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Inorganic chemistry > 24.1 acid base chemistry of aqueous transition metal ions > Flashcards

24.1 acid base chemistry of aqueous transition metal ions Flashcards

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1
Q

How are metal aqua complex ions formed

A

. When metals dissolve in water, the water molecules form coordinate bonds with the central transition metal ions

Eg 6 water molecules form co ordinate bonds with the metal ion by donating a non bonding pair of electrons from their oxygen

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2
Q

What are the 4 metal complex ions we need to know

A

Fe2+ and Cu2+ as the 2+ ions

so [Fe(H2O)6]2+ and [Cu(H2O)6]2+

Al3+ and Fe3+ as the 3+ ions

So [Al(H2O)6]3+ and [Fe(H2O)6]3+

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3
Q

What 3 bases do we need to know the reactions of metal ions with

A

OH-

NH3

CO3 2-

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4
Q

What is the copper metal aqua ion

What colour is it

A

[Cu(H2O)6 ]2+

Each H2O ligand donates a pair of electrons from its oxygen to the copper ion , forming 6 co ordinate bonds

It is blue

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5
Q

What is the iron (II) metal aqua ion

What about iron (III) metal ion

What is the aluminium metal aqua ion

A

[Fe(H2O)6]2+
It is green

[Fe(H2O)6]3+
It is pale violet when it is not oxidised, but turns brown when it is

[Al(H2O)6]3+
It is colourless

The charge of the overall complex is the same as the transition metal ion because the H2O ligand is neutral.

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6
Q

Do these 4 metal aqua complexes form acidic or basic solution

A

They all form acidic solutions, however aluminium is ampholytic so can also form basic solutions

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7
Q

What is the hydrolysis reaction of the metal aqua 2+ ions

A

Any solution that contains metal aqua 2+ ions

There is a reaction between the metal aqua ion and the water, and this reaction is called hydrolysis

The metal aqua 2+ ions release H+ ions, so an acidic solution is formed

But there is only a slight dissociation so the solution is only a weak acid

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8
Q

What happens when the complex

[Fe(H2O)6]2+ reacts with water in a hydrolysis reaction

This can also occur with [Cu(H2O)6]2+

A

It will dissociate in water to form

[Fe(H2O)6] 2+(aq) + H2O(l) —>
[Fe(OH)(H2O)5]+(aq) + H+

The complex has an overall charge of + instead or 2+ because the OH- ion makes it that

This can keep repeating, eg

[Fe(OH)(H2O)5]+ —>
Fe(OH)2(H2O)4(s) + H+

Fe(OH)2(H2O)4(s) —>
[Fe(OH)3(H2O)3]- (aq) + H+

Eventually it stops when

[Fe(OH)5(H2O)]3-(aq) —>
[Fe(OH)6]4- + H+

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9
Q

How do the metal 3+ion complexes have hydrolysis reactions

Eg [Fe(H2O)6]3+
or
[Al(H2O)6]3+

A

[Fe(H2O)6]3+ —>
[Fe(H2O)5(OH)]2+ + H+

then
[Fe(H2O)5(OH)]2+ —>
[Fe(H2O)4(OH)2]+ + H+

then

[Fe(H2O)4(OH)2]+ —>
Fe(H2O)3(OH)3(s) + H+

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10
Q

How do metal aqua 3+ ions react in water compared to 2+ ions

Give an example, eg of [Al(H2O)6]3+

A

They react in the same way but form more acidic solutions

[Al(H2O)6]3+ + H2O(l) —>
[Al(OH)(H2O)5]2+(aq) + H3O+

And H3O+ is formed because inside the brackets there are 11 H and 6 O as one H2O is turned into OH so there is a spare H which is H+ ion in the water

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11
Q

Why are metal aqua 3+ solutions more acidic than 2

A

. Metal 3+ ions are very small and have a big charge so they’ve got a high charge density

The metal 2+ ions have a much lower charge density

  • This makes the 3+ ions much more polarising than the 2+ ions
  • So they attract electrons from the oxygen atoms of the coordinatly bonded water molecules more strongly
  • Weakening the O-H bond, so it is more likely a hydrogen will be released when the bond breaks

More hydrogen ions means more acidic solution, so metal 3+ ions are more acidic than metal 2+ ions

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12
Q

Describe what happens when limited OH- ions are added the a 2+ metal ion complex [Fe(H2O)6] 2+

What is the colour change

A

[Fe(H2O)6]2+(aq) + 2OH-(aq) —>
Fe(H2O)4(OH)2 + 2H2O(l)

Remember that the number of OH- ions added are the same as the charge of the metal ion

Colour changes from green solution to green precipitate

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13
Q

Describe what happens when limited OH- ions are added the a 2+ metal ion complex [Cu(H2O)6]2+

What is the colour change

A

[Cu(H2O)6]2+ (aq) + 2OH-(aq) —>
[Cu(H2O)4(OH)2] (aq) + 2H2O(l)

Colour goes from blue solution to blue precipitate

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14
Q

describe what happens when reacting a metal 3+ ion eg Fe3+ with limited OH- ions

What is the colour change

A

[Fe(H2O)6] 3+(aq) + 3OH-(aq) —>
[Fe(H2O)3(OH)3] (s) + 3H2O

Remember that the number of OH- ions you add is the same as the metal ion charge

Colour goes from a violet solution to a BROWN precipitate

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15
Q

Describe what happens when a metal ion 3+ complex , eg aluminium is reacted with limited OH- ions from NaOH

What are the colour changes

A

[Al(H2O)6]3+(aq) + 3OH-(aq) —>
Al(H2O)3(OH)3(s) + 3H2O(l)

Goes from colourless solution to a white precipitate

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16
Q

What happens when limited ammonia (NH3) is added to metal aqua ion complexes

What type of reaction is it

A

This is an acid base reaction

Water molecules can be replaced by the neutral ligand ammonia, so coordination number won’t change

The ammonia molecule gains a proton so acts as a base

However it is basic so has the same effect as OH- ions at removing protons

17
Q

Describe the reaction for when limited ammonia (NH3) is added to the metal ion complex
[Cu(H2O)6]2+

(NH3) acts as OH-

What is the colour change

A

[Cu(H2O)6]2+(aq) + 2NH3(aq) —>
Cu(H2O)4(OH)2+ 2NH4+

They have the same colour changes as the OH- ions

So from blue solution to a blue precipitate

18
Q

Describe the reaction for when limited ammonia (NH3) is added to the metal ion complex

[Fe(H2O)6]2+

What is the colour change

A

[Fe(H2O)6]2+ (aq) + 2NH3(aq) —>
Fe(H2O)4(OH)2(s) + 2NH4+(aq)

The colour changes from a green solution to a green precipitate

19
Q

Describe the reaction for when limited ammonia (NH3) is added to the metal ion complex

[Fe(H2O)6]3+

A

[Fe(H2O)6]3+(aq) + 3NH3(aq) —>
Fe(H2O)3(OH)3(s) + 3NH4+(Aq)

Colour changes from a purple solution to a brown precipitate

20
Q

Describe the reaction for when limited. ammonia (NH3) is added to the metal ion complex

[Al(H2O)6]3+

A

[Al(H2O)6]3+ (aq) + 3NH3(aq) —->
Al(H2O)3(OH)3 (s) + 3NH4+

Colour changes from colourless to white precipitate

21
Q

What happens when excess OH- ions are reacted with a metal 2+ ion complex

[Fe(H2O)6]2+

A

It is the same reaction as before, but with the product you add one more OH- ion to it

Eg [Fe(H2O)6]2+(aq) + 2OH-(aq) –>
Fe(H2O)4(OH)2(s) + 2H2O

Then Fe(H2O)4(OH)2 (s) + OH-(aq) —>
[Fe(OH)3]- + 4H2O

So it kicks off the H2Os

22
Q

What happens when excess OH- ions are reacted with a metal 3+ ion complex

Eg [Al(H2O)6]3+

What is the colour change

A

An extra OH- is added onto the original product

so Al(H2O)3(OH)3 + OH- —>
[Al(OH)4]- + 3H2O

And it turns from a white precipitate to a colourless solution

23
Q

Aluminium aqua ion is amphoteric

What does this mean

A

It reacts with both acids and bases, unlike the other ones we have studied which just react with bases

24
Q

Describe the reaction between
Al(H2O)3(OH)3 and an acid

A

It is acting as a base to accept H+ ions

Al(H2O)3(OH)3 + 3H+ —>
[Al(H2O)6]3+

Here, it forms its aqua ion complex again

So colour changes from white precipitate to colourless solution

25
Q

Describe reaction of copper hydroxide with excess ammonia

What type of reaction is it
What are the colour changes

What type of base does ammonia act as

A

It is an incomplete ligand substitution reaction

Cu(H2O)4(OH)2(s) + 4NH3(aq) —->
[Cu(H2O)2(NH3)4]2+ + 2H2O + 2OH-

As you can see, not all the H2Os are replaced by NH3

Ammonia acts as a lewis base

Goes from blue precipitate to blue solution

26
Q

Describe the reaction of metal aqua 2+ ions with carbonate ligands, eg from Na2CO3

What is the name of the reaction

[Cu(H2O)6]2+

Describe the colour changes

A

Precipitation reaction

There is only 1 mol of CO3 2- required to react
[Cu(H2O)6]2+(aq) + CO3 2-(aq) —->
CuCO3(s) + 6H2O

From blue solution to blue precipitate

27
Q

What is the name of the reaction

Describe the reaction of metal aqua 2+ ions with carbonate ligands, eg from Na2CO3

[Fe(H2O)6]2+

A

Precipitation reaction

[Fe(H2O)6]2+(aq) + CO3 2-(aq) —>
FeCO3(s) + 6H2O

Colour changes from a green solution
to a green precipitate

28
Q

Describe the reaction of metal aqua 3+ ions with carbonate ligands

Eg [Fe(H2O)6]3+

What will you see

A

Fe(H2O)6]3+(aq) + 3CO3 2-(aq) —>
Fe(H2O)3(OH)3(s) + 3CO2(g) + 3H2O

There will be effervescence / bubbling

Colour change from violet solution to brown precipitate

29
Q

Describe the reaction of metal aqua 3+ ions with carbonate ligands

Eg [Al(H2O)6]3+

What do you see

A

[Al(H2O)6]3+ + 3CO3 2- —>
Al(H2O)3(OH)3 + 3CO2(g) + 3H2O

colour changes from colourless solution to a white precipitate

Effervescence / bubbling

Inorganic chemistry (19 decks)

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