12.3A Aqueous Inorganic Chemistry Flashcards
Arrhenius theory of Acids and Bases
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.
Limitations of Arrhenius theory
- 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)
The Brønsted-Lowry theory of Acids and Bases
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
Amphoteric substances
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)
Problems with the Brønsted-Lowry theory
- Has no definition of Neutrality
- Still concerned with the traffic of protons
- Favors polar solvents
The Lewis theory of Acids and Bases
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).
In an ion with 3+ charges:
- 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
Hexaaqua ions
[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
Why [M(H2O)6]3+ solution has a low pH value?
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.
Why the solution of hexaaquairon(ll) ions is less acidic that the hexaaquairon(lll) one?
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)
NEUTRAL COMPLEX has no charge, so it doesn’t dissolve in water to any extend, and a precipitate(ppt.) is formed.
[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)
Reaction of dilute Ammonia (Brønsted-Lowry base) with Hexaaqua ions
[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+
Ligand exchange reaction
A reaction in which one ligand in a complex ion is replaced by a different ligand.
Water ligands in [Cu(H2O)6]2+ can also be exchanged for chloride ligands if we add concentrated hydrochloric acid drop by drop
blue solution <=> yellow(or olive-green) solution
[Cu(H2O)6]2+ (aq) + 4Cl- (aq) <=> [CuCl4]2- (aq) + 6H2O
Reaction with excess NH3
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)
Ligand exchange example:
dark blue -> pale blue
EDTA 4- (aq) + [Cu(NH3)4(H2O)2]2+ (aq) -> [Cu(EDTA)]2- (aq) + 4 NH3 (aq) + 2 H2O (l)
3+ hexaaqua ions reacting with carbonate ions
release CO2 gas
[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)
Carbonate ions combine with hydrogen ions in 2 stages:
1) (CO3) 2- (aq) + H+ (aq) <=> (HCO3)- (aq) + H2O (l)
2) (HCO3) - (aq) + H+ (aq) <=> CO2 (g) + H2O (l)
2+ hexaaqua ions reacting with carbonate ions
- 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)
Complex Ion
a central transition metal ion surrounded by ligands
Co-ordinate Bond
a covalent bond in which both electrons in the bond come from the same atom
Co-ordination Number
the number of co-ordinate (dative) bonds formed by ligands to the central transition metal ion
Ligand
a molecule or ion with one or more lone pairs of electrons avaliable to donate to a transition metal ion
Monodentate
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
Bidentate
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
Multidentate
ligands that can form only more than two co-ordinate bond from each ion or molecule to the cenral transition metal ion
Ligand Substitution reaction
- 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
Ammonia
- 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
Ligand exchange in [Co(H2O)6]2+ (pink colored) ions
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
Addition of conc. HCl drop by drop to an aqueous [Co(H2O)6]2+ results in tetrahedral complex
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-
Stability Constant
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
Kstab = {complex} / {hexaaqua ion} * {reagent2} ^coefficient
the higher (log) Kstab, the higher the stability of complex
[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
Chelate (crab’s claw) effect
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)
