The `Inconvenient Truth’ about the Arrhenius model: basic salts

EXAMPLE: Na2CO3

(sodium carbonate)

No H+ or OH-….=> salt only

CLASSICAL REACTIONs OVERVIEW continued)

The `Inconvenient Truth’ about the Arrhenius model: basic salts (continued)

•Turns pink in presence of phenolphthalein

• gas-forming reaction with HCl, pink disappears

experimental results of adding to water:

EXAMPLE: Na2CO3 (sodium carbonate)

=> A base !!!!????? Where’s OH ???

CLASSICAL REACTIONs OVERVIEW(continued)

The `Inconvenient Truth’ about the Arrhenius model: basic salts (continued)

Other examples of `basic’ salts

Na3PO4 K2SiO3 NaC2H3O2

Basic salts cannot be rationalized using Arrhenius’ acid/base model

CLASSICAL REACTIONs OVERVIEW (continued)

Bronsted to the rescue…

Young Bronsted…Swedish chemist circa 1910…

Young James Dean…American actor circa 1955…(“Rebel Without a Cause,”“East of Eden”, “Giant” )

Winner of the Bronsted look alike contest….

Bronsted a few years after marriage and kids

CLASSICAL REACTIONs (continued)

Bronsted Model of Acids/Bases3 new concepts (see p. 654)

Acid=

HF, HCl, HBrHNO3 HClO4 H2SO4 H2CO3

H3PO4

No real change from Arrhenius

proton donor (=H+)

CLASSICAL REACTIONs OVERVIEW(continued)

ARRHENIUS ACIDS = BRONSTED ACIDS

New Concept #1: Follow the protons

BASE=

OH-

CO3-2 ,PO4

-3, C2H3O2-,SiO3

-2…

proton acceptor

CLASSICAL REACTIONs OVERVIEW(continued) BRONSTED’S MODEL OF ACIDS/BASES: New Concept #1: following the protons (cont.)

The set of Bronsted bases includes Arrhenius bases + ….

…nearly any negative ion (=anion) that even weakly attracts protons

Examples of Bronsted Bases

How the proton acceptor creates the basic salt effect:

CO32- + H-OH

..Bronsted Base is basic in water because it splits water to make OH- !

Hydrolysis reaction

CLASSICAL REACTIONs OVERVIEW(continued) BRONSTED’S MODEL OF ACIDS/BASES:

New Concept #2

Hydrolysis…Bronsted bases `split’ water !!!

EXAMPLE

HCO3- + OH-

Conjugate idea is like going drag…Men=acid Women=base

BASE ACIDCONJUGATE ACID

CONJUGATEBASE

Identify `acids’ and bases

BRONSTED’S MODEL OF ACIDS/BASES: New Concept #3a: conjugate

acid/base pairs

CO32- + H-OH

Acid conjugate baseBase conjugate acid

Base Acid

Conjugate acid

Conjugate base

CLASSICAL REACTIONs OVERVIEW(continued) BRONSTED’S MODEL OF ACIDS/BASES:

New Concept #3a: conjugate acid/base pairs

HCO3- + OH-

Identify `acid’ and baseIdentify conjugate acid and base

PO43- + HCO3 - HPO4

2- + CO32-

Acid conjugate baseBase conjugate acid

acidbaseconjugate base

conjugateacid

new concept #3a: Conjugates (continued)

When acid and base reacts….

EXAMPLE: IDENTIFY ACID, BASE AND THEIR CONJUGATE BASE AND ACID ABOVE

ACID + BASE CONJUGATE BASE + CONJUGATE ACID

Bronsted Model

Contrast to Arrhenius Model

ACID + BASE SALT + WATER

CLASSICAL REACTIONs OVERVIEW(continued) BRONSTED’S MODEL OF ACIDS/BASES:

New Concept #3b:

spontaneous acid + base reactions produce weaker acids + bases

Both products `neutral’

ARRHENIUS

ACID-BASE THEORIES SUMMARIZED

BRONSTED

ACID = H+ DONOR PROTON DONOR

BASE= OH- DONOR PROTON DONOR

ACID + BASE

SALT + WATER

CONJUGATE ACID + CONJUGATE BASE

What observation required Bronsted to create a new Acid/Base Theory different than Arrhenius’s model ?

A. Many salts formed conjugate acids.

B. Spontaneous acid+base reactions only occurred if weaker acids and bases formed.

C. H2O spontaneously decomposed to OH- and H+ in presence of various salts.

D. Many salts without OH- somehow created OH- in water.

Many sa

lts fo

rmed co

nju...

Spontaneous a

cid+base

r...

H2O spontaneously

dec...

Many sa

lts w

ithout O

H-...

0%

100%

0%0%

Which part of the Bronsted salt K3PO4 is the proton acceptor ?

A. K+

B. O2-

C. P3-

D. PO43-

E. K3PO4

F. OH-

K+O2-

P3-

PO43-

K3PO4OH-

0% 0% 0%0%

100%

0%

What new concept of Bronsted’s most directly explains why many salts are bases?

A. The anionic part of salts can hydrolyze water.

B. Spontaneous acid/base reactions lead to weaker conjugate acids/bases.

C. Bronsted bases are proton acceptors.

D. Bronsted acids are proton donors.

The anionic part

of salts

...

Spontaneous a

cid/b

ase r.

..

Bronste

d bases a

re pro

to...

Bronste

d acids a

re pro

to...

0% 0%0%0%

In-class reaction Jeopardy

Low heat, precipitates, cation swap

metathesis

WHAT IS…

Na2CO3 for example A Bronsted base

What’s my reaction type:

HCl + KOH KCl + H2O

Acid/base neutralization (Arrhenius)

Pick the Arrhenius acids

H2O HNO3 NaHCO3 HBr NaCl HNO3 HBr

In-class reaction JeopardyWHAT IS…

What’s my reaction type:K3PO4 + H2O HK2PO4 + KOH

Acid/base hydrolysis(Bronsted)

Write my products ?

AgNO3(aq) + NaCl(aq) ????? AgCl(s) + NaNO3(aq)

Pick the Bronsted base(s)

NaOH HBr NaHCO3 K3PO4

NaOH NaHCO3 K3PO4

Pick the Arrhenius bases

NaOH

OXIDATION-REDUCTION (REDOX) REACTIONS

see pages text 458-466REDOX REACTIONS ALL INVOLVE CHANGES IN ELECTRON OWNERSHIP

EXAMPLE #1:

Mgo + 2H+ Mg2+ + H2

0 +2

+1 0

Mg loses 2 e-

Each H+ gains 1 e-

Losing is oxidation

Gaining is reduction