CHEM 2.5 - TRANSITION METALS

transition element

transition element = is a d-bloc element that can form at least one stable ion with a partially filled (incomplete) d subshell

for period 4 d block elements, scandium and zinc are not transition metal because they dont form a stable ion with a partial filled d subshell

chromium and copper are exceptions to the electron configuration

loose electrons from 4s first then 3d

scandium and zinc not a transition metal

transition metals have to have at least one stable ION with a partially filled d subshell

scandium forms a stable 3+ ion but it does not have a partially full d subshell. the d sub shell is empty for Sc³⁺

zinc forms a stable 2+ ion but it does not have a partially full d subshell. the d subshell is full for Zn²⁺

properties of transition metals

transition metals have specific properties which include variable oxidation states, form coloured ions in solution and are good catalysts

transition metals have variable oxidation states. this is because the electrons sit in 4s and 3d sub shells which are very close in energy. therefore electrons are gained and lost using a similar amount of energy when they form the ions.

they form many different ions

they form coloured ions in solution

colours of transition metals

these ions form coloured ions in solution (aqueous in water)

metal complexes

transition metals can form complex ions where a central transition metal ion is surrounded by ligands bonded by dative covalent/ coordinate bonds

ligand = a molecule or ion that forms a dative covalent bond with a transition metal by donating a pair of electrons

ligands have at least 1 lone pair of electrons which are used to form a dative covalent bond with the metal

types of ligands

monodentate = a ligand that forms one coordinate bond to the metal ion.

-examples: H₂O, NH₃, Cl^-

bidentate = a ligand that forms two coordinate bonds to the metal ion

-example: ethanedioate C₂O₄^2-, ethane-1,2-diamine C₂H₈N₂

multidentate = a ligand that forms multiple coordinate bonds to the metal ion

-example: EDTA^4- (forms 6 coordinate bonds)

complex shapes

the shape is dependant on the coordination number and the size of ligands

coordination number = the coordination number is the number of coordinate bonds in a complex, NOT the number of ligands

size of ligands = can fit a certain no of ligands around a metal ion depending on its size

-can fit 6 of H₂O and NH₃

-can fit 4 of Cl^-

-can fit 3 of ethanedioate C₂O₄^2- and ethane-1,2-diamine C₂H₈N₂

naming complex shapes

expressing the shapes [Co(H₂O)₆]²⁺

for single atom ligands they dont need to be written with extra brackets around them

octahedral shapes = complexes with a coordination number of 6.l bond angles is 90

tetrahedral shapes = complexes with a coordination number of 4. bond angle is 109.5

square planar shapes = also complexes with a coordination number of 4. bond angle is 90. only example you need to know is cisplatin [Pt(NH₃)₂Cl₂] which is an anti cancer drug. memorise the shape and expression

linear shapes = complexes with a coordination number of 2. bond angle is 180. example you need to know is tollens reagent [Ag(NH₃)₂]⁺. memorise the shape and expression

calculating oxidation state of the metal of complex shapes

oxidation state of metal = total oxidation state of complex shape - total oxidation states of ligands

haemoglobin

is an octahedral shape

haem is a multi dentate ligand found in haemoglobin, the 4 nitogens bound to Fe come from the haem group (the circle). the 5th nitrogen comes from the globin protein

in the lungs when oxygen concentration is high, oxygen substitutes the water ligand and is transported around the body, forming oxyhaemoglobin. the oxygen is removed where its needed in the body and is replaced by water again. the haemoglobin returns to the lungs and process repeats

haemoglobin and carbon monoxide

carbon monoxide is dangerous because it can replace the water ligand in haemoglobin and the bond it forms is very strong so it cannot be replaced by water or oxygen. this leads to a lack of transport of oxygen around the body

optical isomerism

optical isomerism = a type of stereoisomerism where molecules have the same structural formula but are non-superimposable mirror images of each other

non superimposable = they cannot be placed exactly on top of each other so that everything matches perfectly

octahedral shapes with 3 bidentate ligands show optical isomerism (image)

cis trans isomerism

cis-trans isomerism = molecules have the same structural formula but different arrangement in space around a rigid bond/structure

octahedral complexes with 4 of the same ligands and 2 other different ligands display cis-trans isomerism

if the 2 different ligands are opposite each other you have a trans isomer

if the 2 different ligands are adjacent each other you have a cis isomer

square planar complexes with 2 of the same ligand and 2 other different ligands display cis-trans isomerism

cis trans isomerism 2

square planar complexes with 2 of the same ligand and 2 other different ligands display cis-trans isomerism

if the 2 different ligands are opposite each other you have a trans isomer

if the 2 different ligands are adjacent each other you have a cis isomer

example: cisplatin cures cancer, transplatin cannot

d-orbital splitting

when a ligand bonds onto the central atom, the d-subshell splits into 2. when the ligands attach some orbitals gain energy, creating an energy gap

when electrons absorb light energy some move from the lowest energy level (ground state) to a higher energy level orbital (excited state). for this to happen, the light energy must equal ^E

the size of ^E is dependant on: (calculation in next flashcard)

-the central metal ion and its oxidation state

-the type of ligand

-coordination number

calculating ^E = change in energy

this is the formula for the energy absorbed by electrons

coloured complexes

the larger the change in energy ^E, the higher the frequency of light absorbed

any frequencies which are not absorbed are reflected or transmitted

the colour observed is the complementary colour to the colour/frequency of light absorbed

transition metal complexes are coloured because ligands cause the d orbitals to split into different energy levels. when the electrons absorb visible light energy and get excited, a colour change occurs. this explains why Sc and Zn are not transition metals and do not produce this colour change

colour is dependant on:

-the type of ligand

-the shape of the ligand

-the oxidation state of the central metal ion

ligand substitution

ligand substitution can change:

-colour

-coordination number

-shape

colour changes because different ligands change the d-orbital splitting (ΔE)

memorise the second one for Cu

colorimetry

monochromatic light = light of a single wavelength/ frequency, so one colour of light only

the calorimeter must be set to zero. to do this we measure the absorbance of a blank sample, normally the solvent you are using to dissolve your transition metal ion (water). white light is filtered into a narrow range of frequencies to produce monochromatic light (single colour). the sample is held in a vessel called a cuvette. the monochromatic light passes through the sample and some the light is absorbed. light not absorbed travels to the detector. the detector will measure the level of absorbance by comparing it to absorbance in the blank sample

more concentrated sample = more light absorbed = darker colour

calibration curve

plotting concentrations of known solution to work out the concentration of an unknown

more ligand substitution

memorise the second one

is in excess ammonia

ligands of a similar size are being exchanged

more ligand substitution 2

larger ligans replacing smaller ones

more ligand substitution 3

when ligand substitution occurs, some ligand-metal bonds are stronger than other, which means its harder to reverse some reactions

multi dentate ligands form more stable complexes than monodentate

entropy in ligand substitution

increased entropy helps favour formation of a more stable complex

often the energy needed to break the bonds is similar as the energy released when new ones are formed, so the enthalpy change (AH) is small like the reaction in the image

entropy in ligand substitution 2

chelate effect = multidentate ligands form more stable complexes than equivalent monodentate ligands. this is because of an increase in entropy

when we substitute monodentate ligand with bidentate or multidentate ligand we create a solution with more particles in it = increase in entropy. reactions that have an increase in entropy are more likely to happen. it's difficult to reverse these reactions as this would mean a decrease in entropy

multidentate ligands bond to the metal ion and replace several smaller ligands already attached, this releases more free molecules into the solution = entropy

vanadium

has a wide range of oxidation states and colours

when transition metals change oxidation state a redox reaction occurs

memorise those colours

vanadium reactions - reduction using zinc

VO₂⁺ (+5) to V²⁺(+2)

VO₂⁺ → VO²⁺ → V³⁺ → V²⁺
yellow → blue → green → violet
+5 → +4 → +3 → +2

redox potentials

redox potentials tell us how easily an ion is reduced which is the same as electrode potentials. the least stable ions have the largest redox potential and are more likely to be reduced

must have:

-temperature at 298K

-pressure at 100kPa

-concentrations of ions at 1 moldm-3

there may be a difference in redox potential to the standard values seen in a data book. it is dependent on the environment the ions are in

what affects redox potentials

ligands:

the standard electrode potentials is always measured in aqueous solutions. the metal ion is surrounded by water molecules. however ligands other than water can form stronger bonds to the metal ions with different oxidation states. this means the redox potential can be higher or lower than the standard value

pH:

the pH of the solution affects the size of redox potentials with some reactions. the more acidic the solution the larger the electrode potential. this means the ion is more easily reduced. pH affects redox potentials because H⁺ ions are involved in some redox equilibria. changing the H⁺ concentration shifts the equilibrium and changes the electrode potential

tollens reagent

[Ag(NH₃)₂]⁺

formed by ligand substitution

• Ag⁺ is the metal ion

• NH₃ ligands replace water ligands

redox titrations

redox titrations are used to work out the titration of a reducing or oxidising agent

have an oxidising/reducing agent in burette with a known concentration. have your reducing/ oxidising agent solution with an unknown concentration but known volume in conical flask. add excess dilute sulfuric acid into flask too, to ensure you have sufficient H+ ions to allow the reduction of the oxidising agent. add the ions in the burette to the conical flask until you see the faint colour appear = end point. add drop by drop. record your results to 2 decimal places and repeat until you get 2 results that are concordant (within 0.10cm of each eachother)

make sure in titration to record values with 2dp

types of catalyst

heterogenous:

a catalyst that is in a different phase/ state from the reactants

example: haber process

N₂(g) + 3H₂(g) > 2NH₃(g). a solid iron catalyst is used for this reaction which is a different state the gaseous reactants

Increasing the surface area of the heterogeneous catalyst will increase the rate of reaction. More particles can react with the catalyst at the same time.

homogenous:

a catalyst that is in the same phase as the reactants

generally homogeneous catalysts are aqueous in aqueous reactants. for example using sulphuric acid to make an ester.

homogeneous catalysts forms intermediate species by reactants combining with the catalyst. the catalyst is reformed by the end

the contact process

the contact process uses vanadium (V) to make sulfuric acid. in the manufacture of sulfuric acid we use V₂O₅ (Vanadium V) as a catalyst. it's used to catalyse SO₂ to SO₃. this is a heterogeneous catalyst

first V₂O₅ oxidises SO₂ to SO₃ and is itself reduced to V₂O₄:

V₂O₅ + SO₂ → V₂O₄ + SO₃

second V₂O₄ is oxidised by oxygen in the air to reform the catalyst V₂O₅:

V₂O₄ + ½ O₂ → V₂O₅

poisoned heterogenous catalysts

impurities can bind to the surface of a catalyst and block the active sites for reactants. when an impurity blocks a site we call this poisoning. catalytic poisoning reduces the surface area of the catalyst for the reactants bind to. this slows down the reaction

a poisoned catalyst means

-less product is made

-the catalyst needs to be replaced or cleaned more often

-increased cost of the chemical

in the harber process the hydrogen is made from methane which contains sulfur impurities, which can form iron sulfide and block the iron catalyst less effective

homogenous catalyst energy diagram

has two peaks for activation energy because the reaction occurs in more than one step. the catalyst reacts with the reactants to form an intermediate. each step has its own activation energy

example of homogenous catalyst pathway

the original reaction is very slow because it involves 2 negative ions reacting and they repel, so Fe²⁺ is used as a homogenous catalyst to react peroxodisulphate S₂​O₈^2- and iodide ions. Fe²⁺ is oxidised to Fe³⁺, which is the intermediate formed and S₂​O₈^2- is reduced to SO₄^2-

Fe³⁺ then reacts with iodine ions in the second step to reform the catalyst

autocatalysis

autocatalysis is another form of homogeneous catalysis where the product catalyses the reaction/ it catalyses itself

an example of autocatalysis is where Mn²⁺ is the catalyst in the reaction between C₂O₄^2- and MnO₄^- . Mn²⁺ is a product and a catalyst

the original reaction is very slow because it involves 2 negative ions reacting and they repel. Mn²⁺ catalyses the conversion of MnO₄^- to Mn³⁺. then Mn³⁺ reacts with C₂O₄^2- and Mn²⁺ are reformed