catalytic activity of transition elements

7/28/2019 Catalytic Activity of Transition Elements

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Catalytic activity of

transition elements


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Transition metals and their compounds are often good catalysts.

The small differences in ionization energies make variable oxidation numbers poss

When transition metals act as catalyst they use their empty orbitals to form tempo

covalent bond with reactants molecules, weakening the bonds and hence provide energy to break the bonds.

There are two types of catalysts.

In a heterogeneous reaction, the catalyst is in a different phase from the reactants

In a homogeneous reaction, the catalyst is in the same phase as the reactants.


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Homogeneous catalyst

This has the catalyst in the same phase as the

reactants.

Typically everything will be present as a gas or

contained in a single liquid phase.

Transition metals can act as a homogeneous catalysbecause they exhibit variable states.


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The reaction between peroxodisulphate ions and iodide ions

The reaction needs a collision between two negative ions.

The catalyst can be either iron(II)

The peroxodisulphate ions oxidise the iron(II) ions to iron(III) ions.

In the process the peroxodisulphate ions are reduced to sulphate ions.

The iron(III) ions are strong enough oxidising agents to oxidise iodide ions to iodine

process, they are reduced back to iron(II) ions again.


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Heterogeneous catalysis

This involves the use of a catalyst in a different phase from the reactants.

Typical examples involve a solidcatalyst with the reactants as either liquidsor gases.

Heterogeneous catalysis reactions occur on the surface of the catalyst.

There are active sites on the surface of catalyst, where reactions occur.

Transition metals act as heterogeneous catalysts due to vacant d-orbital.


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How the heterogeneous catalyst works

Diffusion

Reactant molecules move to the activesites


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Adsorption

The surface of the catalyst attracts thereactant molecules by intermolecularforces and form temporary dative covalent

bonds. Adsorption is the surface process.


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Weakening of the bondsHence the bonds which are the

reactant molecules become weaker orsome bonds might be broken. Due to

weakness of bonds, energy is requiredto break the bonds is very weak. Thus

activation energy is low.

DesorptionNew bonds are formed on the surface ofthe catalyst to form product molecules

and then product molecules move awayfrom active sites of catalyst, that is

products are desorbed and the diffuseaway from the catalyst.


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Autocatalysis

In autocatalysis, the reaction is catalysed by one of its products.

The oxidation of ethanedioic acid by manganate(VII) ions

The reaction is very slow at room temperature.

It is used as a titration to find the concentration of potassium manganate(VII)solution and is usually carried out at a temperature of about 60C.


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Substance Reaction catalysed

Iron Haber process to convert nitrogen and hydrogen toammoniaNickel Margarine production to hydrogenate unsaturated

hydrocarbonsVanadium(V)

oxide Contact process to convert oxygen and sulphur diox

to sulphur trioxide


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Ligand Exchange Processes It is possible for one type of ligand to be replaced by another type.

Ligand exchange reactions often have a colour change associated with them.

Thiocyanate ions can replace one of the water ligands in [Fe(H2O)6]3+.

[Fe(H2O)6]3+ + SCN- [Fe (H2O)5SCN]

2+ + H2O

An aqueous Copper(II) sulphate solution is blue in colour because of the presence of [Cu(H 2O

concentrated hydrochloric acid is added to this solution the colour changes from blue to gre

happens because the [CuCl4]2- ion is produced. The Cl- ions have replaced the H2O molecule

exchange.

[Cu(H2O)6]2+ + 4Cl- [CuCl4]

2- + 6H2O

When ammonia is added a further change from green to deep blue takes place as ammonia m

replace the chloride ions.

[CuCl4]2- + 4NH3 + 2H2O [Cu(NH3)4(H2O)2]

2+ + 4Cl-

If EDTA is then added yet another ligand exchange takes place and the solution turns pale bl


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These changes take place because the complexes become more stable.

The stability of the [Cu(edta)]2+ complex can be understood in terms of entropy.As the one edta molecule replaces the six smaller ligands, the small ligands are released intosolution and therefore have greater freedom of movement, so greater disorder andconsequently the entropy increases.

[Cu(H2O)6]2+ [CuCl4]

2- [Cu(NH3)4(H2O)2]2+ [Cu(edta)]2+

Increasing stability


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Deprotonation reactions

Deprotonation reactions involve water ligands losing hydrogen ions (proton) toa proton acceptor such as an hydroxide ion.

[Cu(H2O)6]2+ + OH- [Cu(OH)(H2O)5]+ + H2O

Deprotonation reactions often result in the formation of a precipitate.


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Reaction of complex ions with sodiumhydroxide and ammonia solutions

Sodium hydroxide and ammonia solutions contain hydroxide ions.

When these are added to solutions containing transition metal ions a

precipitate of the metal hydroxide is formed.

If further quantities of these reagents are added to the mixture, theprecipitate, in certain cases, dissolves


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Ion insolution

Reaction with a few dropsof NaOH(aq) or NH3(aq)

Reaction with excessNaOH(aq)

Cr3+ Pale green ppt Ppt dissolves to form a deep

green solutionMn2+ Beige ppt No further reaction

Fe2+ Dirty green ppt No further reaction

Fe3+ Red-brown ppt No further reaction

Ni2+ Green gelatinous ppt No further reaction

Cu2+ Blue ppt No further reaction

Zn2+ White gelatinous ppt Ppt dissolves to form acolourless solution


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A simple way of looking at these is as hydroxide ions adding to the transition metalnumber of hydroxide ions being equal to the charge on the ion.

[M(H2O)x]y+ + yOH- [M(H2O)x-y(OH)y] + yH2O

In fact rather than a water molecule leaving and a hydroxide ion joining, the proce

consists of a hydrogen ion moving


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Vanadium chemistry

Vanadium forms stable compounds in 4different oxidation states, +2, +3, +4 and

+5.

In aqueous solution, the ions formed are:

+5 oxidation state is mildly oxidizingagent.

+4 oxidation state is more stable oxidationstate

+3 oxidation state is unstable in thepresence of oxygen

+2 oxidation state is strong reducingagent.

V +4 and +5 oxidation states have smallsize and high charge, so they are highlypolarizing and deprotonate aqua ligands.

Thus they do not exist as simple ions suchas V4+ and V

+5, instead exist as VO

2+and

VO2+

ions.

Oxidation state +5 +4 +3 +2

Colour Yellow Blue green Violet

Ion VO2+ VO2+ V3+ V2+


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The reduction from +5 to +4

The reduction from +4 to +2


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Production of VO2+ in the laboratory

Dissolve about 2 g of ammonium metavanadate (NH4

VO3

) in abcm3 of sodium hydroxide aqueous solution (1 M) in a 500 cm3 b

While stirring the solution formed, add 80 cm3of dilute sulphur

Transfer the solution into a 250-cm3 volumetric flask and add d

water to the calibration mark. Shake the volumetric flask vigo

record the color of the solution.

NH4VO3 + H2SO4 NH4+ + H2O + VO2

+ + SO42-

VO3-

+ 2H+

+ H2O + VO2+


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Oxygen is expected to oxidize all the vanadium species t o VO2+ but it oxidize, V2+

V3+ to VO2+ not V2+ due to kinetic stability of the VO2+


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Question When zinc power is added to aqueous Vanadium(V) ions, yellow green, blue,

green and violet colours can be seen successively. Explain these observations.

VO2+(aq) VO2+(aq) V3+ V2+

Yellow.........greenblue..............green..violet

Step 1 Step 2 Step 3

Step 1= E red- Eoxid

=(+1.00)-(-0.76)

=+1.76V


=(+0.34)-(-0.76)=+1.10V


=(-0.26)-(-0.76)=+0.50V


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Conclusion

As E0cell value of each step is positive, all the three steps arefeasible to occur spontaneously.

Thus zinc can reduce VO2+ to V2+. Therefore colour change isfrom yellow to green to blue to green to violet. The final

solution is violet.

In between yellow and blue in the first step, green colourcan be seen due to a mixture of VO2+ and VO2+ ions


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COPPER

Copper [Ar]3d104s1

Copper, 3d10, is the only member of the transition series to have a significant +1

oxidation state, and even here the +1 state is only stable if in a complex ion, or inan insoluble compound in solution, it disproportionates.

The +1 state, with a full d sub-shell, is not coloured (apart from Cu2O).

The +2 state, with its familiar blue and green complexes, is the normal stable state.


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Cu(I)

Cu2O, as made by reduction of Fehlings or Benedicts solution with a reducing

sugar, is a red insoluble solid.

CuCl and Cu2SO4 are white solids. Both of these, when dissolved in waterdisproportionate:

2Cu+(aq) Cu(s) + Cu2+(aq)

This can be understood in terms of the redox potentials:

Cu2+ + e- Cu+ Eo =+0.15V

Cu+ + e- Cu Eo =+0.52V

There is a reaction between the two underlined species, i.e. thedisproportionation.

So when a copper(I) compound is dissolved in water a blue solution (Cu2+(aq))and a red-brown solid (Cu(s)) are formed.


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Many copper(I) compounds and copper(I) complex ions do not show the samevariety of colour you see in copper(II) compounds and complex ions.

The lack of scope for a variety of coloured compounds arises from the

fundamental electronic configuration of the Cu

+

ion, namely [Ar]3d

10

,giving a completely filled 3d sub-shell (identical to that of the zincion Zn2+, and zinc compounds and complex ions tend to be white or

colourless).

ie there is no electron that can be promoted to a higher level when the

3d sub-shell is split when the central metal ion interacts with the ligands.


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Cu(II)

Most copper(II) compounds are blue, and in solution they give blue Cu(H2O)62+ ions.


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Reactions of copper(II) ions in solution

The reaction of hexaaquacopper(II) ions with hydroxide ions

Hydroxide ions (from, say, sodium hydroxide solution) remove hydrogen ionsfrom the water ligands attached to the copper ion


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Reactions of hexaaquacopper(II) ions with ammonia solution

The ammonia acts as both a base and a ligand.

With a small amount of ammonia, hydrogen ions are pulled off the hexaaquaion exactly as in the hydroxide ion case to give the same neutral complex

That precipitate dissolves if you add an excess of ammonia

The ammonia replaces water as a ligand to give tetraamminediaquacopper(II) ions.


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A ligand exchange reaction involvingchloride ions

Concentrated hydrochloric acid to a solution containing hexaaquacopper(II)ions, the six water molecules are replaced by four chloride ions.

The reaction taking place is reversible.

Because the reaction is reversible, you get a mixture of colours due to both of the complex ion


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Using this reaction to find theconcentration of copper(II) ions in solution

pipette a known volume of a solution containing copper(II) ions into a flask,and then add an excess of potassium iodide solution.

Find the amount of iodine liberated by titration with sodium

thiosulphate solution.


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As the sodium thiosulphate solution is run in from a burette, the colour of theiodine fades.

When it is almost all gone, you add some starch solution.

This reacts reversibly with iodine to give a deep blue starch-iodine complex

which is much easier to see

the last few drops of the sodium thiosulphate solution slowly until the bluecolour disappears

the reacting proportions through the two equations, you will find that for

every 2 moles of copper(II) ions you had to start with, you need 2 moles of

sodium thiosulphate solution.

If you know the concentration of the sodium thiosulphate solution, it is easy

to calculate the concentration of the copper(II) ions.


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CHROMIUM

Chromium forms stable ions in three different oxidation states, +2, +3 and +6.

In aqueous solutions, the ions formed are

The actual formula of the +2 and +3 complex ions depends on the acid used(Cl- and SO4

2- will also behave as ligands).

Oxidation state +6 +3 +2

Colour orange Green blue

Ion Cr2O72- Cr3+ Cr2+

I lk li l ti th i f d


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In aqueous alkaline solutions, the ions formed are:

The +6 chromium complexes can be readily interconverted using acid and

alkali:

2CrO42-(aq) + 2H+(aq) Cr2O72-(aq) + H2O(l)

Cr2O72-(aq) + 2OH-(aq) 2CrO42-(aq) + H2O(l)

Oxidationstate

+6 +3

Colour yellow GreenIon CrO4

2- Cr(OH)63-


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a) In acidic solution

All chromium (VI) compounds can be reduced to the +3 and then the +2oxidation state by strong reducing agents such as zinc in acid solution.

The chromium is first reduced to the +3 oxidation state:

Cr2O72-(aq) + 14H+(aq) + 6e == 2Cr3+(aq) + 7H2O(l)

Zn(s) == Zn2+(aq) + 2e

Overall : Cr2O72-(aq) + 14H+(aq) + 3Zn(s) 2Cr3+(aq) + 7H2O(l) + 3Zn

2+(aq)


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It is then further reduced:

Cr3+(aq) + e == Cr2+(aq)

Zn(s) == Zn2+(aq) + 2e

Overall: Zn(s) + 2Cr3+(aq) Zn2+(aq) + 2Cr2+(aq)

The colour change observed on adding zinc in acid solution to dichromateions is orange (Cr2O7

2-) to green (Cr3+) to blue (Cr2+).


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Other, milder reducing agents will reduce the +6 oxidation state to +3 but no

further.

Fe2+ is an important example:

Fe2+(aq) == Fe3+(aq) + e

Overall: Cr2O72-(aq) + 14H+(aq) + 6Fe2+(aq) 2Cr3+(aq) + 7H2O(l) + 6Fe3+(aq)

This reaction can be used in titrations to determine the concentration of Fe2+

ions in a sample.


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b) In alkaline solution

In alkaline solution, it is possible to oxidise the +3 oxidation state to the +6oxidation state.

. Hydrogen peroxide, which is a reducing agent in acidic solution, is anoxidizing agent in alkaline solution.

[Cr(OH)6]3-(aq) + 2OH-(aq) == CrO4

2-(aq) + 4H2O(l) + 3e

H2O2(aq) + 2e == 2OH-(aq)

2[Cr(OH)6]3-(aq) + 3H2O2(aq) 2CrO4

2-(aq) + 8H2O(l) + 2OH-(aq)

The green [Cr(OH)6]3- ion is oxidised to the yellow CrO4

2- ion in alkalinesolution.

In general, oxidation is favoured by alkaline conditions and

reduction is favoured by acidic conditions.


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Ligand exchange reactions involvingchloride or sulphate ions

The hexaaquachromium(III) ion is a "difficult to describe" violet-blue-greycolour.

Replacement of the water by sulphate ions

One of the water molecules is replaced by a sulphate ion.

Two of the positive charges are cancelled by the presence of the two negative

charges on the sulphate ion.


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Replacement of the water by chloride ions

In the presence of chloride ions (for example with chromium(III) chloride), the

most commonly observed colour is green.

This happens when two of the water molecules are replaced by chloride ionsto give the tetraaquadichlorochromium(III) ion - [Cr(H2O)4Cl2]

+.


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Uses of Cr

Chromium metal is used in making stainless steel is much more expensivethan mild steel, resists corrosion effectively, but lacks some other useful

properties (e.g strength, hardness) and so cannot always be substituted fornormal steel.

Chromium is added to iron in smaller amounts to make alloy steels which arevery hard (used for example in ball bearings).

O h U f T i i El


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Other Uses of Transition Elements

Cancer involves cells dividing uncontrollably forming tumours.

Cis Pt(NH3)2Cl2 was found to be able to inhibit cell division and could

therefore be used as a treatment for cancer.

Cisplatin as the material was called is now one of the most widely used anti-cancer drugs.

One of the problems with it is its toxicity, so research continues and a new

drug, carboplatin, has been developed.

S l


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Sunglasses

Photochromic sunglasses which become darker as the light intensity increasesuse a redox reaction.

The lenses contain silver(I) chloride and copper(I) chloride. Strong lightcauses the following reactions to take place:

Cu+ + Ag+ Ag + Cu2+

The silver produced in this reaction turns the glasses darker.

When the light intensity decreases, the reverse reaction takes place and theglasses become less dark.

catalytic activity of transition elements

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