Transition Metals (Unfinished)

Why do the characteristics of transition metals arise?

Transition metals have a partially filled 3d energy sub-level in their atoms or ions

Why is zinc not considered a true transition metal?

• When any transition metal loses electrons, they lose them for their 4s sub-level first

• Zinc’s outer electron configurations is 3d¹⁰,4s²

  • Zinc only forms 1+ and 2+ ions meaning that it will never have a partially filled 3d sub-level which is a characteristic of true transition metals

What are the characteristic properties of transition metals?

• Ability to form complexes

• Form coloured ions

• Exists in variable oxidation states

• Good catalysts

What is meant by the term ligand?

Species that can donate one or more lone pairs of electrons to a transition metal or ion

What is meant by the term complex?

A complex is formed when transition metals or their ions form dative covalent bonds with ligands

What is meant by the term co-ordinate/dative covalent bonds in relation to transition metals?

Where the ligand provides both electrons in the covalent bond

What is meant by the term co-ordinate number?

The number of coordinate bonds formed to the central metal ion

What is meant by a monodentate ligand and give some examples

• A ligand that forms one co-ordinate bond with the metal ion

• H₂O, Cl^-, NH₃, -CN

Which two uncharged monodentate ligands are similar in size?

Water and ammonia

Which monodentate ligand is bigger than water and ammonia?

Cl^-

What is meant by the term bi-dentate?

Ligands that form two coordinate bonds since they donate two pairs of electrons (two donor atoms)

Name, write the formula and draw the two bidentate ligands that you should recall?

• Ethane-1,2-diamine

  • NH₂CH₂CH₂NH₂

• Ethanedioate ion

  • C₂O₄^2-

What is a multidentate ligand?

Ligands forming 3 or more dative covalent bonds since they donate at least 3 lone pairs of electrons

How many coordinate bonds does EDTA^4- form?

6- it is a hexadentate ligand

How many coordinate bonds does porphin form?

4

What does EDTA stand for?

Ethane diamine tetra acetic acid

Recall the four most common shapes for transition metal complexes and their bond angles

• Octahedral- 90^o

• Tetrahedral- 109.5^o

• Square planar- 90^o

• Linear- 180^o

What are some ways we can form octohedral transition metal complexes?

• 6 monodentate ligands- water or ammonia, NOT Cl- as it is too large

• 3 bidentate ligands- ethane-1,2-diamine and the ethanedioate ion

• 1 hexadentate ligand- EDTA^4-

How can we form tetrahedral transition metal complexes

Formed when a transition metal is bonded to 4 chloride ligands

Why do chloride ligands form tetrahedral complexes instead of octohedral?

• Chloride ions are larger than other monodentate ligands

• We can’t fit as many around the central metal atom- only enough space for four

Draw the structure of [Cu(NH₂CH₂CH₂NH₂)₃]²⁺

Draw the structure of [Cu[C₂O₄)₃]^4-

When do we normally form optical isomers of transition metal complexes?

Octahedral complexes with 3 bidentate ligands

What are optical isomers?

• These exist as pairs of enantiomers

• They are non-superimposable mirror images of each other

Why does cis-trans isomerism occur in transition metal complexes?

The ligands cannot rotate around the central metal ion

In which complex shapes can Cis-Trans isomerism occur and with what ligands?

• Square planar with two of one type of ligand and two of another

• Octahedral with 4 of one type of ligand and 2 of another

What is the difference between a cis and a trans isomer?

• Cis- pair are next to each other at a 90^o angle

• Trans- the pair are on opposite sides to each other at a 180^o angle

Explain with reference to d sub levels and electronic transitions why many transition metals appear coloured

• When a transition metal is bonded to ligands, the d-orbitals split

• When visible light is shone on the transition metal, the electrons are excited and an electron moves to a d-orbital higher in energy- this is called a transition

• The light energy absorbed to excite the electron corresponds to certain parts of the visible electromagnetic spectrum

  • Visible light includes red, orange, yellow, green, blue, indigo and violet light with the red end being lower in energy and having longer wavelengths and the blue end being greater in energy and shorter wavelengths

• If the change in energy is small, then lower energy wavelengths of light are absorbed (red, orange, yellow) and so shorter wavelength, high energy visible light is emitted (blue, indigo, violet) meaning that the compound will appear to be blue (for example

  • This is the inverse for transition energy changes that are larger

Why do some transition metals appear colourless?

• Some like zinc have a full d sub level so transition cannot take place

• Doesn’t absorb any visible light- it is all transmitted

What factors affect the colour of transition metals and how?

• Type of ligand

• Shape of the complex

• Co-ordination number

• Oxidation state of the metal

• Metal

They affect the energy change that occurs when a transition happens. If any of these things change then the colour of the complex will change because the change in energy is different.

What equation give us the change in energy upon a transition?

ΔE = h x f (or v)

• ΔE = The change in energy in Joules

• h = Planks constant = 6.63 × 10^-34 Js

• f/v = Frequency (Hz/hertz or s^-1)

What is the equation to give frequency?

f = c/λ

• f = Frequency (Hz/hertz or s^-1)

• c = The speed of light 3.00 ×10⁸ ms^-1

• λ = Wavelength in m

How can we link the two equations ΔE = h x f (or v) and f = c/λ to find the change in energy upon transition of an electron in a transition metal?

ΔE = (h x c) / λ

• ΔE = The change in energy in Joules

• h = Planks constant = 6.63 × 10^-34 Js

• c = The speed of light 3.00 ×10⁸ ms^-1

• λ = Wavelength in m

We have just replaced frequency with the second equation

How do we make a calibration graph for a coloured transition metal solution?

• Solutions of known concentrations are placed into the colorimeter and their absorption values are recorded

• From these values, we can plot a graph of concentrations against absorption (a calibration graph)

What should a calibration graph look like and how would we use it to determine an unknown concentration of transition metal solution?

• Graph of absorbance against concentration

• Should be linear

• If we can place a solution of unknown concentration is and measure the absorbance

  • We can then read across from absorbance and down to find the concentration

What happens in full ligand substitution reactions?

• Ligands bonded to a central metal ion substituted for new ligand(s)

• A complex with 6 monodentate ligands like water can be replaced by another 6 monodentate ligands or 1 hexadentate ligands for example

What happens in a ligand substitution reaction where chloride ligands are substitute monodentate ligands?

• 6 monodentate ligands can normally form a octahedral complex with a central metal ion

• 4 chloride ligands can be substituted for these monodentate ligands

  • Remember that chloride ligands are larger than ammonia and water so we can’t fit as many around the central metal ion

How is a haemoglobin molecule formed?

• A central Fe²⁺ ion is bonded to the tetradentate ligand porphin

• A globin molecule also form a coordinate bond to the central Fe²⁺ ion

• This forms haemoglobin

How does haemoglobin transport different substances (oxygen, water, carbon monoxide) in the blood through ligand substitution reactions?

• O₂ form a dative covalent bond with Fe²⁺ to form oxyhaemoglobin

  • The bond is relatively weak so it breaks/dissociates at cells

• At cells Fe²⁺ picks up water by forming a dative covalent bond with the oxygen and forms deoxyhaemoglobin

• C≡O/carbon monoxide can form a dative covalent bond with Fe²⁺ but this covalent bond is very strong and does not dissociate from the new complex, carboxyhaemoglobin

What is entropy?

The measure of disorder in a system

• This exists because everything in the universe tends towards disorder

Give the equation linking entropy, enthalpy change, temperature and free energy?

ΔG = ΔH - (T * ΔS)

• ΔG: Change in Gibbs free energy- J or J/mol^-1

• ΔH: Enthalpy change- KJmol^-1

• T: Temperature (must be in Kelvin)

• ΔS: Entropy change- JK^-1mol

For a reaction to be feasible/spontaneous, what ΔG be?

Negative

When is the enthalpy change almost zero for ligand substitution reactions?

When you a breaking the same number of the same type of bonds

What is the chelate effect?

The greater stability of bidentate or multidentate ligand complexes compared to monodentate ligands for the same metal

What is en short for?

en is short for NH₂CH₂CH₂NH₂ or ethane-1,2-diamine we’re just using slang guys

Take the reaction:

[Co(NH3)₆]³⁺ + 3en -> [Co(en)₃]³⁺ + 6NH₃

What happens to the entropy change of this reaction? What is the effect on Gibbs free energy and why does this mean the reaction is feasible?

• 6 Co-N bonds are broken and 6 Co-N bonds are formed so the enthalpy change is 0

  • This means that the reaction only happens due to the entropy change

• When a reaction has fewer reactant particles producing more product particles, the entropy change increases

  • We know that ΔG must be negative for a reaction to be feasible/spontaneous

• ΔG = ΔH - (T * ΔS) but since the enthalpy change is 0, ΔG = 0 - (T * ΔS)

  • Because our reaction has more product particles than reactant particles, our entropy change is positive meaning that 0 take away a positive number will give us a negative value for Gibbs free energy

  • Therefore the reaction is feasible

Take the reaction:

[Co(NH3)₆]³⁺ + 3en -> [Co(en)₃]³⁺ + 6NH₃

Explain with reference to entropy why it’s difficult to reverse the above reaction?

• If we attempt to reverse this reaction, the number of reactant particles in now greater than product particles- we are going from 7 particles to 4

• This means that the entropy change is negative

  • ΔG = ΔH - (T * ΔS) but since the enthalpy change is 0, ΔG = 0 - (T * ΔS)

  • 0 take away are negative value gives us a positive value for Gibbs free energy meaning the reaction is not feasible

In what scenarios does entropy change increase?

• When going from a solid to a liquid to a gas

• When we go from fewer reactant particles to more product particles

Recall the colours vanadium species 2+ to 5+ and an acronym to remember it

• V⁵⁺ - Yellow

• V⁴⁺ - Blue

• V³⁺ - Green

• V²⁺ - Purple

You’d Better Get Prepared

Will the reduction of Co²⁺ occur easier with water ammonia or ligands?

• [Co(NH₃)₆]²⁺ + 2e^- ⇌ Co + 6NH₃ E^o = -0.43V

• [Co(H₂O)₆]²⁺ + 2e^- ⇌ Co + 6H₂O E^o = -0.28V

• Water ligands

• The electrode potential is more negative with ammonia ligands than it is with water

• For the water ligands, it is more likely that the reaction moves forwards to form cobalt atoms/Co than Co²⁺

• With ammonia ligands, it is more likely the reaction moves backwards to form the Co²+ ions rather than Co

What is the half equation for the oxidation of Fe²+ to Fe³+?

Fe²⁺ → Fe³⁺ + e⁻

What is the half equation for the oxidation of ethanedioate ions (C₂O₄^2-)?

C₂O₄²⁻ → 2CO₂ + 2e⁻

What is the half equation for the reduction of manganate ions/MnO₄^- to Mn²⁺ ions?

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Give the full equation for the reaction of Fe²⁺ with MnO₄^- and the molar ratio of these two ions

• MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺

• Fe²⁺ : MnO₄^- = 5:1

Give the full equation for the reaction of C₂O₄^2- with MnO₄^- and the molar ratio of these two ions

• 2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 8H₂O + 10CO₂

• C₂O₄^2- : MnO₄^2- = 5:2

What does aliquot mean?

A sub portion of a large sample

What is a heterogenous catalyst?

A catalyst that is in a different state to the reactants

How do heterogenous catalysts work?

• The reactant particles diffuse to the surface of the catalyst

• The reactants adsorb to the surface forming weak IMFs/attractive forces

• This weakens the bonds in the reactant particles

  • This it takes less energy to break these bonds so the activation energy is lowered

• Atoms then desorb from the surface

What is the point where reactants adsorb to the surface of a heterogenous catalyst?

The active site

What is catalyst poisoning?

impurities present in the reaction mixture can ‘poison’ the catalyst by blocking the active site

Why is catalyst poisoning bad for industrial reaction?

• Reduces the efficiency of the catalyst

• Increases cost of the catalyst- you may need to buy more to replaced the poisoned catalyst

Describe the structure of industrial catalysts and give two reasons for this structure

• Structure- honeycomb support

  • Large surface area

  • Less precious metal needs to be used so reduced costs

Why did leaded petrol reduce the efficiency of cars?

The platinum and rhodium catalyst in a catalytic converter are poisoned by lead compounds

Give the equation for the Haber process and state the catalyst

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

• Uses a catalyst of iron filings (essentially iron powder)

How can we produce the hydrogen needed for the Haber process and which is the method used most commonly?

• Hydration of hydrocarbons with steam- this is used most frequently

• Electrolysis of water

Equation for the hydration of methane with steam

CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g)

Why is using hydrogen produced from the hydration of methane with steam in the Haber process a problem?

• Methane is usually contaminated with sulfur so the hydrogen produced will also be contaminated with sulfur

• The iron filing catalyst used in the Haber process can be poisoned by sulfur

What is the contact process used for?

Producing sulfuric acid

Write the equation for the production of sulfur trioxide for sulfur dioxide in the contact process

SO₂ (g) + ½ O₂ (g) → SO₃ (g)

Why does the contact process need a catalyst?

• All the substances are in the gas phase

• The particles are spread apart so there are fewer successful collisions than with a solid or a liquid

• We could increase the rate by increasing temperature and pressure but these methods are expensive

What is the catalyst used in the contact process?

V₂O₅

How is the contact process catalysed? Use equations to show this

• SO₂ + V₂O₅ → SO₃ + V₂O₄

• ½ O₂ + V₂O₄ → V₂O₅

Overall equation: SO₂ (g) + ½ O₂ (g) → SO₃ (g)

Keep in mind the two equations before are also full equations

What is a homogenous catalyst?

A catalyst in the same physical state as the reactants

What happens to a homogenous transition metal catalyst during a reaction?

• The reaction often proceeds via an intermediate species

• The transition metal temporarily changes oxidation state

Give the reaction between peroxodisuphate ions and iodine and identify a catalyst for this reaction?

S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq)

• Fe²⁺ or Fe³⁺

Why is the reaction between S₂O₈²⁻ and 2I⁻ slow?

• Both species are negatively charged

• They will repel each other

Give the two overall equations that occur during the catalysis of this reaction

Step 1: Reduction of Peroxodisulphate by Iron(II): S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺

Step 2: Oxidation of Iodide by Iron(III): 2I⁻ + 2Fe³⁺ → I₂ + 2Fe²⁺

If we cancel out species on either side then the overall equation is: S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq)

We can do either step first if we just change what products are added together first

What is auto-catalysis?

When one of the products of a reaction acts as a catalyst for the reaction?

In the reaction with MnO₄^- ions and C₂O₄^2- ions, why does it start off slowly?

Two negatively charged ions repel each other

In the reaction with MnO₄^- ions and C₂O₄^2- ions, what is the species that is an auto-catalyst?

Mn²⁺

What is the half equation to show the oxidation of Mn²⁺ to Mn³⁺

Mn²⁺ → Mn³⁺ + e^-

Give equations to show how the autocatalysis of the reaction between MnO₄^- and C₂O₄^2- works

• Step 1: 4Mn²⁺ + MnO₄⁻ + 8H⁺ → 5Mn³⁺ + 4H₂O (x2)

  • Mn³⁺ acts as an intermediate compound and is produced from the oxidation of Mn²⁺

• Step 2: 2Mn³⁺ + C₂O₄²⁻ → 2Mn²⁺ + 2CO₂ (x5)

Overall: 2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 8H₂O + 10CO₂

Step 1 has to be multiplied by 2 and step 2 multiplied by 5 in order to get the same amount of Mn³⁺ on each side of the equation

• This allows us to eliminate Mn³⁺ from the overall equations which is important as it never shows up in the overall equations

Describe and explain a concentration against time graph for an autocatalysis reaction (Use MnO₄^-’s reaction with C₂O₄^2- as an example)

Explain how this would be different if we added powdered Mn²⁺ at the start

• At the start, the concentration of Mn²⁺ is building up but the negatively charged species are sill repelling each other

• In the middle, the rate is the fastest as Mn²⁺ is acting as a catalyst

• At the end, the concentration of reactants is low so the rate slows down as the collision frequency (and therefore the frequency of successful collisions, decreases

Adding powdered Mn²⁺

• There would be no lag phase at the start because the Mn²⁺ can immediately begin catalysing the reaction

Give the general equation for the reaction of Cu²⁺ and Fe²⁺ in their metal aqua ion complex with OH^- ions?

[M(H₂O)₆]²⁺ + 2OH^- → M(H₂O)₄(OH^-) + 2H₂O

Give the general equation for the reaction of Fe³⁺ and Al³⁺ in their metal aqua ion complex with OH^- ions?

[M(H₂O)₆]³⁺(aq) + 3OH⁻(aq) → [M(H₂O)₃(OH)₃](s) + 3H₂O(l)

What type of reaction is occurring when hydroxide ions are added to metal aqua ions complexes?

Hydrolysis

Give the colours of the following complexes:

[Fe(H₂O)₆]²⁺

[Cu(H₂O)₆]²⁺

[Fe(H₂O)₆]³⁺

[Al(H₂O)₆]³⁺

[Fe(H₂O)₆]²⁺ : Pale green solution

[Cu(H₂O)₆]²⁺ : Blue solution

[Fe(H₂O)₆]³⁺ : Yellow-brown solution

[Al(H₂O)₆]³⁺ : Colourless solution

What observations do we make when adding hydroxide ions to metal aqua complexes

Fe(OH)₂(H₂O)₄ : Dark green ppt and then turns brown on standing

Cu(OH)₂(H₂O)₄ : Blue ppt

Fe(OH)₃(H₂O)₃ : Brown ppt

Al(OH)₃(H₂O)₃ : White ppt

What observations do we make when adding hydroxide ions to metal aqua complexes in excess?

Fe(OH)₂(H₂O)₄ : No visible change

Cu(OH)₂(H₂O)₄ : No visible change

Fe(OH)₃(H₂O)₃ : No visible change

Al(OH)₃(H₂O)₃ : White precipitate dissolves

Why does Fe(OH)₂(H₂O)₄ turn brown on standing?

The iron 2+ ion is oxidised to iron 3+ which is brown

Why does Al(OH)₃(H₂O)₃ dissolve when excess OH^- is added? Use equations to show how this occurs

• Compounds of aluminium are amphoteric and so can react with OH^- ions

  • Al(OH)₃(H₂O)₃ + 3OH^- → [Al(OH)₆]^3- + 3H₂O

• The aluminium complex produced is charged so now it can form interactions with the water, causing it to dissolve

Why do neutral (not charged) complexes not dissolve in water?

They have no charge so they cannot form interactions with the water, therefore they do not dissolve- they are insoluble

Give the general equation for the reaction of 3+ metal aqua ion complexes when ammonia is added dropwise

[M(H₂O)₆]³⁺ + 3NH₃ → [M(H₂O)₃(OH)₃] + 3NH₄⁺

Give the general equation for [M(H₂O)₆]²⁺ reaction when ammonia is added dropwise?

[M(H₂O)₆]²⁺(aq) + 2NH₃(aq) → [M(H₂O)₄(OH)₂](s) + 2NH₄⁺(aq)

What type of reaction is occurring when ammonia is added dropwise to metal aqua ion complexes?

• Hydrolysis

Give the observations made when ammonia is added to the following 4 compounds dropwise:

[Fe(H₂O)₆]²⁺

[Cu(H₂O)₆]²⁺

[Fe(H₂O)₆]³⁺

[Al(H₂O)₆]³⁺

[Fe(H₂O)₆]²⁺: Dark green precipitate

[Cu(H₂O)₆]²⁺: Blue precipitate

[Fe(H₂O)₆]³⁺: Brown precipitate

[Al(H₂O)₆]³⁺: White precipitate

Give the observations made when ammonia is added to the following 4 compounds in excess:

[Fe(H₂O)₆]²⁺

[Cu(H₂O)₆]²⁺

[Fe(H₂O)₆]³⁺

[Al(H₂O)₆]³⁺

[Fe(H₂O)₆]²⁺: No visible change

[Cu(H₂O)₆]²⁺: Deep blue solution

[Fe(H₂O)₆]³⁺: No visible change

[Al(H₂O)₆]³⁺: No visible change

Why does [Cu(H₂O)₆]²⁺ form a deep blue solution when ammonia is added in excess? Use equations in your explanation

• [Cu(H₂O)₆]²⁺ + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 4H₂O(l) (This is an overall equation)

• When ammonia is added dropwise, the copper aqua complex undergoes hydrolysis reactions with two of the water ligands at the axial positions

• Ammonia can then also undergo ligand substitution with the rest of the water ligands

  • It breaks the O-H bonds at equatorial positions

What two equations occur when [Cu(H₂O)₆]²⁺ form a deep blue solution when ammonia is added in excess and what kind of reactions are occurring?

• [Cu(H₂O)₆]²⁺(aq) + 2NH₃(aq) → [Cu(H₂O)₄(OH)₂](s) + 2NH₄⁺(aq) - Hydrolysis

• [Cu(H₂O)₄(OH)₂](s) + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2OH⁻(aq) + 2H₂O(l) - Ligand substitution

What is the name of the shape of the complex that is formed when [Cu(H₂O)₆]²⁺ reacts with excess ammonia?

Elongated octahedral