12.3A Aqueous Inorganic Chemistry

Question 1

Arrhenius theory of Acids and Bases

Answer 1

ACID – a substance that ionizes in water to produce hydrogen ions, H+.
ALKALI – a soluble base, ionizes in water to produce hydroxide ions, OH-.

The combination of an Acid and Base – NEUTRALISATION, combination of the H+ ion and the OH- ion.

Question 2

Limitations of Arrhenius theory

Answer 2

• doesn’t explain the reaction between ammonia NH3 and HCl gas, as ammonia doesn’t not contain OH- ions.
• NH3 (g) + H2O (l) —> NH4+ (aq) + OH- (aq)
  Thus, aqueous ammonia is an alkali
  Ammonia accepted H+ ion to become NH4+
  So, a wider definition of a base needed (Base – a substance that accepts H+ ions)

Question 3

The Brønsted-Lowry theory of Acids and Bases

Answer 3

An ACID is a proton (H+) DONOR.

A BASE is a proton (H+) ACCEPTOR.

• acid-base pairs related to each other – Conjugate Acid-Base pairs
• a theory of proton transfer

Question 4

Amphoteric substances

Answer 4

Substances like water, which can act as either acids or bases
Base:
HCl (g) + H2O (l) —> H3O+ (aq) + Cl- (aq)

Acid:
NH3 (g) + H2O (l) <=> NH4+ (aq) + OH- (aq)

Question 5

Problems with the Brønsted-Lowry theory

Answer 5

• Has no definition of Neutrality
• Still concerned with the traffic of protons
• Favors polar solvents

Question 6

The Lewis theory of Acids and Bases

Answer 6

A Lewis ACID is a substance that can ACCEPT a pair of electrons to form a new bond (electrophiles).

A Lewis BASE is a substance that can DONATE a pair of electrons to form a new bond (nucleophiles).

An ACID is an electron pair acceptor.
A BASE is an electron pair donor.

• theory is based of the transfer of electrons
• COORDINATE COVALENT BOND

NEUTRALISATION – a reaction between acid and base that results in the formation of an additional compound, in which the electron pair that constitutes chemical bond comes from only one reactant (Dative bond).

Question 7

In an ion with 3+ charges:

Answer 7

• Stronger attraction of electrons
• The electrons in the O–H bonds will be pulled further away
• Hydrogen atoms in the water ligand will have greater positive charge in a 3+ ion, so can be pulled off in a reaction involving water molecules in a solution (hydrolysis)
  (cause greater polarization of water molecules than 2+ ions)
• So the 3+ ions are more acidic

Question 8

Hexaaqua ions

Answer 8

[M(H2O)6]3+ pH is 2-3
[M(H2O)6]2+ pH is 5-6

• the smaller the radius of the metal ion, the stronger the acid (the greater the distorting effect on the electrons in the O–H bonds

Question 9

Why [M(H2O)6]3+ solution has a low pH value?

Answer 9

The low pH value is due to the formation of the hydroxonium ions, which are just hydrogen ions carried by water molecules. The graeater the concentration of hydrogen ions, the lower the pH.

Question 10

Why the solution of hexaaquairon(ll) ions is less acidic that the hexaaquairon(lll) one?

Answer 10

The 2+ ion at the centre of the complex will have less pulling effect on electrons in the co-ordinate bonds tha a 3+. This will mean that there will be less distortion in the O-H bonds, and so the hydrogens won’t be quite so positive. That means that they won’t be pulled off by other water molecules quite so easily, thus fewer hydroxonium ions will be formed, and so the solution is less acidic (as there ia less hydrogen ions)

Question 11

NEUTRAL COMPLEX has no charge, so it doesn’t dissolve in water to any extend, and a precipitate(ppt.) is formed.

Answer 11

[M(H2O)6]3+ (aq) + H2O (l) <=> [M(H2O)5(OH)]2+ (aq)+ H3O+ (l)
[M(H2O)5(OH)]2+ (aq) <=> [M(H2O)5(OH)2]+ (aq) + H+
[M(H2O)5(OH)2]+ (aq) <=> [M(H2O)5(OH)3] (s) + H+

[M(H2O)6]2+ (aq) + H2O (l) <=> [M(H2O)5(OH)]+ (aq) + H3O+ (l)
[M(H2O)5(OH)]+ (aq) <=> [M(H2O)5(OH)2] (s) + H+ (aq)

Question 12

Reaction of dilute Ammonia (Brønsted-Lowry base) with Hexaaqua ions

Answer 12

[M(H2O)6]2+ (aq) + 2NH3 <=> [M(H2O)5(OH)2] (s) + 2NH4+
[M(H2O)6]3+ (aq) + 3NH3 <=> [M(H2O)5(OH)3] (s) + 3NH4+

Question 13

Ligand exchange reaction

Answer 13

A reaction in which one ligand in a complex ion is replaced by a different ligand.

Question 14

Water ligands in [Cu(H2O)6]2+ can also be exchanged for chloride ligands if we add concentrated hydrochloric acid drop by drop

Answer 14

blue solution <=> yellow(or olive-green) solution
[Cu(H2O)6]2+ (aq) + 4Cl- (aq) <=> [CuCl4]2- (aq) + 6H2O

Question 15

Reaction with excess NH3

Answer 15

blue solution -> deep ink blue solution
[Cu(H2O)6]2+ (aq) + 4NH3 (aq) <=> [Cu(NH3)4(H2O)2]2+ (aq) + 4H2O (l)
- ammonia as a base

pale blue ppt in blue solution redissolves when more ammonia added and ligand exchange reaction occurs, resulting in deep ink blue solution.
Cu(OH)2(H2O)4 (s) + 4NH3 (aq) <=> [Cu(NH3)4(H2O)2]2+ (aq) + 2H2O (l) + 2OH- (aq)

Question 16

Ligand exchange example:

Answer 16

dark blue -> pale blue
EDTA 4- (aq) + [Cu(NH3)4(H2O)2]2+ (aq) -> [Cu(EDTA)]2- (aq) + 4 NH3 (aq) + 2 H2O (l)

Question 17

3+ hexaaqua ions reacting with carbonate ions
release CO2 gas

Answer 17

[M(H2O)6]3+ (aq) + (CO3) 2- (aq) -> [M(H2O)5(OH)]2+ (aq) + CO2 (g) + H2O (l)

2[M(H2O)6]3+ (aq) + 3 (CO3) 2- (aq) -> 2[M(H2O)3(OH] (s) + 3 CO2 (g) + 3 H2O (l)

Question 18

Carbonate ions combine with hydrogen ions in 2 stages:

Answer 18

1) (CO3) 2- (aq) + H+ (aq) <=> (HCO3)- (aq) + H2O (l)

2) (HCO3) - (aq) + H+ (aq) <=> CO2 (g) + H2O (l)

Question 19

2+ hexaaqua ions reacting with carbonate ions

Answer 19

• The 2+ hexaaqua ions are not strongly acidic enough to release CO2 from carbonates
• Forms a ppt of what is loosely described as the “metal carbonate”
  M2+ (aq) + (CO3)2- (aq) -> MCO3 (s)

Question 20

Complex Ion

Answer 20

a central transition metal ion surrounded by ligands

Question 21

Co-ordinate Bond

Answer 21

a covalent bond in which both electrons in the bond come from the same atom

Question 22

Co-ordination Number

Answer 22

the number of co-ordinate (dative) bonds formed by ligands to the central transition metal ion

Question 23

Ligand

Answer 23

a molecule or ion with one or more lone pairs of electrons avaliable to donate to a transition metal ion

Question 24

Monodentate

Answer 24

ligands such as water and ammonia, that can form only one co-ordinate bond from each ion or molecule to the cenral transition metal ion

Question 25

Bidentate

Answer 25

ligands that can form only two co-ordinate bond from each ion or molecule to the cenral transition metal ion
OR
Ligands that have two lone pairs per ligand molecule avaliable for complex formation

Question 26

Multidentate

Answer 26

ligands that can form only more than two co-ordinate bond from each ion or molecule to the cenral transition metal ion

Question 27

Ligand Substitution reaction

Answer 27

• Ligand Substitution involves the exchange of one ligand for another, with no change in oxidation state of central metal ion
• a type of substitution reaction
• happens if the new complex formed is more stable than the original complex

Question 28

Ammonia

Answer 28

• water and ammonia molecules are very similar in size, and so there is no change in co-ordination
• ammonia as a BASE:
  small amount of ammonia will act as a base by accepting a proton (H+) and precipitates of the metal hydroxide is formed.
• ammonia as a LIGAND:
  in some cases, more ammonia redissolves the ppt of ligand exchange reactions

Question 29

Ligand exchange in [Co(H2O)6]2+ (pink colored) ions

Answer 29

pink solution -> blue ppt
[Co(H2O)6]2+ + 2OH- -> Co(OH)2(H2O)4 (s)

pink solution -> blue ppt redissolves in excess of ammonia -> straw solution
[Co(H2O)6]2+ + 6NH3 <=> [Co(NH3)6]2+ + 6 H2O

Question 30

Addition of conc. HCl drop by drop to an aqueous [Co(H2O)6]2+ results in tetrahedral complex

Answer 30

pale pink solution <=> blue solution
[Co(H2O)6]2+ (aq) + 4Cl- (aq) <=> [CoCl4]2- (aq) + 6 H2O (l)

• chloride ions are bigger than water molecules, and there is no room to fit 6 of them around the central cobalt ion
• aqueous cobalt(ll) ions usually form tetrahedral complexes with monodentate anionic ligands such as Cl- , SCN- and OH-

Question 31

Stability Constant

Answer 31

is an equllibrium constant for the formation of a complex in a solution
OR
is the equilibrium constant for the formation of the complex ion in a solvent from its constituent ions/molecules

Question 32

Kstab = {complex} / {hexaaqua ion} * {reagent2} ^coefficient

the higher (log) Kstab, the higher the stability of complex

Answer 32

[Cu(H2O)6]2+ + 4 NH3 <=> [Cu(NH3)4(H2O)2]2+ + 4 H2O

Kstab = {[Cu(NH3)4(H2O)2]2+} / {[Cu(H2O)6]2+} * {NH3 }^4
Kstab = 1.20 * 10^13 mol^-4 dm^12
log Kstab = 13.1

Question 33

Chelate (crab’s claw) effect

Answer 33

an effect which happens when you repace water(or other simple ligands) around the central metal ion by multidentate ligands like EDTA

• chelate is more stable than than ions with only monodentate ligands (as each multidentate ligand displaces more than one water molecule, this leads to an increase of species present in the system, therefore results in an increase in entropy which makes the formation of chelated complex more favourable)