Transition metals Flashcards
What is meant by a transition metal? (1)
Forms at least one stable ion with a partially filled d sub shell
What two elements found in the d-block period are not considered transition metals? (2)
Zinc
Scandium
Why are zinc and scandium not transition metals? (3)
In Zn2+ the d sub shell is full
In Sc3+ the d sub shell is empty
Do not form stable ion with partially filled d sub shell
What are the two transition metals that do not follow the regular electron configuration? (2)
Chromium
Copper
What are the 4 properties of transition metals? (4)
Form complex ions
Form coloured ions
Variable oxidation states (partially filled d sub shell so can lose 4s and 3d electrons)
Catalytic activity/properties
What is a ligand? (1)
An ion or molecule that forms a co-ordinate bond with a transition metal ion by donating a pair of electrons
What is a co-ordinate bond? (1)
Shared pair of electrons which have both come from the same atom
What is a complex? (1)
Central metal atom or ion surrounded by ligands that are co-ordinate bonded
What is a monodentate ligand? (1)
Each ligand forms 1 coordinate bond
What is a bidentate ligand? (1)
Each ligand forms 2 coordinate bonds
What is a multidentate ligand? (1)
Each ligand forms 3 or more coordinate bonds
Example of monodentate ligands (3)
H2O
NH3
Cl-
What are the two similarities between the NH3 and H2O ligands? (2)
Similar in size
Both are uncharged
What is the difference between Cl- and NH3 and H2O ligands? (1)
Cl- ligand is larger than the uncharged ligands NH3 and H2O
What are the two bidentate ligands? (2)
ethane-1,2-diamine OR 1,2-diaminoethane
ethanedioate ion
What are the 4 shapes which transition metal complexes form? (4)
Octahedral
Tetrahedral
Square planar
Linear
How does EDTA4- form a complex with a metal? (2)
2 coordinate bonds from the N atoms
4 coordinate bonds from O- atoms
Why do chloro complexes tend to be tetrahedral? (1)
Chloride ligand is too big to fit any more than 4Cl- ligands around the metal ion
What is the chelate effect? (1)
When monodentate ligand is substituted by a bidentate or a multidentate ligand from a complex to form a more stable complex ion
Explain chelate effect in terms of entropy and enthalpy change (5)
Enthalpy change is zero
Because same number of bonds broken and formed
Lower number of molecules on the left and higher number of molecules on the right
Large increase in entropy
Reaction is feasible as delta G value will be negative
Describe the structure of haemoglobin (3)
4 coordinate bonds between Fe2+ and the N atoms in the haem structure
1 coordinate bond between the Fe2+ and the protein globin
1 coordinate bond between the Fe2+ and the O2 molecules
Why is carbon monoxide toxic? (2)
Carbon monoxide permanently coordinates to and bonds more strongly to Fe2+
This prevents O2 from binding to Fe2+, causing suffocation
How does haemoglobin allow oxygen to be transported around the body? (2)
O2 forms a coordinate bond to Fe2
The coordinate bond breaks releasing the O2 to where it is needed in the body
What type of isomerism can be shown by transition metal complexes? (2)
Stereoisomerism (cis-trans and optical isomerism)
How is cis platin used in anti-cancer drugs? (2)
Cis platin binds to the DNA in cancer cells and stops DNA replication
The two Cl- ions on cisplatin are substituted for two N atoms on adjacent guanine bases
What type of isomerism do octahedral complexes with monodentate ligands display? (1)
Cis-trans
What type of isomerism do octahedral complexes with bidentate ligands display? (1)
Optical isomerism
Why are transition metal compounds coloured? (1)
Partially filled d sub shell
What is ground state? (1)
The most stable electronic configuration of an atom which has the lowest amount of energy
What is excited state? (1)
When electrons move from a lower energy d orbital to a higher energy d orbital
Explain why complexes formed from transition metal ions are coloured (3)
Absorb some wavelength of visible light
To excite electrons in d orbital
Remaining wavelength of light reflected
Why does a red solution appear red (2)
Large difference between ground state and excited state for d sub shell electron
High energy part of visible light spectrum (blue, indigo, violet) absorbed more strongly and red, orange, yellow part is transmitted making the solution appear red
Light with low energy and long wavelength (3)
Red
Orange
Yellow
Light with high energy and short wavelength (3)
Blue
Indigo
Violet
Why does CuSO4 solution appear blue? (3)
Partially filled d orbital
Electrons absorb specific amount of energy from visible light: red, orange yellow is more strongly absorbed. Blue, indigo, violet is transmitted
D sub shell electrons become excited and move from ground state to excited unstable state
How to measure the energy difference between the ground state and the excited state of the d electrons? (2)
Delta E = hv
Delta E = h x c/wavelength of light
What does it mean when a transition metal compound has a large change in energy between d sub shell? (2)
High energy light is absorbed to excite the electrons (blue, indigo, violet)
Low energy light is reflected (red, orange, yellow)
What does it mean when a transition metal compound has a small change in energy between d sub shell? (2)
Low energy light is absorbed to excite the electrons (red, orange, yellow)
High energy light is reflected (blue, indigo, violet)
What changes can alter the colour of a compound? (4)
Change in ligands
Change in oxidation state of the metal
Change in the coordination number of the complex
Change in the shape of the complex
Why is it possible that some complexes may be colourless? (2)
No d electrons or full d-sub shell
What can colorimetry be used to determine? (1)
The unknown concentrations of some transition metal ions
Why is a filter used in the colorimeter? (1)
Only allows one colour of light through the sample
What can the amount of absorption be affected by in colorimetry (1)
The length of the container the solution is in
How to find the concentration of an unknown sample from the calibration curve? (3)
Measure the absorbance for a range of known concentrations
Plot a graph of absorbance against concentration
Read the value of the concentration for the measured absorbance from the graph
What is a catalyst and how do they work, in general terms? (2)
A substance which increases the rate of reaction without being used up
Provide an alternative reaction pathway with a lower activation energy
What is a heterogenous catalyst and state an advantage of using heterogenous catalyst? (1)
The catalyst is in a different phase to the reactants
No need for separation of products from catalyst
What is a homogenous catalyst? (1)
The catalyst is in the same phase as the reactants
With a heterogenous catalyst, where does the reaction occur? (1)
At active sites on the surface
How does a heterogenous catalyst work? (3)
Reactants adsorb onto the surface of the catalyst on an active site
Reaction occurs on the surface of the catalyst
Products desorb from the surface of the catalyst
How are catalysts made more efficient? (2)
Increase the surface area (to increase the number of active sites present)
Spread the catalyst over an inert support medium
How can catalysts become poisoned and what implications does this have? (2)
Impurities block the active sites which prevents the reactants from adsorbing
Reduces efficiency and therefore, has cost implications
How to prevent catalysts from being poisoned? (1)
Purify the reactants
What is the contact process, what catalyst is used? (1)
What are the two equations involved (2)
Why is the catalyst good to use in this process? (1)
Makes H2SO4
Heterogenous catalyst - V2O5
SO2 + V2O5 –> SO3 + V2O4
2V2O4 + O2 –><— 2V2O5
Changes oxidation state from +5 to +4 and back to +5 again
What is the Haber process, what catalyst is used and what can it be poisoned by? (2)
N2(g) + 3H2(g) –> 2NH3(g) (making ammonia)
Uses heterogenous catalyst Fe
Poisoned by sulphur impurities
Why can transition metals act as catalysts? (1)
Variable oxidation states
Why type of catalysts are autocatalysis and what does this mean? (2)
Autocatalysis is an example of homogenous catalysis
It is when the catalyst for the reaction is a reaction product
What ions are involved in autocatalysis? (2)
MnO4-
C2O42-
How to monitor concentration of MnO4- ions?
Colorimeter
Which transition metal ions catalyse the reaction between I- and S2O82- and write the two equations? (3)
Fe2+
S2O82- + 2Fe2+ —> 2Fe3+ + 2SO42-
2Fe3+ + 2I- —> 2Fe2+ + I2
What colour are MnO4- ions and what colour is Mn2+? (2)
MnO4-: Deep purple colour
Mn2+: pink
Explain the concentration vs time graph of MnO4- ions (4)
The rate starts off slow as there is no catalyst initially
The two negatively charged reactants collide with a very high Ea
As some Mn2+ ions are formed, the rate increases as the reaction is being increasingly catalysed
The rate then decreases and levels off as the reactants used up
State the oxidation state and colour of the following vanadium species:
VO2+
VO^2+
V3+
V2+
VO2+ = +5 - yellow
VO^2+ = +4 - blue
V3+ = +3 - green
V2+ = +2 - violet
What can be used to reduce vanadium (1)
Zinc
What colour is [Cr(H2O)6]2+ and [Cr(H2O)6]3+
[Cr(H2O)6]2+: Blue
[Cr(H2O)6]3+: red/violet
What colour is Cr2O72- and Cr3+
Cr2O72-: orange
Cr3+: green
What colour is [Co(NH3)6]2+ and [Co(NH3)6]3+
[Co(NH3)6]2+ : yellow
[Co(NH3)6]3+: brown
What happens to aqua metal ions in acidic and alkaline conditions (2)
Acidic: reduced
Alkaline: oxidised
Why are redox titrations with transition metal compounds said to be self indicating (1)
Usually involves a colour change as the metal is changing oxidation state
Why is the titration between MnO4- and Fe2+ self indicating? (3)
As MnO4- reacts to form Mn2+ it turns from deep purple to colourless.
As the MnO4- is added to the Fe2+ solution it decolourises as it reacts.
As soon as all the Fe2+ is reacted, the reaction mixture will start to turn purple as the MnO4- has nothing to react with.
Whenever a transition metal redox reaction takes place in acidic conditions what should never be used and why?
Suggest another reagent that can be used (4)
HCl
Both MnO4- and Cr2O72- can oxidise the Cl- ions to Cl2. This results in the formation of Cl2 gas which poses an obvious safety risk
It also affects the titration results, and it make it seem like more oxidising agent has been used than it should have
Sulfuric acid should be used
