Acids & BasesLesson 2

Strong and Weak(Bases)

Review of Bronsted-Lowry Acids

FORMING HYDRONIUM IONS

The proton (H+) has been transferred from the HCl molecule to a water molecule. form a hydronium (H3O+) ion and a Cl- ion.  This type of reaction is called ionization (because ions are being formed).

• Complete Question 1 on page 4 of your notes.

BRONSTED-LOWRY DEFINITIONOF ACIDS AND BASES

An acid is any substance which donates (gives) a proton (H+) to another substance.

A base is any substance which accepts (takes) a proton from another substance.

A Bronsted Acid is a proton donor

A Bronsted Base is a proton acceptor

BRONSTED-LOWRY DEFINITIONOF ACIDS AND BASES

We see that the HCl is donating the proton and the water is accepting the proton.

Therefore HCl is the Bronsted acid and H2O is the Bronsted base.

HCl + H2O H3O+ + Cl-

acid base

BRONSTED-LOWRY DEFINITIONOF ACIDS AND BASES

Let’s look at another example:

NH3 + H2O NH4+ + OH-

base acid acid base 

Bronsted Bases

Strong Bases- yellow

Oxide O2-

Amide NH2-

PO43-

Weak Bases

SO32-

CN-

CO32-

NH3

Incr

easi

ng

stre

ngt

hacids bases

The two Bronsted strong bases. They are found on the right bottom area of the acid chart -p 334

H+

H+

O2- + H2O OH-

NH2- + H2O NH3

Again, all bases make OH-

means strong

+ OH-

+ OH-

Bronsted Weak Bases

H+

H+

H+

+ OH-HF+ H2OF-

+ OH-HC2O4-⇄+ H2OC2O4

2-

+ OH-H2CO3⇄+ H2OHCO3-

⇄

Arrhenius bases

Strong Bases- yellow

LiOH

NaOH

KOH

Sr(OH)2

Ba(OH)2

Arrhenius Bases

RbOH

CsOH

FrOH

Ra(OH)2

Zn(OH)2

Arrhenius Bases

Be(OH)2

Weak Bases-most other metals

AgOH

Strong Bases

Note all bases make OH-

Ra(OH)2 Ra2+ + 2OH-

Sr(OH)2 Sr2+ + 2OH-

KOH K+ + OH-

LiOH Li+ + OH-

means strong

There are 11 Arrhenius strong bases - Group I and Group II hydroxides

Strong bases completely ionize in water to produce OH-

Be(OH)2(s) ⇄ Be2+ + 2OH-

Zn(OH)2(s) ⇄ Zn2+ + 2OH-

Arrhenius Weak Bases

A weak base is one that partially reacts with water to produce OH-.

 

 

Whether you consider Arrhenius or Bronsted-Lowry, BASES act the same way: They all make OH- by accepting a proton from water!

NH3 + H2O ⇄ NH4+ + OH-

C6H5O73- + H2O ⇄ HC6H5O7

2- + OH-

H+

H+

Weak bases

• At equilibrium, use

• Use Kb – Base ionization constant

Write the Kb expression for the following 3 weak bases

Kb =

[HF][OH-]

[F-]

[HC2O4-][OH-]

[C2O42-]

Kb =

Kb =[H2CO3][OH-]

[HCO3-]

AMPHIPROTIC SUBSTANCES

Some substances (ex: H2O) are capable of acting as an ACID (when surrounded by a stronger base) OR acting as a BASE (when surrounded by a stronger acid).

Substances that act as acids or bases are called amphiprotic.

AMPHIPROTIC SUBSTANCESOther amphiprotic substances:

• H2O

• H2PO4-

• HCO3-

 

+ H+ - H+

• Example: H3PO4 H2PO4

- HPO42-

 

AMPHIPROTIC SUBSTANCES

In general, amphiprotic substances…

Have a negative charge and

An easily removable hydrogen.

 

AMPHIPROTIC SUBSTANCES

Identify the acid and base in the reactants of the following reactions:  

H2S + HCO3- H2CO3 + HS-

 

 

NH4+ + H2O H3O+ + NH3

 

 

HCOOH + HSO3- H2SO3 + HCOO-

CONJUGATE ACID-BASE PAIRS

Conjugate acid – the species with one more proton (ex. HIO3)

Conjugate base - the species with one less proton (ex. IO3

-)

*There will always be 2 conjugate pairs

POLYPROTIC ACIDSThe formula of an acid tells us how many protons (H+) the acid can

donate.

 

An acid that can supply:

– ONE proton (ex: HCl) = monoprotic acid

– TWO protons (ex: H2SO4) = diprotic acid

– THREE protons (ex: H3PO4) = triprotic acid

– More than ONE proton = polyprotic acid

WRITE THE BRONSTED-LOWRY ACID-BASE EQUILIBRIA WHEN THIS PAIR IS MIXED:

HCN and F-

Homework:

Hebden: pg 115 #10, pg 117 #11-12,

pg 119 #13-14, pg 121 #16-19