Transition metals 3.2.5 Flashcards
What is a transition metal?
An element that has an incomplete d sub shell in either its atoms or one of its common ions
Where are the transition metals found in periodic table?
D-block
is Zn = [Ar] 3d10 4s2 a transition metal?
No as it has a full d sub shell
What are the four common properties of transition metals?
1) formation of complexes
2) forming coloured compounds
3) variable oxidation states
4) catalytic action
What is a ligand?
A particle the lone pair of electrons that bonds to metals by a coordinate bond
What is a complex?
A metal ion with coordinately bonded ligands
What is the coordination number?
The number of coordinate bonds from ligands to the metal ion
What is a Lewis base?
A lone pair donor or a nucleophile
What is a Lewis acid?
A lone pair acceptor of electrophile
What is a unidentate ligand?
examples?
a ligand which forms one coordinate bond so has one lone pair
e.g. Cl , OH-, H2O, NH3
What is a bidentate ligand?
examples?
a ligand which forms two coordinate bonds so has two lone pairs
e.g. C2O4 2- (ethanedioate ion)
What is a multidentate ligand?
examples?
a ligand which forms more than two coordinate bonds so has more than two lone pairs
e.g. EDTA4-
What is a ligand substitution?
When one ligand is replaced by another ligand
What happens if the ligand substitution reaction occurs with similar sized ligands?
They’ll be no change in the coordination number or shape
What happens if the ligand substitution reaction occurs with different sized ligands?
The coordination number and size will change
Can ligands be replaced by one that forms more coordinate bonds?
Yes
For example, six unidentate ligands can be replaced by three bidentate ligands
What changes if the ligands are replaced by ones that form more coordinate bonds?
There’ll be an increase in entropy
In substitution reactions, what is the value of ΔH?
ΔH = negligible
What does the feasibility of substitution reaction depend on?
What makes a reaction feasible
ΔG = ΔH - ΔS
If the ΔH is negligible and ΔS is very positive then ΔG is very negative and the reaction is feasible
What is the chelate effect?
Entropy change making the reaction feasible and more stable when more coordinate bonds are formed
What are chelating agents
ligands that form more than one coordinate bonds as the increase entropy
What is the coordination number of a linear complex?
example?
2
Ag + in tollens reagent
What is the coordination number of a square planar complex?
example?
4
Pt 2+ in cisplatin
What is the coordination number of a tetrahedral complex?
example?
4
Cl - in [CuCL4]
What is the coordination number of an octahedral complex?
example?
6
H2O in [Cu(H2O)6]2+
What is the haemoglobin?
A globular protein that contains 4 Fe2+ centres each with a multidentate that takes up 4/6 coordination sites
How does oxygen bind to haemoglobin?
The other two sites that are not bonded to the porphyrin ligand allow oxygen to bind
Why is carbon monoxide or cyanide ions toxic?
carbon monoxide or cyanide ions are better ligands than oxygen so bind more easily and prevent the transport of oxygen
What kind isomerism can complexes form?
What is the definition?
Stereoisomerism
- complexes that have the same structural molecular formula but the bonds are arranged differently in space
What are the two types of Stereoisomerism ?
cis-trans and optical
In what type of complexes can cis-trans isomerism form?
octahedral and square planar
In what type of complexes can optical isomerism form?
Octahedral with three bidentate ligands
do the 5 D orbitals in transition metals all have the same energy?
no - They are split into higher and lower energy orbitals
Why are transition metal ions coloured?
- the gap between the higher and lower energy orbitals corresponds to the energy of visible light
- the electrons in the d orbitals absorb light and use the energy to excite electrons into the higher energy level
- the colour of visible light is reflected I’m not absorbed to excite electrons is the colour that the ion will b
What are the four factors that affects the colour of the transition metal ions?
1) the identity of the metal
2) the oxidation state of the metal
3) the identity of the ligands
4) the coordination number
If any of the four factors that affect the colour of the transition metal ions are changed what will happen?
The size of the energy gap between the higher and lower D orbitals will change so the frequency of light that is absorbed will change which changes the colour of light reflected so changes the colour of the ion
What is the equation for calculating the energy gap between the higher and lower d orbitals
ΔE = h x f = (h x c)/λ
What do all the symbols in the energy gap calculation mean?
ΔE = energy gap (J) h = Planck's constant f = frequency of light (s-1) c = speed of light (ms) λ = wavelength (m)
What error is often made when calculating the energy gap?
Wavelengths are often given in nanometres so you need to convert this value into meters by dividing it by 10^9
How can frequencies of light absorbed be measured?
With a UV/ visible light spectrometer
How does a UV/ visible light spectrometer work?
- Light is passed through the complex and the frequencies of light passing through are detected and those that do not pass through are absorbed
- a graph of wavelength versus absorption is then plotted
What method is used to work out the concentration of a solution?
Colorimetry
How does colorimetry work?
- A solution of known concentration containing a complex ion is put into curvette which is then put into a colorimeter
- the colorimeter measures how much light is absorbed
- repeat this the different concentrations
- a calibration curve of absorption versus concentration is plotted
Do transition metals have one or many oxidation states?
They have many variable oxidation states
What an example of a transition metal having many oxidation state?
Tollen’s reagent testing for aldehydes
- Tollen’s reagent contains [Ag(NH3)2]+ ions
- To begin withm the silver is Ag+1 but when it’s reduced it becomes Ag and forms a silver mirror
What the factors that affect how easy it is to change the oxidation state of the transition metal?
1) the pH
2) the identity of the ligand
What conditions are required to oxidise a transition metal and what are the reagents?
Alkaline conditions using H2O2/NaOH
What conditions are required to reduce a transition metal and what are the reagents?
Acidic conditions using Zn/H2SO4
How is how easy it is to reduce the transition metal shown?
Using electrode potentials
What are electrode potentials and what do they indicate?
Electrode potentials are also called redox potentialS and indicate how easily and ion or atom is reduced
What does a more positive electrode potential indicate?
The transition metal will be more easy to reduce
What does a more negative electrode potential indicate?
The transition metal will be more easy to oxidise
In what conditions does vanadium in ammonium vanadate have different oxidation states?
Acidic conditions (HCl or H2SO4) with zinc as a reducing agent
In the experiments to show how vanadium has variable oxidation states, what step in the method is essential?
Using a cotton wool plug in the neck of the flask rather than a bung (so it pressure doesnt build up and explode) to keep her out so the formation of V(+2) is able to occur
What are the colours and oxidation states of the species present throughout the vanadium experiment and what are the equations each step?
1) VO2 + = V(+5) = Yellow
2) VO 2+ = V(+4) = Blue
3) V 3+ = V(3+) = Green
4) V 2+ = V(2+) = Violet
Between 1 and 2: 2VO2 + +Zn + 4H+ = 2VO 2+ + Zn 2+ + 2H2O Between 2 and 3: 2VO 2+ + Zn + + 4H+ = 2V 3+ + Zn 2+ + 2H2O Between 3 and 4: 2V 3+ + Zn = Zn2+ + 2V2+
What is the acronym to remember the colours in the vanadium experiment?
You - yellow
Better - blue
Get - green
Vanadium - violet
Is potassium manganate (VII) a reducing or oxidising agent in redox titrations?
What is the equation?
Oxidising agent
MnO4- + 8H+ + 5e- = Mn2+ + 4H2O
When using potassium manganate (VII) in a redox titration, what conditions are required?
Acidic conditions using dilutes sulphuric acid
Why can you not use hydrochloric acid, concentrated sulphuric/nitric acid or ethanoic acid when using potassium manganate (VII) in a redox titration?
- the Cl- in hydrochloric acid would also be oxidised to Cl2 by the potassium manganate (VII) which would affect the volume of potassium manganate required and titration and produce chlorine gas which is toxic
- The Cl- in HCl will also reduce MnO4-
- concentrated sulphuric/nitric acid are already oxidising agents so would affect the volume of potassium manganate (VII) required in the titration
- ethanoic acid is a weak acid so would not provide enough hydrogen ions
In a redox titration using potassium manganate (VII) what solution is in the burette and what solutions is in the pipette?
burette = potassium manganate (VII) pipette = the sample being analysed which has been acidified
What are the observations in a redox titration using potassium manganate (VII)?
What will the colour change be in a redox titration using potassium manganate (VII)?
- the dark purple potassium manganate (VII) is added to the colourless sample
- the dark purple colour will disappear in a colour sample until the end point is reached and a permanent pale pink colour is formed
- Colourless to pale pink
What the metal is commonly analysed in redox titrations with acidified potassium manganate (VII)
Fe 2+
What is the equation for the reduction of Fe 2+
Fe 2+ = Fe 3+ + e-
What is the equation for a redox titration with Fe 2+ and potassium manganate (VII)
5Fe2+ + MnO4- + 8H+ = Mn2+ + 4H2O + 5Fe3+
What is what is done to iron if it is not in the Fe2+ state?
1) if it is the element Fe(0) = it is reacted with sulphuric acid to oxidise it to Fe 2+
2) is it is Fe(3+) = it is reacted with zinc to reduce it to Fe 2+
What compound is commonly analysed in redox titrations with acidified potassium manganate (VII)
The ethanedioate ion (C2O4 2-)
What is the equation for the dissociation of the ethanedioate ion?
C2O4 2- = 2CO2 + 2e-
What is the equation for a redox titration with the ethanedioate ion and potassium manganate (VII)
5C2O4 2- + 2MnO4- + 16H+ = 10CO2 + 2Mn2+ + 8H2O
Why is the reaction in a redox reaction between potassium manganate (VII) and ethanedioate slow?
Both ions are negative so repel each other
How is the reaction in a redox reaction between potassium manganate (VII) and ethanedioate sped up?
By warming up the mixture at the beginning
Why does the reaction in a redox reaction between potassium manganate (VII) and ethanedioate only need to be sped up at the beginning?
This reaction is an example of autocatalysis so one of the products (Mn2+) is a catalyst which then will speed up the reaction
Write an equation for the reaction of EDTA 4- with aqueous copper ions in [Cu(H2O)6]+2
[Cu(H2O)6]+2 + EDTA 4- = [Cu(EDTA)]-2 + 6H2O
Given example of a linear complex ion formed by transition metal?
[Ag(NH3)2]+
Write an equation to the substitution reaction in which the complete replacement of ligands causes a change in the coordination number and the overall charge of the complex ion:
[Co(H2O)6]+2 + 4Cl- = [CuCl4]-2 + 6H2O
Write an equation to the substitution reaction in which the complete replacement of ligands causes NO change in the coordination number and the overall charge of the complex ion:
[Co(H2O)6]+2 + 6NH3 = [Cu(NH3)]+2 + 6H2O
What is a catalyst?
The substance which increases the rate of a chemical reaction by providing an alternative to chemical pathway with a lower activation energy
Does a catalyst to change the value of Kc and why?
No - it increases the rate of the forwards and backwards reaction equally
Why are transition metals good catalysts?
They have variable oxidation states so can transfer electrons from D orbital to speed up reactions
What are the two types of catalyst?
Heterogeneous or homogeneous catalysts
What is a heterogeneous catalyst?
When the catalyst is in a different phase to the reactant
What are 3 examples of heterogeneous catalysts with equations?
1) The Haber process 3H2 + N2 --> 2NH3 using an Fe catalyst 2) The Contact process 2SO2 + O2 --> 2SO3 using a V2O5 catalyst 3) Catalytic converters NO + CO --> 1/2 N2 + CO2
How does a heterogeneous catalyst work?
3 things
1) one of the reactant molecules adsorbs to the surface of the catalyst
2) the location that is the molecule adsorbs it is called the active site
3) the reactant molecules can move around the surface and bond to different active sites to speed up the reaction
What is the difference between adsorption and absorbtion?
- adsorption is when molecules go ONTO the catalyst
- absorption is when molecules go INTO the catalyst
How does the adsorption of reactants onto the surface of a catalyst increase the rate?
(3 things)
1) it concentrates the reactants increasing the likelihood of collisions
2) it weakens some of the bonds in the reactants to make the reaction easier
3) it may position the reactants in a more favourable orientation
What are the conditions for adsorption for a catalyst to work?
adsorption cannot be too weak:
Not many molecules will be able to adsorb so the catalyst will have very little effect
adsorption cannot be too strong:
Molecules will not be able to move around the active sites so be less like to collide
How do you increase the adsorption?
4 things
1) increasing the surface area
2) increasing the pressure
3) increasing the temperature
4) increasing the concentration of reactants
How is increasing the surface area of a catalyst economic?
- It decreases the quantity of catalyst needed to do the same effect
- catalysts are also very expensive
How is the cost of the catalyst decreased?
By using a honeycomb shaped inert substances to increase the surface area and spraying a thin layer of the catalyst on top
What is catalyst poisoning?
Some substances can adsorb and bind irreversibly to the surface of a catalyst which blocks the active sites or reduces the surface area and prevents a catalyst from increasing the rate of reaction
What are two examples of catalyst poisoning?
1) lead poisoning of catalytic converters in cars
- Rh or Pt catalysts are poisoned by lead from leaded petrol
2) sulphur poisoning in the Haber process
- the hydrogen is obtained from natural gas which is contaminated by sulphur
- the sulphur come poison the iron catalyst
What is the contact process?
The manufacturing of sulphuric acid using a vanadium oxide catalyst
What are the two equations of the contact process?
1) V5O2 + SO2 = V2O4 + SO3
2) V2O4 + 1/2 O2 = V2O5
What is the overall equation for the contact process?
SO2 + 1/2 O2 = SO3
Is the contact process a slow or fast reaction?
Very slow which is why it needs a catalyst
What is a homogeneous catalyst?
The catalyst and reactants are in the same physical state
In what physical state to most homogeneous catalyst reactions take place?
in solution (aq)
Why do most reactions involving a homogeneous catalyst produce intermediate species?
The activation energy to form an intermediate is lower than the product
In what reaction does an acid act as a homogeneous catalyst and how does it work?
Esterification reactions
1) the acid catalyst protonates one of the reactants
2) the protonated species reacts with the other reactants to get the products and reform the catalyst
3) the protonated species becomes deprotonated by doing step 2
What are the two examples of transition metals acting as homogeneous catalysts?
1) the reaction between iodide ions and peroxodisulphate ions using an iron catalyst
2) autocatalysis in the reaction between manganate ions and ethanedioate ions
Why is the reaction between iodide ions and peroxodisulphate ions so slow?
- Both ions are negatively charged which means they repel each other
- this increases the activation energy as collisions are difficult
What are the two equations for the reaction between iodide ions and peroxodisulphate ions?
1) 2Fe 2+ + S2O8 2- = 2Fe 3+ + 2SO4 2-
2) 2FE 3+ + 2I - = 2 Fe 2+ + I2
What is autocatalysis?
When one of the products of the reaction is a catalyst
Why is the reaction between manganate ions and ethanedioate ions so slow?
- Both ions are negatively charged which means they repel each other
- this increases the activation energy as collisions are difficult
What are the two equations for the reaction between manganate ions and ethanedioate ions?
1) 4Mn 2+ + MnO4 - +8H+ = 3Mn3+ + 4H2O
2) 2Mn 3+ + C2O4 2- = 2CO2 + 2Mn 2+
