Unit 2: Section 4 - Transition Metals Flashcards
Define a transition element
An element which forms at least one stable ion with a partially full d-shell of electrons
What are some physical properties of transition metals?
- Metallic
- Good conductors of heat and electricity
- Hard
- Strong
- Shiny
- High m.p
- High b.p
- Low reactivity
What are some uses of iron?
Vehicle bodies, to reinforce concrete
What are some uses of titanium?
Jet engine parts
What are some uses of copper?
Water pipes
What are the characteristic chemical properties of transition metals (4)?
- Variable oxidation states -> take part in many redox reactions
- Coloured compounds/ions in solution
- Good catalysts
- Form complex ions
Define the term complex ion
Central transition metal ion surrounded by ligands (other ions/molecules) that are co-ordinately bonded to it
Give some examples of transition metals catalysts amd the processes/reactions they catalyse? (3)
Iron - Haber process
Vanadium (V) oxide - contact process
MnO2 - decomposition of H2O2
Which electrons do transition metals lose first when forming ions?
4s
Define the term ligand
An ion or molecule with at least one lone pair of electrons, that donates them to a transition metal ion to form a co-ordinate bond and thus a complex ion
Define the term mono/unidentate ligands
A ligand that forms one co-ordinate bond to the central metal ion (one lone pair to donate)
Define the term bidentate ligand
A ligand that forms two co-ordinate bonds to the central metal ion (2 lone pairs to donate)
Define the term multidentate ligand
A ligand that forms three or more co-ordinate bonds to the central metal ion
Give some examples of common monodentate ligands (4)
Cl- H2O NH3 CN-
How many co-ordinate bonds does EDTA4- form?
6
Define the term coordination number?
The number of co-ordinate bonds the metal ion has formed to surrounding ligands
What is the chelate effect?
Chelate complexes with multidentate ligands are favoured over monodentate ligans or ligands that form fewer co-ordinate bonds per molecule
Explain the chelate effect in terms of entropy and the reaction that is occurring
- Number of molecules increases when multidentate ligands, e.g. EDTA, displace ligands that form fewer co-ordinate bonds per molecule
- Significant increase in entropy -> Gibbs’ free energy change , 0 -> feasible reaction
- A more stable complex ion is formed
What ion is usually formed when a transition metal compound is dissolved in water? What shape is it?
Aqua ion 6 H2O ligands around the central metal ion. Octahedral complex is formed
If a transition metal ion has 2 ligands, what shape is it usually?
Linear
If a transition metal ion has 4 ligands, what shape is it usually?
Tetrahedral
Name an exception to the general rule that ions with 4 ligands is generally tetrahedral. What shape is it?
Platin is square planar -> forms cisplatin
What shape is a complex ion if it has 6 ligands?
Octahedral
How can complex ions display E-Z or cis-trans isomerism? What shapes of ion does this apply to?
- Ligands differ in the way in which they are arranged in space
- 2 ligands of the same type can be on the same side of the metal ion (next to each other), which forms the E or cis isomer
- 2 ligands of the same type can be on opposite sides of the metal ion (not next to each other), which forms the Z or trans isomer
- Applies to square planar and octahedral complex ions
What conditions are needed for a complex ion to display optical isomerism?
Usually applies to octahedral molecules with 2 or more bidentate ligands, so that the mirror images are non-superimposable
What happens to Co2+ Cu2+ and Fe3+’s coodrination numbers whe Cl- ligands replace NH3 or H2O ligands?
Decreases from 6 to 4 as Cl- is a much larger ligand than H2O and NH3
What is haem - its metal ion, coordination number and ligands?
- A molecule which makes up protein chains, with an Fe2+ central metal ion, which has a coordination number of 6
- 4 of these bonds are to a ring system called porphyrin
- 1 is to the nitrogen of a globin (protein) molecule and one is to the oxygen in an O2 molecule
How does haemoglobin transport oxygen?
O2 forms a weak coordinate bond to the metal ion, then is transported around the body
The bond breaks when haemoglobin reaches cells and oxygen is released
Why is CO toxic?
- CO also coordinately bonds to the Fe2+ and is a better ligand, so bonds more strongly than O2
- Stops O2 from bonding to haemoglobin, so O2 cannot be transported around the body
Why are transition metal compounds coloured?
- They have partially filled d-orbitals and electrons are able to move between the d-orbitals
- In compounds (when ligands coordinately bond to the ion), the d-orbitals split into different energy levels
- Electrons can absorb energy in the form of photons to become excited and move to a higher energy level (excited state)
- Energy of photon = energy difference between levels
- Energy of photon is related to frequency of light by E=hf
- The colour corresponding to the frequency of the energy change is missing from the spectrum, so we see a combination of all the colours that aren’t absorbed
What affects the colour of a transition metal compound?
ΔE affects the frequency of absorbed photons, so determines the colour
ΔE is changed by oxidation state of the metal, number and type of ligands, shape, co-ordination number
What are the colours and oxidation states for vanadium species?
- VO2+ 5+ Yellow
- VO^2+ 4+ Blue
- V^3+ 3+ Green
- V^2+ 2+ Violet
What can you use to reduce vanadium?
Zinc
What does a colorimeter do?
Measures the absorbance of a particular wavelength of light by a solution
How would you use colorimetry experimentally?
Use solutions of known concentration to create a calibration graph; find unknown concentration
What information can a colorimeter give you?
The concentration of a certain ion in the solution
Why can transition metals have variable oxidation states?
They have partially filled d-orbitals, so can can use 4s and 3d electrons
Which oxidation states do all transition metals have (except Sc). Why?
+2 due to loss of electrons from 4s orbital
When oxidation state is high, do the transition metals exist as simple ions?
No, after oxidation state of about III, metal ions covalently bond to other species
What is the use of the complex [Ag(NH3)2]^+ ion?
Tollens’ reagent to test for aldehydes/ketones (silver mirror formed with aldehyde, no visible change with ketone)
Wrie a half equation for the reduction of MnO4- to Mn^2+
MnO4- + 8H+ + 5e- –> Mn^2+ + 4H2O
Why are redox titrations with transition metal compounds said to be self-indicating?
They usually involve a colour change as the metal is changing oxidation state; sometimes an indicator is still needed/useful
What colour is Cr2O7^2-?
Orange
What colour is Cr^3+?
Green
Write a half equation for the reduction of Cr2O7^2- to Cr^3+
Cr2O7^2- + 14H+ + 6e- –> 2Cr^3+ + 7H2O
What happens to aqua metal ions in acidic conditions?
They get reduced
What happens to aqua metal ions in alkaline conditions?
They get oxidised
What happens to metal ions in neutral conditions?
No change
What does whether reduction/oxidation occurs and the readiness of the reaction depend on?
E∘ values
What can change these E∘ values?
pH, ligands involved
Define a catalyst
A substance that increases the rate of a reaction without being chemically changed at the end of the reaction
How do catalysts usually work?
Provide an alternative reaction pathway with a lower activation energy
Why are transition metals good catalysts?
They can exist in variable oxidation states, so can provide alternative pathways easily
Why are group 1, 2 and 3 metals not as good catalysts?
Only exist in one oxidation state
What are advantages of using a catalyst for a reaction?
Allows reactions to proceed at lower temperatures and pressures -> saves valuable energy and resources
What metals are used in a catalytic converter?
Pt, Rd, Pd
Define a heterogenous catalyst
A catalyst that is present in the reaction in a different phase to the reactants (usually a solid, with gas/liquid reactants)
What is an advantage of using a heterogenous catalyst?
No need for separation of products from catalyst
How do heterogenous catalysts work?
- Reactants adsorb to the catalyst’s surface at active sites
- This weakens bonds within the reactants - holds reactants close together on the surface and/or in the correct orientation to react
- Once the reaction has occurred, products desorb from the active sites
What properties does the catalyst need to have to make it a good catalyst?
- Can’t adsorb too strongly, otherwise the products will not desorb
- Can’t adsorb too weakly as reactant would not be held in place for long enough and bonds would not be sufficiently weakened
- Need a good balance between desorption and adsorption
How can you increase the efficiency of heterogeneous catalysts?
- Increase the surface area to increase the number of active sites that are present - use a honeycomb surface
What is catalyst poisoning?
- Unwanted impurities adsorb to the catalyst’s active sites and do not desorb
- This blocks the active sites on the catalyst’s surface
What effect does catalyst poisoning have on catalytic activity?
Decreases the effectiveness of the catalyst over time
What is the Haber process? What catalyst is used?
N2(g) + 3H2(g) -> 2NH3(g)
Makes ammonia, uses iron (Fe) catalyst
What size/shape is the catalyst for the Haber process?
Pea sized lump to increase surface area
How long does the catalyst last for the Haber process? What is it poisoned by
- About 5 years
- Poisoned by sulfur impurities in the gas streams
What is the Contact Process? What is the catalyst?
Makes H2SO4 - catalysed by vanadium (v) oxide V2O5
S (s) + O2 (g) → SO2 (g)
2SO2(g) + O2(g) -> 2SO3(g)
H2O (l) + SO3 (g) → H2SO4 (aq)
What are the 2 reactions that are involved in the contact process?
SO2 + V2O5 -> SO3 + V2O4
V2O4 + 1/2O2 -> V2O5
Why is V a good catalyst in the case of the contact process?
Can change oxidation state from 5+ to 4+ and back to 5+ (so can be used again)
Define homogeneous catalyst
A catalyst that is in the same phase as the reactants
How do homogeneous catalysts work?
Form intermediates to give a different reaction pathway with lower Ea
What is the reaction between S2O8^2- ions and I- ions?
S2O8^2- + 2I- -> 2SO4^2- + I2 (all aq)
Why does the reaction between S2O8^2- ions and I- ions have a high Ea in normal conditions?
- Two negative ions are reacting
- They repel each other so Ea is high
Which transition metal ions catalyse the reaction between S2O8^2- ions and I- ions?
Fe^2+
Equation 1 Fe^2+ -> Fe3+ and S2O8^2- -> SO4^2-
Equation 2 I- -> I2 and Fe^3+ to Fe^2+
Define the term autocatalysis
When the product of a reaction is also a catalyst for that reaction
What does a concentration of reactant against time graph for an autocatalysed reaction look like? Explain each stage
- Initially slow, uncatalysed as not much of catalyst has been formed
- Rate increases as catalyst is made; catalysed reaction is faster
- Slows down as reactants are used up
Write a half equation for the conversion of C2O4^2- ions into CO2
C2O4^2- -> 2CO2 + 2e-
What do C2O4^2- ions and MnO4- ions go to in half equations?
MnO4- -> Mn^2+
C2O4- -> C2O
How does Mn^2+ autocatalyse the reaction of C2O4^2- ions and MnO4- ions?
1st stage: MnO4- + 4Mn^2+ + 8H+ -> 4H2O + 5Mn^3+
2nd stage: 2Mn^3+ + C2O4^2- -> 2CO2 + 2Mn^2+
How can you monitor the concentration of MnO4- ions?
Using a colorimeter
Why does an aqueous 3+ ion have a lower pH than an aqueous 2+ ion?
- +3 ions are more acidic than 2+ ions
- 3+ ions have a higher charge density
- 3+ ions are more polarising
- O-H bond in ligand is weaker and so is broken more easily
