Transition metals Flashcards
What is a transition metal?
Forms one or more stable ions which have incompletely filled d subshells.
State the properties of transition metals:
- Form coloured ions
- Have variable oxidation states
- Are good catalysts
- Exist as complex ions
What is a heterogenous catalyst?
Catalyst is in a different phase from the reactants.
What is a homogenous catalyst?
Catalyst is in the same phase as the reactants.
Why are transition metals able to catalyse reactions?
Most transition metals have multiple stable condition states as they have an incompletely filled d-subshell so they can be the middle man and accept/lose electrons.
Having variable oxidation states makes it a good catalyst
Describe the 3 steps in heterogenous catalysis:
- Adsorption
Reactants adsorb by forming bonds with the atoms at the active sites on the surface of the catalyst. - Bond breaking and making
As a result, bonds in the reactants are weakened and break followed by bond making (between the 2 reactants). - Desorption
This is turn weakens bonds between products and catalyst so the product can leave (desorb).
Why can adsorption of reactants at active sites on the surface lead to catalytic action (faster rate of reaction).
Will be a higher concentration of reactants at the solid surface so leading to a higher collision frequency.
Also the bond between the reactants molecules are weaker.
What’s the problem with too much adsorption?
The products won’t be able to leave because the stick is too strong.
What’s the problem with too little adsorption?
Means reactant’s won’t stick properly to the active site catalyst surface. Can’t form the appropriate bonds etc.
What is one example of a heterogenous catalyst?
What is the catalyst?
Catalytic converter
Catalyst is Pt or Rh.
What is the purpose of the catalytic converter?
To remove CO and NO from exhaust fumes.
The CO comes from the incomplete combustion of hydrocarbons in fuels.
NO comes from the air.
How does the structure of a catalytic converter improve its efficiency?
Large surface area.
A support medium is often used to maximise the surface area and minimise the cost.
Powdered catalyst:
Increasing the surface area of a solid by having a thin layer - also Pt is expensive!
Ceramic support:
Can withstand high temperatures.
What is meant by the term catalyst poisoning?
Give an example
Catalysts can become poisoned by impurities and consequently have reduced efficiency.
The impurities adsorb to the active sites and there are fewer active sites available for the desired reactants.
Catalysts may need to be replaced, imposing a cost implication.
Example:
Lead in catalytic converters, it adsorbs too strongly to the catalyst surface.
What is the heterogenous catalyst used in the Haber process?
Fe
What is autocatalysis?
The product of the reaction is the catalyst for the reaction.
A +B —> C + D
D is the catalyst.
The reaction rate is very slow to begin with as there is not a lot of D but as more D is made, the faster the reaction.
What is a complex ion?
A central metal ion surrounded by ligands.
What is a ligand?
An atom, ion or molecule which can donate a lone pair of electrons.
What is a co-ordination number?
The number of co-ordinate bonds to the central metal atom or ion.
The shape of a complex ion depends on its….
Co-ordination number.
Usual co-ordination numbers are 6 and 4.
If the ligands are small and neutral, 6 of H2O or NH3 can fit around the central metal ion.
If the ligands are large and charged like CL- only 4 can fit around the central metal ion. (Due to space and repulsion of negative ions).
Define a monodentate ligand:
One donor atom with a lone pair that can from one co-ordinate bond per ligand.
Define a bidentate ligand:
Give examples:
Have 2 donor atoms per ligand with lone pairs, and each atom can form one co-ordinate bond. Therefore, forms 2 coordinate bonds per ligand.
Examples:
1,2 - diaminoethane
ethanedioate ion
Define a multidentate ligand:
Can form six co-ordinate bonds per ligand
Explain why water is not a bidentate ligand:
Bidendate ligands are classified as having 2 donor atoms.
Water is a monodentate ligand as it has only one donor atom.
The other lone pair of electron doesn’t form a co-ordinate bond.
Write an equation to show how ammonia can act as an alkali in water:
NH3 + H2O –> NH4+ + OH-
Explain why Fe 3+ is more acidic than Fe 2+
Fe 3+ has a higher charge density than Fe 2+
Fe 3+ more strongly attracts the electrons from O and H weakening the OH bonds.
Therefore, the water ligand will readily release H+
Therefore, Fe3+ is more acidic than Fe2+.
What colour are transition metals with a full d subshell?
Colourless
Why are transition metals coloured?
The partially filled d orbitals cause colour to be absorbed in the visible region.
What happens to the d orbitals once ligands bond to transition metals?
The 5 d orbitals no longer have the same energy.
There’s a lower energy state.
And a higher energy state.
Energy is absorbed to excite electrons from the lower d orbitals to the higher d orbitals.
The electron has been promoted from the ground state to the excited state.
This energy is in the visible region
Any energy not absorbed will be reflected.
The colour of the complex will be complementary to…
To the colour of light absorbed.
With reference to electrons, explain why aqueous copper ions are blue:
Transition metals have incompletely filled d - orbitals.
Ligands cause the d orbitals to split into high and low energy states.
When visible light hits transition metal ions, some of the wavelengths of light are absorbed by electrons , which are excited up to higher level energy states.
The complementary colour light to the wavelength of light absorbed, which is transmitted is the light we see.
Why are elements/ions with completely full or empty d subshells white/colourless.
Empty d-subshells - no electrons to be excited
Full d-subshells - nowhere in the d subshells for electrons to be excited to.
No excitation = no light can be absorbed
So all light is reflected and therefore, we see white/colourless.
The size of the energy gap between the d orbitals affects the colour.
What are the 4 factors that affect this?
- The metal
- The oxidation state
- The ligands
- The co-ordination number
