Catalyst/Transition metals

Transition metals and their compounds can act as…

heterogenrous and homogeneous catalyst

• heterogeneous - catalyst in a different phase from the reactants, reaction occurs at the surface of the catalyst

• homogenous - catalyst in the same phase as the reactants, catalysed reaction will proceed via an intermediate species

• transitions metal ions are effective as catalyst because they have variable oxidation states and easy to gain or lose electrons

How does V2O5 acts as a catalyst in the contact process

• is reduced to VO2 but regenerated so oxidised back by reacting with oxygen

Explain how a catalytic converter decrease CO and NO (use of catalyst)

• adsorption of CO and NO onto the surface of catalyst at active sites

• weakening of bonds, holding reactants close together on the surface in the correct orientation to react so chemical reaction takes place on the surface

• then desorbs CO2 and N2 from the surface of catalyst

What is the role of Fe2+ in catalysing the reaction between I- and S2O8 2-

what is the role of Mn2+ in autocatalysing the reaction between MnO4- and C2O42-

• oxidised to Fe3+ and then reacts with I- and reduced to Fe2+, so regenerated, acts as catalyst. This reaction is quite slow due to collisions between two -ve charged ions, so has repulsion

• MnO4- is reduced to Mn 2+, so Mn2+ as the product is able to acts as a catalyst for the reaction

Which types of binding exist within complex ion [Cr(H2O)]?

dative covalent and covalent only

How can impurities on reactant make catalyst less effective

• impurities also get adsorb onto the catalyst surface

• but impurities prevent bond weakening in the reactants and less surface area available for the reaction

• impurities less likely to desorb from the surface as they form strong bond to the surface

Define a transition element

An element which forms at least one stable ion with a partially full d-shell of electrons

physical properties of transition metals

• metallic

• good conductor of heat and electricity

• hard, strong, shiny

• high melting point and boiling point

• low reactivity - compare with group1/2

chemical properties of transition metals

• variable oxidation states

• coloured compounds/ions in solution

• good catalysts

• form complex ions

Define complex ion

central transition metal ion surrounded by ligands that are bonded by coordinate bonds

Define ligand

an ion or molecule with at least one lone pair of electrons that donate to a transition metal ion to form a coordinate bond and a complex ion

Define mono dentate ligands

a ligand that forms one coordinate bond to the central metal ion (bidentate forms 2, multi-dentate forms multiple)

name some common mono-dentate/bidentate/multidentate ligands

• water, NH3, CN-, OH-

• C2O4^2-, H2NCH2CH2NH2

• EDTA^4- (hexadentate)

define coordination number

the number of coordinate bonds the metal ion has formed to the surrounding ligands

what is a chelate

name of the complex ion with multi dentate ligands

what ion is usually formed when a transition metal compound is dissolved in water? What shape?

• aqua ion

• 6 water ligands around the central metal ion, so octahedral shape

tetrahedral, square planar complex ion and linear complex ion example

• tetrahedral: [CuCl4]^2-, Cl- is much bigger than water, so can only fit 4 around the central metal ion

• Square planar: [Pt(NH3)2(H20)2] - cisplatin

• linear:

How can complex ions display E/Z or cis/trans isomerism?

what shapes of ion does this apply to

• 2 ligands of the same type can be on the same/opposite sides of the metal ion, which forms E/Z or cis/trans isomers

• applies to square planar and octahedral complex ions

What conditions are needed for a complex ion display optical isomerism

applies to octahedral molecules with 2 or more bidentate ligands so mirror images are non-superimposable

What is harm - its metal ion, coordination number and ligands?

• a molecule which makes up protein chains

• with Fe2+ central metal ion, coordination number of 6

• 4 bonded to haem, 1 to nitrogen of global, one to oxygen

Why is CO toxic

CO coordinately bonds to the Fe2+, acts as a ligand, ligand exchange occurs

CO can bond more strongly to the central metal ion than oxygen

Why are transition metal coloured?

• they have partially filled d-orbitals with free electrons between the d-orbitals

• when ligands coordinately bond to the ion, d-orbitals split into different energy levels

• electrons can absorb energy in the form of photons to become excited and move to higher energy level

• we see the colours that are not absorbed

why is there a lack of colour in some aqueous ions and other complex ions

• ions that have completely filled 3d energy levels or with 0 are not coloured

• must have a partially filled 3d energy level

vanadium colours and chromium colours

• You -yellow, 6+

• better - blue, 5+

• get - green, 3+

• violent - violet, 2+

• Or - orange, 6+ (Cr2O7^2-)

• You - yellow, 6+

• Get - green, 3+

• Burnt - blue, 2+

colours of complex ions!!

why can transition metals have variable oxidation states

which oxidation states do all transition metals have, why?

they have partially filled d-orbitals, can lose 4s and 3d electrons

+2 due to loss of electrons from 4s orbital

why are transition metals good catalysts

they can exist in variable oxidation states, so can provide alternate pathways easily

What is an advantage of using a heterogeneous catalyst

• easy to separation of products from catalyst

What properties does the catalyst need to have to make it a good catalyst

• can’t adsorb too strongly, otherwise products will not desorb

• cant adsorb too weakly as reactants would not be held in place for long enough and bonds would not be sufficiently weakened

• so need a good balance between adsorption and desorption

how to increase efficiency of heterogeneous catalysts

• increase surface area to increase number of active sites present

• spread onto an inert support medium to increase surface/mass ratio e.g. ceramic honeycomb

define autocatalysis

when the product of a reaction is also a catalyst for that reaction