Transition Metals Flashcards
What are transition elements
An element that forms one or more stable ions with an incomplete d sub-shell.
Why is zinc classified as a D block element but not as a transition element
Zinc is a d block element because the last electron in the zinc atom goes into a d orbital. Zinc only forms the Zn 2+ ion where the d-sub shell is full, therefore it is not a transition element.
Characteristics of transition elements
They have variable oxidation states
Form coloured ions
Complex formation
Show catalytic activity
Why do transition metals have several oxidation states
In transition metals the increase in successive ionisation energies is small because the 3d and 4s energy levels is close together. This increase can be offset by the lattice energy. Therefore transition metals have several oxidation states.
Colours of all Vanadium ions
V 2+ = violet/purple
V 3+ = Green
VO 2+ = Blue
(VO2)+ and (VO3)- = Yellow
What is a complex
A central metal atom or ion surrounded by ligands
What is a ligand
A atom, molecule or ion that donates a lone pair of electrons to a metal ion to form a coordinate bond
What is coordination number
Number of coordinate bonds to the central metal atom or ion
What is a monodentate ligand
A ligand that donates one electron pair and forms one coordinate bond to a metal ion
What are the most common monodentate ligands
H2O
NH3
OH-
Cl-
CN-
What is a bidendate ligand
Two atoms that each donate a lone pair (of electrons) / coordinate bonds from two atoms
A co-ordinate bond is formed when a transition metal ion reacts with a ligand.
Explain how this co-ordinate bond is formed
An electron pair on the ligand is donated from the ligand to the central metal ion
Most common bidentate ligands
1,2-diaminoethane (en)
ethanedioate ion (C2O4 2-)
What type of ligand is EDTA 4-
Hexadentate ligand
How many coordinate bonds does EDTA form
6
Name the type of bond between the ligand and the vanadium ion in the complex [V(H2O)6]3+ and state the feature of the ligand that enables this bond to be formed
A coordinate bond is formed between the ligand and the vanadium ion.
There is a lone pair of electrons on the oxygen atom of a water molecule that enables this bond to be formed as it is donated to the central Vanadium ion.
State three characteristic features of the chemistry of cobalt and its compounds.
Variable oxidation state
Act as catalysts
Form complexes
Form coloured ions/compounds
What is the rule for naming neutral and positive complexes
- Write the name of the ligand
- Write the name of the transition metal and its oxidation number in roman numerals
- If there are several ligands, they are listed in alphabetical order
Name [Co(NH3)6]3+
hexaamminecobalt (III) ion
Name [CrCl2(H2O)4]+
tetraaquadichlorochromium(III) ion
What are the similarities between the NH3 and H2O ligands
They are similar in size and uncharged
What happens to the coordination number in the exchange of the ligands NH3 and H2O
Exchange occurs without a change of co-ordination number e.g. Co2+ and Cu2+
Reaction between [Co(H2O)6]2+ and NH3
[Co(H2O)6]2+ + 6NH3 = [Co(NH3)6]2+ + 6H2O
Reaction when aqueous ammonia is added to aq copper (II) ions
Reaction is incomplete
[Cu(H2O)6]2+ + 4NH3 (aq) = [Cu(NH3)4(H2O)2]2+ + 4H2O
Colour change in the reaction between aqueous ammonia and aq copper (II) ions
pale blue solution to deep blue solution
What happens when a high conc of chloride ions e.g. from conc HCl is added to an aq ion
Ligand substitution
Why is there a change in co-ordination number when an aq ion reacrs with chloride ions
The Cl- ligand is larger than the uncharged H2O and NH3 ligands
Reaction when concentrated HCl is added to aq copper (II) ions
[Cu(H2O)6]2+ + 4Cl- = [CuCl4]2- + 6H2O
What is the change in coordination number when conc HCl is added to aq ions of Cu and Co
6 to 4
What shape are chloride complexes such as [CuCl4]2- and [CoCl4]2-
Tetrahedral
Colour of [CuCl4]2- ion
yellow/green solution
Colour of [CoCl4]2-
Blue solution
What is formed if a solid metal (e.g. copper) chloride is dissolved in water
Aq [Cu(H2O)6]2+ complex NOT chloride complex [CuCl4]2-
Reaction between aq copper ion and ethane-1,2-diamine
[Cu(H2O)6]2+ + 3NH2CH2CH2NH2 = [Cu(NH2CH2CH2NH2)3]2+ + 6H2O
Reaction between aq copper ion and ethanedioate ions
[Cu(H2O)6]2+ + 3C2O4 2- = [Cu(C2O4)3]4- + 6H2O
Reaction when a dilute aq solution containing ethanedioate reacts with aq copper (II) ions
[Cu(H2O)6]2+ + 2C2O4 2- = [Cu(C2O4)2(H2O)2]2- + 4H2O
What is haem
An iron (II) complex with a multidentate ligand
Explain what enables oxygen to be transported in blood
Oxygen forms a coordinate bond to Fe(II) in haemoglobin
Explain why CO is toxic to humans
CO can form a strong coordinate bond with haemoglobin. This is a stronger bond than that made with oxygen and so it replaces the oxygen and attaches to the haemoglobin
Write an equation for the equilibrium that is established when hydrated calcium ions react with EDTA4– ions.
Explain why the equilibrium in part (c)(i) is displaced almost completely to the right to form the EDTA complex
[Ca(H2O)6]2+ + EDTA4– = [CaEDTA]2– + 6H2O
2 mol of reactants form 7 mol of products (Allow more moles/species of products)
2 mol of reactants form 7 mol of products
Therefore disorder increases
Entropy increases and free-energy change is negative
What is the chelate effect
When a monodentate ligand is replaced with a bidentate or multidentate ligand, leading to the formation of a more stable complex. A more stable complex is formed as there are more moles of products than reactants and so entropy increases and disorder increases.
What ion does a haemoglobin molecule consist of
Fe 2+ ion with a coordinate number of 6
What shaped ligands are commonly formed with small ligands such as NH3 and H2O
Octahedral
What shape and bond angle do most complex ions with coordination number 4 form
Tetrahedral with bond angles of 109.5 degrees
What shape and bond angle do a few complex ions with coordination number 4 form
Square planar with bond angles of 90 degrees
What shape and bond angle do most complex ions with coordination number 2 form
Linear structure with bond angles of 180 degrees
What shape and bond angle do complex ions with coordination number 6 form
Octahedral with bond angles of 90 degrees
Give 2 examples of linear complex ions
[CuCl2]-
[Ag(NH3)2]+
Give an example of a tetrahedral complex ion
[CoCl4]2-
Give an example of a square planar complex ion
[PtCl2(NH3)2]
THIS IS CISPLATIN
Give an example of a octahedral complex ion
[Fe(H2O)6]2+
State and explain the shape of the [Fe(H2O)6]2+ ion
It is octahedral and has bond angles of 90 degrees.
There are six bonding pairs of electrons around the central Fe2+ ion.
These electron pairs repel each other as far apart as possible to a position of minimal repulsion
Name the reagents used to convert a solution of VO2 + ions to vanadium (II) ions
Zinc and dilute sulfuric acid
What type of isomerism do square planar complexes display
Cis-trans
What does cis-isomers look like
The identical groups are adjacent to each other
What do trans-isomers look like
The identical groups are opposite to each other (180 degrees apart)
What type of isomerism do octahedral complexes show with monodentate ligands
Cis-trans isomerism
What type of isomerism do octahedral complexes show with bidentate ligands
Optical isomerism
Describe the bonding and structure of cisplatin
It is a transition metal complex of a platinum (II) ion with two chloride ion and two ammonia molecule ligands.
The ligands are arranged around the central Pt2+ ion in a square planar arrangement
Describe how the nitrogen atoms in DNA can bond to the platinum ion
The lone pair of electrons on the nitrogen from the DNA forms a dative covalent bond to the platinum ion
Why is the enthalpy change small in some ligand substitutions
There are similar numbers of bonds in both complexes.
The same number and type of bonds are broken and formed
What 3 factors cause the colour change of ions
The change in:
Oxidation state
Co-ordination number
Ligand
How does colour arise
From electronic transitions from the ground state to excited states between different d orbitals.
A portion of visible light is absorbed to promote d orbitals to higher energy levels.
d-d transition takes place.
Remaining light is transmitted which is the colour seen.
Colour of the ion is the complimentary colour to the light absorbed in the d-d electron transmission
In terms of bonding, explain the meaning of the term complex.
atom or ion or transition metal bonded to one or more ligands by co–ordinate bonds / donation of
an electron pair
State the origin of the colour of transition-metal complexes.
Electron transitions/electrons excited in d shell (1) or d-d transition
(Energy in) visible range (1)
How can transition metals be identified
By their colour
Explain why complexes formed from transition metal ions are coloured.
absorb (some) wavelengths/frequencies/colours/energies of (visible) light to promote/excite electrons in d-orbitals remaining/complementary wavelengths/frequencies/colours/energies of (visible) light transmitted (to give colour seen)
Equation for the energy difference between the ground state and the excited state of d electrons
E = hv = hc / lambda
Explain why the hydrated V2+ and V3+ ions have different colours
The energy gap between the ground state and the excited state of the d-orbitals is different for each transition metal ion.
Explain why solutions of scandium (III) ions are colourless
The Sc3+ ion has an empty d sub-shell, so there are no electrons to make the d-d electron transition when some visible light is absorbed
Suggest why the strong interactions between the F- ligands and the Fe3+ ion result in complex [FeF6]3- being colourless
The energy gap between the ground state and the excited state of the d-orbitals is large.
The energy absorbed in the d-d electron transition is outside the visible region of the spectrum
What is used to determine the concentration of coloured ions in solution
A simple colorimeter
Copper(II) compounds may be used as fungicides in vineyards. When used in this way, copper(II) ions can enter the water supply and cause problems because they are toxic in high concentrations.
The water supply near a vineyard can be tested for copper(II) ions by forming a blue aqueous complex with EDTA4− ions. The concentration of this complex can be determined using a colorimeter.
Outline the practical steps that you would follow, using colorimetry, to determine the concentration of this complex in a sample of water.
Calibrate a colorimeter / produce a calibration curve.
By testing the colorimeter with solutions of copper-EDTA complex of known concentration.
Add excess EDTA salt to the sample.
Why does changing a ligand or the coordination cause a change in colour
It alters the energy split between the d-orbitals and changes the delta E and hence changes the frequency of light absorbed
Describe the method for spectrophotometry
Add an appropriate ligand to intensify colour
Make up solutions of known concentration
Measure absorption or transmission
Plot graph of absorption vs concentration
Measure absorption of unknown and compare
What influences the redox potential for a transition metal changing from a higher to a lower oxidation state
pH and the ligand
How are vanadium species with oxidation states IV, III and II formed
Reduction of vandate(V) ions by zinc in acidic solution
Use of [Ag(NH3)2]+
Used in Tollens’ reagent to distinguish between aldehydes and ketones.
Aldehyes reduce the silver in Tollens reagent to silver
What do catalysts do and how
Increase rate of reaction without getting used up.
They do this by providing an alternative reaction route with a lower activation energy
What is a heterogenous catalyst
It is in a different phase from the reactants
What is a homogenous catalyst
It is in the same phase as the reactions
Define Lewis Base
Electron pair donor
Describe how a calibration graph is produced and used to find the concentration of the iron(III) complex.
measure absorbance for (a range of) known concentrations
plot graph absorbance v concentration
read value of concentration for the measured absorbance from this graph
Why is a support medium used
To maximise the SA of a heterogenous catalyst and minimise the cost
What is the catalyst in the contact process
V2O5
What are the equations for the contact process
S1. V2O5 + SO2 → V2O4 + SO3
S2. 2V2O4 + O2 → 2V2O5
Overall Equation :
2SO2 + O2 → 2SO3
What is the catalyst used in the manufacture of methanol from carbon monoxide and hydrogen
Cr2O3
What type of catalyst is V2O5
Heterogenous
Give one reason why impurities in the reactants can cause problems in processes that use heterogeneous catalysts.
Impurities poison the catalyst / block the active sites
Implications of catalyst poisoning
Decreased efficiency
Cost
Describe the steps in heterogenous catalysis
Reactants form bonds with atoms at active sites on the surface of the catalyst (adsorbed onto the surface)
As a result bonds in the reactants are weakened and break
New bonds form between the reactants held close together on catalyst surface.
This in turn weakens bonds between product and catalyst and product leaves (desorbs).
Why can transition metals act as homogenous catalysts
They have variable oxidation states
Why is the uncatalysed reaction between I- and S2O8 2- very slow
The reaction needs a collision between 2 negative ions.
Repulsion between the ions hinders this and therefore there is a high activation energy
Condition for a substance to act as a homogenous catalyst
Electrode potential must lie in between electrode potentials of the 2 reactants.
It first reduces reactant with more positive E value and then oxidises the reactant with more negative E value.
What are the equations for the catalysed reaction between I- and S2O8 2-
stage 1: S2O8 2- + 2Fe2+ = 2SO42- + 2Fe3+
stage 2: 2I- + 2Fe3+ = 2Fe2+ + I2
What is autocatalysis
Where one of products of the reaction can catalyse the reaction.
Describe the reaction rate between ethanedioate and manganate ions
The initial uncatalysed reaction is slow because the reaction is a collision between two negative ions which repel each other leading to a high activation energy.
The Mn2+ ions produced act as an autocatalyst and therefore the reaction starts to speed up because they bring about the alternative reaction route with lower activation energy. Reaction also speeds up due to attraction between negatively charged ion and positively charged catalyst.
The reaction eventually slows as the MnO4 concentration drops.
Why do chemists prefer to use heterogenous catalysts
It is easier to separate a solid catalyst from the reaction mixture by filtration as it is insoluble
Colour of MnO4-
Purple
Colour of Mn2+
Colourless
What is the colour at the end of the titration if manganate is in the burette
Pink
What acid should be used for ALL manganate titrations
dilute sulfuric acid
What happens if insufficient volume of dilute H2SO4 or a weak acid is used in manganate redox titrations
The solution will not be acidic enough and MnO2 will be produced instead of Mn2+ and the brown MnO2 will mask the colour change and lead to a greater volume of manganate being used
Equations for the catalysed reaction between ethanedioate and manganate ions
4Mn2+ + MnO4- + 8 H+ = 5Mn3+ + 4 H2O
2Mn3+ + C2O4 2- = 2Mn2+ + 2 CO2
What is the rule for Ligand substitution reaction
For all complexes EXCEPT Cu2+ and Zn2+ :
6 H2O ligands replaced by 6 NH3 ligands
6 H2O ligands replaced by 4 Cl- ligands (applies to all)
With Cu2+ and Zn2+:
6 H2O ligands replaced with 4 NH3 ligands and 2 H2O ligands remain
What colour are Ag complex solutions
Colourless
Describe how silver does and does not behave as a transition metal
Can form complexes
Shows catalytic behaviour
Does not form coloured compounds
Does not have variable oxidation states
What leads to catalytic action
Adsorption of reactants at active sites on the surface may lead to catalytic action.
The active site is the place where reactants adsorb onto the surface of the catalyst
Reaction and observation for silver chloride dissolving in dilute ammonia
AgCl + 2NH3 = [Ag(NH3)2]+ + Cl-
Colourless solution formed
Reaction and observation for silver bromide dissolving in concentrated ammonia
AgBr + 2NH3 = [Ag(NH3)2] + Br-
Colourless solution formed
Why does silver iodide not react with ammonia
It is too insoluble
