Lecture 5- Acids and bases and solvent systems

Question 1

What is a solvent?

Answer 1

A solvent is a substance
that dissolves a solute (a chemically different liquid, solid or gas),
resulting in a solution.

Question 2

What state is a solvent usually in?

Answer 2

A solvent is usually a liquid but can be a solid or a
gas. The maximum quantity of solute that can dissolve in a specific
volume of solvent varies with temperature.

Question 3

Is water always a suitable solvent?

Answer 3

Although many inorganic reactions take place in aqueous solution, water
is not always the most suitable solvent; some reagents react violently or
decompose in water (e.g. the alkali metals) and non-polar molecules are
often insoluble in water.

Question 4

Utility of a solvent depends on?

Answer 4

1. Its auto-dissociation properties
2. Its acidity or basicity auto-dissociation : electron or proton donor/acceptor properties
3. Its dielectric constant
4. Temperature over which it is a liquid

Question 4

What happens in auto dissociation?

Answer 4

A proton is transferred from one solvent molecule to the next,
generating the protonated cation of the solvent and the deprotonated anion of the solvent.
These solvents that contain ionizable protons and are called protic solvents.

Question 5

What are examples of autodissociation?

Answer 5

2H2O(l) ⇌ H3O+
(aq) + OH-
(aq)
2HCl(l) ⇌ H2Cl+
(sol) + Cl-
(sol)

Question 6

Write the auto-dissociation equation for HF, H2SO4and NH3

Answer 6

2H2SO4
(l) ⇌ H3SO4
+
(sol) + HSO4
-
(sol)
3HF(l) ⇌ H2F+
(sol) + HF2
-
(sol)
2NH3
(l) ⇌ NH4
+
(sol) + NH2
-
(sol)

Question 7

Solvents can be defined as protic or aprotic. What are protic solvents and give examples.

Answer 7

Protic – contains ionizable protons
Examples
H2O, HCl, HF and H2
SO4

Question 8

Solvents can be defined as protic or aprotic. What are aprotic solvents and give examples.

Answer 8

Aprotic- no ionizable protons
1. Non-polar, non ionized: CCl4 and hydrocarbons

2. Polar, nonionized: DMF, DMSO, THF
3. Highly polar, autoionizing: BrF3
   , IF5 auto-dissociation of these are by Ftransfer
   e.g. 2BrF3 (l) ⇌ BrF4
   - (sol) + BrF2
   + (sol)

Question 9

What is molecular autoionization?

Answer 9

Applies to autoionizing solvents that are both protic and aprotic
A reaction between two molecules of the same substance to produce ions.

Auto-dissociation –can also apply to solvents that are aprotic. An ion is transferred from
one solvent molecule to the next, generating the cation form of the solvent and the anion of
the solvent.

Protic Solvents
2H2O(l) ⇌ H3O+(aq) + OH-
(aq)
2NH3
(l) ⇌ NH4
+(sol) + NH2
-
(sol)
3HF(l) ⇌ H2
F
+(sol) + HF2
-
(sol)

Non-Protic Solvents
2PF5
(l) ⇌ PF4
+(sol) + PF6
-
(sol)
2BrF3
(l) ⇌ BrF2
+(sol) + BrF4
-
(sol)

Question 10

What is the solvent system definition?

Answer 10

Solvent system definition

▪Acid base properties are closely connected with the properties of
solvents

▪The solvent system definition applies in all cases where the solvent
is capable of autoionization whether protons are involved or not.

▪This model for acid-base reactivity was developed for solvents
that can dissociate into cations (acid) and anions (base)

Question 11

What happens in the solvent system definition? Give an exampls.

Answer 11

In this definition, an acid is a solute that produces +ve solvent ions (increases
the concentration of the solvent cation) and a base is a substance that
produces –vely charged solvent ions (increases the concentration of the
solvent anion

Recall the autoionization of ammonia 2NH3
(l) ⇌ NH4
+(sol) + NH2
-(sol

n liquid ammonia, a
substance that
produces/increases NH4
+
ions is an acid and a
substance that
produces/increases
amide ions, NH2
- would
be a base

Question 12

Look at the following equation: BrF2AsF6 → BrF2
+ (sol) + AsF6- Is the salt an acid or a base?

Answer 12

An acid! It increases the
concentration of the cations
of the solvent

Question 13

Look at the following equation: KBrF4 → K
++ BrF4-
– Is the salt an acid or a base?

Answer 13

A base! It increases the
concentration of the anions
of the solvent,

A salt of the solvent cation is an acid and a
salts of the solvent anion is a base

Question 14

What does the solvent system definition do?

Answer 14

Broadens the understanding of acid/base characteristics:
For some substances, acidic or basic character can only be
specified in relation to a particular solvent

Example:
CH3
COOH(l) + H2
O(l) ⇌ H3
O
+(aq) + CH3
COO-
(aq)
CH3
COOH(l) + H2
SO4
(l) ⇌ CH3
C(OH)OH+(sol) + HSO4
-
(sol)

Question 15

How does acetic acid CH
3
COOH behave in each solvent?

CH3
COOH(l) + H2
O(l) ⇌ H3
O
+(aq) + CH3
COO-
(aq)
CH3
COOH(l) + H2
SO4
(l) ⇌ CH3
C(OH)OH+(sol) + HSO4
-
(sol)

Answer 15

It is an acid in water (increases the concentration of the solvent cation)
It is a base in sulfuric acid! (increases the concentration of the solvent anion)

Question 16

How else can we explain the relative behavior of acetic acid in each? CH3
COOH(l) + H2
O(l) ⇌ H3
O
+(aq) + CH3
COO-
(aq)
CH3
COOH(l) + H2
SO4
(l) ⇌ CH3
C(OH)OH+(sol) + HSO4
-
(sol)

Answer 16

First consider the Auto-dissociation or auto-ionization of solvent:
oFor protic solvents: transfer of a proton one solvent molecule to the next,
generating the protonated cation of the solvent and the deprotonated anion
of the solvent.
oFor aprotic solvents: transfer of an atom (e.g. F
-
in BrF3
) to generate cations
and anions of the solvent
❑The principle of the solvent system definition:
An acid is a solute that produces +ve solvent ions (increases the
concentration of the solvent cation) and a base is a substance that
produces –vely charged solvent ions (increases the concentration of the
solvent anion)

Question 17

What is the Lux Flood definition of acids and bases?

Answer 17

An acid is an oxide ion (O2-) acceptor
A base is an oxide ion donor.

Question 18

How would you classify these oxides based on the Lux
Flood definition?

Na2O (s) + H2O(l) → 2 Na+(aq) + 2 OH-
(aq)
SO2
(g) + H2O(l) → H2SO3
(aq)

Answer 18

❑Oxides are not obvious proton donors or acceptors
❑Their acidic and basic behaviour relate to their reaction
with water, which can generate H3O+(aq) or OH-
(aq).

E.g.
❖Sodium oxide (Na2O) is a basic oxide - it reacts with
water to form an alkaline solution of NaOH
❖Sulfur dioxide (SO2
) is an acidic oxide – it reacts with
water to form an oxoacid

Question 19

Explain the trend of the nature of basic and acidic oxides as you move from left to right across the periodic table.

Answer 19

Oxides of metallic elements
are generally basic oxides,
and oxides of nonmetallic
elements acidic oxides.

Question 20

CaO + CO2 → CaCO3
Identify the acid and base using Lux-Flood definition

Answer 20

The base (CaO) donates O2-
to the acid (CO2) to form CO3
2-

Question 21

What are amphoteric oxides?

Answer 21

❑Some oxides can react with both acids and bases

Question 22

With equations, give an example of an amphoteric oxide.

Answer 22

❑Aluminium oxide is an amphoteric oxide

❑ Al2O3(s) + 6HCl (aq) → 2AlCl3 (aq) + 3H2O (l)
❑Al2O3(s) + 2NaOH (aq) + 3H2O → 2NaAl(OH)4

Note: Many metals (such as copper, zinc, tin, lead, aluminium, and
beryllium) form amphoteric oxides or hydroxides. Amphoterism
depends on the oxidation state of the oxide.

Question 23

Explain the trend in the behaviour of oxides.

Answer 23

❑Basicity of the oxides increase with
increasing atomic number down a
group:
BeO < MgO < CaO < SrO < BaO

❑Acidity increases with increasing
oxidation state of the element:
MnO < Mn2O3 < MnO2 < Mn2O7
in keeping with the increase in
covalent character.