3.2.5/6 Transition metals & reactions of ions

What is the definition of a transition element?

A d-block element that can form at least one stable ion with a partially filled d-subshell

Where are the transition metals located in the periodic table?

d block

Which electrons do transition metals lose first when forming ions?

4s orbital first, then 3d orbital

Why is scandium not a transition element?

• It forms only 1 stable ion of Sc³⁺

• Sc³⁺ has an empty d-subshell (1s²2s²2p⁶3s³3p⁶4s⁰3d⁰), so is not partially filled

Why is zinc not a transition element?

• It only forms 1 stable ion of Zn²⁺

• Zn²⁺ has a full d-subshell (1s²2s²2p⁶3s²3p⁶4s⁰3d¹⁰), so is not partially filled

What are the physical properties of transition metals?

• High density, melting & boiling points

• Hard, strong & shiny

• Good conductors of heat & electricity

• Similar ionic radii & fairly unreactive

What are some of the uses of transition metals?

• Iron & steel alloys: used in buildings & transport, as it is very strong

• Copper: used in water pipes & wires, as it is unreactive & conducts electricity

• Titanium: used in nuclear reactors & jet engines, as it has a very high melting point

What are the chemical properties of transition metals?

• They form complex & coloured ions

• They are good catalysts

  • iron (used in the Haber process to make ammonia)

  • manganese (used in the decomposition of hydrogen peroxide)

• They can exist in variable oxidation states

What is the definition of a complex ion?

A central transition metal ion surrounded by ligands that are bonded by dative covalent bonds to it

What is the definition of a 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 & so, a complex ion

What is the definition of a monodentate ligand?

A ligand that forms one co-ordinate bond to the central metal ion (one lone pair to donate). Examples include:

• H₂O:

• :NH₃

• :Cl^-

What is the definition of a bidentate ligand?

A ligand that forms two co-ordinate bonds to the central metal ion (2 lone pairs to donate). Examples include:

• ethanedioate

• ethane-1,2-diamine (shortened name → en)

What is the definition of a polydentate ligand?

A ligand that forms three or more co-ordinate bonds to the central metal ion. For example:

• EDTA^4- → can form 6 dative covalent bonds with the central metal ion

What is the definition of co-ordination number?

The number of co-ordinate bonds the central metal ion has formed to surrounding ligands

How is the oxidation state of a metal ion with a complex calculated?

Oxidation state of the metal ion = total charge of the complex - total oxidation state of ligands

What is the Chelate effect?

When bidentate/polydentate ligands replace monodentate ligands, to form a more stable complex because there is an increase in entropy

Explain the Chelate effect in terms of entropy & the reaction that is occurring

• Number of molecules increases with bidentate/polydentate ligands (e.g. EDTA^4- displaces 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 ion is formed

If a transition metal ion has 2 ligands, what shape is it usually?

Linear shape (180° bond angle)

If a transition metal ion has 4 ligands, what shape is it usually?

Tetrahedral (109.5° bond angle)

What is an exception to the general rule that ions with 4 ligands is generally tetrahedral. What shape is it?

Platin is square planar shape → forms cisplatin (90° bond angle)

What shape is a complex ion if it has 6 ligands?

Octahedral shape (90° bond angle)

How are transition metals complexes named?

1. State the number of ligands (e.g. di- , tetra- , hexa-)

2. Name the ligand(s) in alphabetical order

3. Name the central atom identifying the overall charge on the complex

• for cations or neutral complexes → the metal name is normal (e.g. copper, silver)

• for anions → use the suffix -ate & the metal is usually referred to by its Latin name (e.g. Cu - cuprate, Ag - argentate & Fe - ferrate)

4. Show the oxidation number of the metal in Roman numerals

What are the formulas, charges & names for the following ligands?

What are the names for the following complexes?

1. Tetraaquacopper(II)

2. Diamminesilver(I)

3. Hexacyanoferrate(III)

4. Pentaaquachlorochromium(III)

5. Tetraamminedichloridocobalt(III)

6. Tetracarbonylnickel(0)

7. Hexaaquacopper(II)

8. Diamminedichloridoplatinum(II)

What are the formulas for the following named complexes?

1. [Ag(CN)₂]^-

2. [Ni(en)₃]²⁺

3. [CuCl₄]^2-

4. [FeCl₂(OH)₂]^2-

5. [CoCl₄]^2-

6. [Ag(S₂O₃)₂]^3-

7. [Ni(EDTA)]^2-

8. [Fe(H₂O)₂]²⁺

How can complex ions display E-Z/cis-trans isomerism & what shapes of ion does this apply to?

• Ligands differ in the way they are arranged in space:

  • 2 ligands of the same type can be on the same side of the metal ion → Z (cis) isomer

  • 2 ligands of the same type can be on the opposite sides of the metal ion → E (trans) isomer

• Applies to square planar & 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⁺ & Fe³⁺ co-ordination numbers when Cl^- ligands replace NH₃ or H₂O ligands?

Decreases from 6 to 4, as Cl^- is a much larger ligand than H₂O & NH₃

What is haem, its metal ion, co-ordination number & ligands?

• A molecule that makes up protein chains, with an Fe²⁺ central metal ion, which has a co-ordination number of 6

• 4 of these bonds are to a ring system called porphyrin

• 1 is to the nitrogen of a globin (protein) molecule & 1 is to an oxygen in an O₂ molecule

How does haemoglobin transport oxygen?

In the body, both water & oxygen bind to Fe²⁺ ions as ligands:

• in the lungs (high O₂ concentration), water ligands are substituted for oxygen molecules → forms oxyhaemoglobin, which is carried in the bloodstream

• in respiring cells (low O₂ concentration), the oxygen molecules are exchanged for water molecules → forms deoxyhaemoglobin

• the haemoglobin then returns to the lungs & the whole process starts again

Why is carbon monoxide toxic?

• CO also co-ordinately bonds to Fe²⁺ & is a better ligand, so bonds more strongly than O₂

• This stops O₂ from bonding to haemoglobin, so O₂ cannot be transported around the body

Why are transition metal compounds coloured?

• When ligands bond to transition metal ions, some orbitals are given more energy than others, splitting the 3d orbitals into different energy levels

• Electrons tend to occupy the lower orbitals (ground state) & to jump up to higher orbitals (excited state), they need energy equal to the energy gap (ΔE) → they get this energy from visible light

• They then transmit this energy as visible light (only 1 colour is transmitted)

  • for complexes with a full or empty 3d sub-shell, no electrons can migrate to the higher orbital, so complexes are seen as colourless or white

What is the equation to calculate the energy absorbed when electrons jump up?

What factors affect the colour of complexes?

• Changes in the oxidation state of a transition metal in a complex ion

• Changes in the co-ordination number of a transition metal in a complex ion

• Changes in the type of ligand → ligands have different crystal field strengths (i.e. amount of energy given out) & will split the d orbitals by differing amounts

Fill in this table for Vanadium species:

(how to remember → “You Better Get Vanadium”)

What is used to reduce Vanadium?

Zinc

Complete this table of the complex ion colours

What is spectroscopy used for & how does it work?

To determine the concentration of a solution by measuring how much light it absorbs:

• white light is shone through a filter, which is chosen to only let the colour of light through that is absorbed by the sample

• the light then passes through the sample to a colorimeter, which shows how much light was absorbed by the sample → the more concentrated a coloured solution is, the more light it will absorb

In calorimetry, what has to be done before the unknown concentration of a sample is found?

A calibration graph/curve has to be produced:

• involves measuring the absorbencies of unknown concentrations of solutions & plotting the results on a graph

• once this is done, the absorbance of the sample is measured & its concentration is read off the graph

Why can transition metals have variable oxidation states?

They have partially filled d-orbitals, so can lose 4s & 3d electrons

Which oxidation states do all transition metals have (except Sc) & why?

+2 due to loss of electrons from 4s orbital

When the oxidation state is high, do transition metals exist as simple ions?

No, after the oxidation state of about +3, 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

What colour is:

• MnO₄^-

• Mn²⁺

• Deep purple

• Pink

Write a half equation for the reduction of MnO₄^- to Mn²⁺

MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 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-

• Cr³⁺

• Orange

• Green

Write a half equation for the reduction of Cr₂O₇^2- to Cr³⁺

Cr₂O₇^2- + 14H⁺ + 6e^- → 2Cr³⁺ + 7H₂O

What happens to aqua metal ions in acidic, alkaline & neutral conditions?

• Acidic conditions: they get reduced

• Alkaline conditions: they get oxidised

• Neutral conditions: no change

What does whether reduction or oxidation occurs & the readiness of the reaction depend on?

E° values (standard electrode potential)

What can change E° values?

pH & the ligands involved

What is the definition of a catalyst?

A substance that increases the rate of a reaction by providing an alternative pathway with a lower activation energy & is not used up

Why are transition metals good catalysts & why are group 1, 2 & 3 metals not as good catalysts?

• Transition metals can exist in variable oxidation states (due to partially filled d sub-shells), so can provide alternative pathways easily

• Group 1, 2 & 3 metals only exist in one oxidation state

What is advantage of using a catalyst for a reaction?

Allows reactions to proceed at lower temperatures & pressures → saves valuable energy, money, time & resources

What is the definition of 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):

• catalytic activity occurs on the solid surface as the reactants pass over it

• no need for separation of products from catalyst

How do heterogenous catalysts work?

1. Reactants adsorb (stick to) the catalyst’s surface at active sites

2. This weakens the bonds within the reactants & holds reactants close together on the surface &/or in the correct orientation to react

3. Once the reaction has occurred, products desorb from the active sites

What properties do catalysts 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 reactants would not be held in place for long enough & bonds would not be sufficiently weakened

  • need a good balance between desorption & adsorption

How can you increase the efficiency of heterogenous catalysts?

• Increase the surface area to increase the number of active sites that are present

• Spread onto an inert support medium (e.g. ceramic) to increase the surface/mass ratio → use ceramic honeycomb/mesh/sponge

What is catalyst poisoning?

Unwanted impurities adsorb to the catalyst’s active sites & do not desorb, which blocks the active sites on the catalyst’s surface

What effect does catalyst poisoning have on the catalytic activity?

Decreases the effectiveness of the catalyst over time

Aside from catalyst poisoning, how else can a catalyst be degraded?

Finely divided catalysts can be gradually lost from their support medium

What is the Haber process & what catalyst is used?

• Makes ammonia & uses iron (Fe) catalyst

• N₂(g) + 3H₂(g) → 2NH₃(g)

What size/shape is the Fe catalyst for the Haber process?

Pea sized lumps to increase the 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 catalyst is used?

2SO₂(g) + O₂(g) → 2SO₃(g)

• used industrially to make sulfuric acid (H₂SO₄)

• catalysed by vanadium (V) oxide (V₂O₅)

What are the two reactions that are involved in the Contact process?

• SO_{2 +} V₂O₅ → SO₃ + V₂O₄

• V₂O₄ + _1/2O₂ → V₂O₅

What is a vanadium a good catalyst in the case of the Contact process?

It can change oxidation state from +5 to +4 & back to +5 (so can be used again)

What is the definition of a homogenous catalyst & how do they work?

A catalyst that is in the same phase/physical state as the reactants:

• form intermediates to give a different reaction pathway with a lower activation energy

What is the reaction between S₂O₈^2- ions & I^- ions?

S₂O₈^2- (aq) + 2I^- (aq) → I₂ (aq) + 2SO₄^2- (aq)

Why does the reaction between between S₂O₈^2- ions & I^- ions have a high activation energy in normal conditions?

Two negative anions are reacting, meaning they repel each other, so the activation energy is high

Which transition metal ion catalyses the reaction between S₂O₈^2- ions & I^- ions? Write two equations to show how

Fe²⁺

What is the definition of autocatalysis?

When the product of a reaction is also a catalyst for that reaction

Draw a concentration of reactant against time graph for an autocatalysed reaction & explain each stage

1. The rate if reaction is initially slow & uncatalysed, as not much of the catalyst has been formed

2. The rate increases as the catalyst is made (catalysed reaction is faster)

3. The rate slows down as the reactants are used up

Write a half equation for the conversion of C₂O₄^2- ions into CO₂

C₂O₄^2- → 2CO₂ + 2e^-

Write an equation for the reaction between C₂O₄^2- ions & MnO₄^- ions. How does Mn²⁺ autocatalyse this reaction?

2MnO₄^- + 16H⁺ + 5C₂O₄^2- → 10CO₂ + 2Mn²⁺ + 8H₂O

• 1st stage: MnO₄^- + 4Mn²⁺ + 8H⁺ → 4H₂O + 5Mn³⁺

• 2nd stage: 2Mn³⁺ + C₂O₄^2- → 2CO₂ + 2Mn²⁺

How can you monitor the concentration of MnO₄^- ions?

Using a colorimeter

Are 2+ ions or 3+ ions more acidic in solution & why?

3+ ions:

• they are smaller & more highly charged, so have a higher charge density

• they attract the electrons from the oxygen of the ligands much more strongly

• this weakens the O-H bonds, so the complex readily releases a H⁺ ion into the solution, making it acidic

Write an equation for the hydrolysis of [Fe(H₂O)₆]³⁺ to release a proton

[Fe(H₂O)₆]³⁺ (aq) ⇌ [Fe(H₂O)₅(OH)]²⁺ (aq) + H⁺(aq)

What kind of acid is the complex ion acting as & why?

Bronsted-Lowry → donates a proton

What is acting as the Lewis acid & base during the formation of complex ions?

The metal ion is acting as the Lewis acid (electron pair acceptor) & the ligands as Lewis bases (electron pair donor)

What is the order of ligand strength between ammine, chloride & aqua ligands?

1. Chloride (strongest)

2. Ammine

3. Aqua (weakest)