2

Hydrochloric acidHydrobromic acidHydroiodic acidNitric acidSulfuric acidPerchloric acid

Lithium hydroxideSodium hydroxidePotassium hydroxideBarium hydroxide

Formula Name Formula Name



Weak acid:Weak acid: A substance that dissociates only partially in water to produce H3O+ ions.

• Acetic acid, for example, is a weak acid. In water, only 4 out every 1000 molecules are converted to acetate ions:

Weak base:Weak base: A substance that only partially reacts with water to produce OH- ions.• Ammonia, for example, is a weak base:

CH3COOH(aq) + H2O(l) CH3COO-(aq) + H3O+(aq)

Acetic acid Acetate ion

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



Conjugate acid-base pair:Conjugate acid-base pair: Any pair of molecules or ions that can be interconverted by transfer of a proton.

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

WaterHydrogenchloride

Hydroniumion

Chlorideion

(base)(acid) (conjugateacid of water)

(conjugatebase of HCl)

conjugate acid-base pair


NH4+CH3COOH CH3COO-

NH3

(base) (conjugate baseacetic acid)

(conjugate acidof ammonia)


+ +Acetic acid Ammonia

(acid)


Acetate ion

Ammoniumion



C2H5OH C2H5O-H2O OH-HPO4

2- PO43-

HCO3- CO3

2-

C6H5OH C6H5O-HCN CN-

NH3NH4+

H2PO4- HPO4

2-

H2S HS-H2CO3 HCO3

-CH3COOH CH3COO-H3PO4 H2PO4

-HSO4

- SO42-

H2OH3O+HNO3 NO3

-H2SO4 HSO4

-HCl Cl-HI I-Hydroiodic acid

Hydrochloric acidSulfuric acid

Dihydrogen phosphateAcetateBicarbonate

Hydrogen phosphateAmmonia

Phenoxide

Carbonate

PhosphateHydroxideEthoxide

Hydrogen sulfide

Nitric acidHydronium ion

Hydrogen sulfate ion

Name of acid Name of ion

Phosphoric acidAcetic acidCarbonic acid

Dihydrogen phosphateAmmonium ion

Phenol

Bicarbonate ion

Hydrogen phosphate ionWaterEthanol

Hydrogen sulfide

AcidConjugate Base

IodideChlorideHydrogen sulfateNitrateWater

Sulfate

StrongAcids

Weak Acids

Weak Bases

StrongBases

Hydrocyanic acid Cyanide



Acids are classified a monoprotic, diprotic, or triprotic monoprotic, diprotic, or triprotic depending on the number of protons each may give up; examples are HCl, H2CO3, and H3PO4.

Carbonic acid

Bicarbonateion

Bicarbonateion

Carbonateion

H2CO3 H2O

HCO3- H2O

HCO3-

CO32-

H3O+

H3O+

+ +

+ +

Amphiprotic: Amphiprotic: A substance that can act as either an acid or a base. The HCO3

- ion, for example, can give up a proton to become CO3

2-, or it can accept a proton to become H2CO3 .



• When a weak acid, HA, dissolves in water

• The equilibrium constant expression, Keq, for its ionization is

• Because water is the solvent and its concentration changes very little when we add HA to it, we treat [H2O] as a constant equal to 1000 g/L or 55.5 mol/L.

• We combine the two constants to give a new constant, which we call an acid ionization constant, Ka

HA H2O A- H3O++ +

[HA][H2O]

[A-][H3O+]Keq =

[HA]

[A-][H3O+]Ka = Keq[H2O] =



H3O+CH3COO-H2OCH3COOH + +

Acetic acid Acetate ion

• Ka for acetic acid, for example is 1.8 x 10-5.

• Because the acid ionization constants for weak acids are numbers with negative exponents, we commonly express acid strengths as pKa where:

• The value of pKa for acetic acid is 4.75.

pKa = -log Ka



H3PO4

HCOOH

CH3CH(OH)COOH

CH3COOH

H2CO3

H2PO4-

H3BO3

NH4+

C6H5OH

HPO42-

HCO3-

HCN

Phosphoric acid

Formic acid

Lactic acid

Acetic acid

Carbonic acid

Dihydrogen phosphate ion

Name

7.21

pKa

9.14

9.25

9.89

12.66

10.25

Boric acid

Ammonium ion

Phenol

Hydrogen phosphate ion

Bicarbonate ion

Acid

7.5 x 10-3

1.8 x 10-4

8.4 x 10-4

1.8 x 10-5

4.3 x 10-7

6.2 x 10-8

Ka

7.3 x 10-10

5.6 x 10-10

1.3 x 10-10

2.2 x 10-13

5.6 x 10-11

2.12

3.75

3.08

4.75

6.37

Hydrocyanic acid 4.9 x 10-10 9.31



Chemical Properties of Acids and Bases:Chemical Properties of Acids and Bases:

• Neutralization

Acid + Base Water + SaltHCl + NaOH H2O + NaCl

• Others :• Reaction with metals• Reaction with Metal Hydroxides• Reaction with Metal Oxides• Reaction with carbonates and bicarbonates• Reaction with Ammonia and Amines



pHpH

pH = -log [H+]

• Acidic solution:Acidic solution: One whose pH is less than 7.0.• Basic solution:Basic solution: One whose pH is greater than 7.0.• Neutral solution:Neutral solution: One whose pH is equal to 7.0.



pH buffer:pH buffer: a combination of substances that act together to prevent a drastic change in pH

• A pH buffer is an acid or base “shock absorber.”• A pH buffer is commonly referred to simply as a

buffer.• The most common buffers consist of approximately

equal molar amounts of a weak acid and a salt of the weak acid; that is, approximately equal molar amounts of a weak acid and a salt of its conjugate base.

• For example, if we dissolve 1.0 mole of acetic acid and 1.0 mole of its conjugate base (in the form of sodium acetate) in water, we have an acetate buffer.



• How does an acetate buffer resist changes in pH?

• If we add a strong acid, such as HCl, added H3O+ ions react with acetate ions and are removed from solution:

• If we add a strong base, such as NaOH, added OH- ions react with acetic acid and are removed from solution:

CH3COOH H2O CH3COO- H3O++ +

Added asCH3COOH

Added asCH3COO-Na+

CH3COO- H3O+ CH3COOH H2O+ +

CH3COOH OH- CH3COO- H2O+ +



The average pH of human blood is 7.4.

• Any change greater than 0.10 pH unit in either direction can cause illness.

To maintain this pH, the body uses three buffer systems:

• Carbonate buffer:Carbonate buffer: H2CO3 and its conjugate base, HCO3-

• Phosphate buffer:Phosphate buffer: H2PO4- and its conjugate base,

HPO42-

• ProteinsProteins.

cmh 121 luca preziati chapter 8: acids and bases acid = produces h + an acid is a compound that: 1....

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