Reactions of ions in aqueous solution Flashcards
What are metal-aqua ions?
When metal ions dissolve in water, water ligands form coordinate bonds with the metal ion. This forms metal-aqua complex ions, where there are 6 water ligands around a central metal ion in an octahedral shape
Describe the acidic nature of metal-aqua ions
[M(H2O)6]^2+ + H2O <-> [M(H2O)5(OH)]+ +H3O+
[M(H2O6)]^3+ + H2O <-> [M(OH)(H2O)5]^2+ +H3O+
This is a hydrolysis or acidity reaction.
The solution is weakly acidic.
pH is around 6.00 for 2+ , 3.00 for 3+
Why does the acidity of a solution increase with the charge on the metal-aqua complex?
Electron pairs are being pulled away from the oxygen towards the metal ion.
Electron pairs on the O-H bonds to be pulled even closer to oxygen than normal.
Hydrogen atoms even more slightly positive than normal so can be pulled off in a reaction with water molecules
How can the effect of charge on acidity be explained using m/z?
Smaller charge ion has smaller charge to size ratio than bigger charge ion, so the smaller ion is less polarising, so less weakening effect on the O-H bond
What are the base compounds used?
CuBr2
Fe(NO3)2
Fe(NO3)3
AlCl3
What happens when a copper bromide salt is dissolved in water?
CuBr2 +6H2O -> [Cu(H2O)6]2+ + 2Br-
pH reading is 6.00 - hydrolysis reaction
Describe the reaction of Cu(II) ions with dilute OH- ions drop by drop
[Cu(H2O)6]2+ + OH- -> [Cu(H2O)5(OH)]+ + H2O
[Cu(H2O)5(OH)]+ + OH- -> [Cu(H2O)4(OH)2] + H2O
Type of reaction: hydrolysis
Observation: Blue solution to blue ppt
What is the general reaction for 2+ ions in excess dilute/ conc. OH- ions
[M(H2O)6]2+ + 6OH- -> [M(OH)6]4- + 6H2O
Describe the reaction of Cu(II) ions with dilute NH3 ions drop by drop
[Cu(H2O)6]2+ +NH3 -> [Cu(OH)(H2O)5]+ + NH4+
[Cu(OH)(H2O)5]+ +NH3 -> Cu(OH)2(H2O)4 + NH4+
Type of reaction: Hydrolysis
Observation: Blue solution to blue ppt
What is the reaction when excess dilute or conc. ammonia is added to Cu(II) ions?
[Cu(H2O)6]2+ + 4NH3 -> [Cu{NH3)4(H2O)2]2+ +4H2O
Type of reaction: Ligand substitution
Observation: Blue solution to deep blue ppt (blue solution forms blue ppt, which then dissolves to form deep blue soln)
Describe the reaction of Cu(II) ions with dilute CO32+ ions drop by drop
[Cu(H2O)6]2+ + CO32- -> CuCO3 + 6H2O
Type of reaction: Ligand substitution
Observation: Blue solution to blue-green ppt
This type of ppt can be identified by adding excess ammonia solution to CuCO3 - which will not dissolve
Describe the reaction of Cu(II) ions with concentrated Cl- ions
[Cu(H2O)6]2+ + 4Cl- -> [CuCl4]2- + 6H2O
Type of reaction: Ligand substitution
Observation: Blue solution to yellow-green solution
What is the reaction when an iron(II) nitrate is dissolved in water?
Fe(NO3)2 + 6H2O -> [Fe(H2O)6]2+ + 2NO3-
pH reading is 6.00 as it is hydrolysis
Describe the reaction of Fe(II) ions with OH- ions that are added drop by drop until in excess
[Fe(H2O)6]2+ + OH- -> [Fe(OH)(H2O)5]+ + H2O
[Fe(H2O)5(OH)]+ + OH- -> Fe(OH)2(H2O)4 + H2O
Type of reaction: Hydrolysis
Observation: Green solution to green precipitate
Reagent: NaOH
What happens when Fe(OH)2(H2O)4 ppt is exposed to oxygen?
Goes to Fe(OH)3(H2O)3
Observation: Green ppt to brown ppt
Type of reaction: Redox
Describe the reaction of Fe(II) ions with NH3 that are added drop by drop
[Fe(H2O)6]2+ + NH3 -> [Fe(OH)(H2O)5]+ + NH4+
[Fe(OH)(H2O)5]+ + NH3 -> Fe(OH)2(H2O)4 + NH4+
Type of reaction: Hydrolysis
Observation: Green solution to green ppt
Reagent: NH3
Describe the reaction of Fe(II) ions with CO32- ions that are added drop by drop
[Fe(H2O)6]2+ + CO32- -> FeCO3 + 6 H2O
Type of reaction: Ligand substitution
Observation: Green solution to green ppt
Reagent: Na2CO3
How do you distinguish between FeCO3 ppt and Fe(OH) ppt?
Leave exposed to air. Fe(OH) turns brown
Describe the reaction of Fe(II) ions with Cl- ions that are added drop by drop
[Fe(H2O)6]2+ + 4Cl- -> [FeCl4]2- + 6H2O
Type of reaction: Ligand substitution
Observation: Green solution to yellow solution
Reagent: Conc. HCl or NaCl
What is the reaction when an iron(III) nitrate salt is dissolved in water?
Fe(NO3)3 + 6H2O -> [Fe(H2O)6]3+ + 3NO3-
pH reading is 3.00 as it is hydrolysis
Describe the reaction of Fe(III) ions with OH- ions that are added drop by drop until in excess
[Fe(H2O)6]3+ + OH- -> [Fe(OH)(H2O)5]2+ + H2O
[Fe(OH)(H2O)5]2+ + OH- -> [Fe(OH)2(H2O)4]+ + H2O
[Fe(OH)2(H2O)4]+ + OH- -> Fe(OH)3(H2O)3 + H2O
Type of reaction: Hydrolysis
Observation: Purple solution to brown ppt
Reagent: NaOH
Describe the reaction of Fe(III) ions with NH3 that are added drop by drop
[Fe(H2O)6]3+ + NH3 -> [Fe(OH)(H2O)5]2+ + NH4+
[Fe(OH)(H2O)5]2+ + NH3 -> [Fe(OH)2(H2O)4]+ + NH4+
[Fe(OH)2(H2O)4]+ + NH3 -> Fe(OH)3(H2O)3 + NH4+
Type of reaction: Hydrolysis
Observation: Purple solution to brown ppt
Reagent: NH3
Describe the reaction of Fe(III) ions with CO32- ions that are added drop by drop until in excess
2[Fe(H2O)6]3+ + 3CO32- -> 2Fe(OH)3(H2O)3 + 3H2O + 3CO2
Type of reaction: Hydrolysis
Observation: Purple solution to brown ppt and effervescence
Reagent:Na2CO3
Why do +2 and +3 metal-aqua ions react differently with CO32-?
3+ ion has a larger charge to mass ratio, so has a larger weakening effect on the O-H bond in water.
CO32- + H3O+ -> HCO3- + H2O
HCO3- + H3O+ -> CO2 + 2H2O
cancel, overall:
CO32- + 2H3O+ -> CO2 + 3H2O
