Transition metals

What is the defining characteristic of transition metals regarding their electron configuration?

They have an incomplete d sub-level in their atoms or ions.

When transition metals form ions, which sub-level electrons are lost first?

The 4s electrons are lost before the 3d electrons.

Why is zinc not considered a transition metal?

Zinc only forms a +2 ion, which has a complete 3d sub-level, failing the criteria of having an incomplete d sub-level.

List four general characteristics of transition metals.

Complex formation, formation of coloured ions, variable oxidation states, and catalytic activity.

Define a complex in coordination chemistry.

A central metal ion surrounded by ligands.

What is a ligand?

An atom, ion, or molecule that can donate a lone electron pair to a central metal ion.

What type of bonding occurs between a ligand and a central metal ion?

Co-ordinate (dative covalent) bonding.

Define coordination number.

The number of co-ordinate bonds formed to a central metal ion.

What is a monodentate ligand?

A ligand that can form only one co-ordinate bond per ligand (e.g., H2O, NH3, Cl-).

Give two examples of bidentate ligands.

Ethane-1,2-diamine (NH2CH2CH2NH2) and the ethanedioate ion (C2O4 2-).

What is a multidentate ligand?

A ligand that can form multiple co-ordinate bonds per ligand, such as EDTA4- which forms six.

What happens to the coordination number when [Cu(H2O)6]2+ reacts with concentrated HCl?

It changes from 6 to 4, forming the tetrahedral [CuCl4]2- complex.

Why does the coordination number change when Cl- replaces H2O ligands?

The chloride ion is significantly larger than the uncharged H2O and NH3 ligands, causing steric hindrance that limits the number of ligands that can fit around the metal ion.

What is the 'chelate effect'?

The increased stability of a complex formed by substituting monodentate ligands with bidentate or multidentate ligands.

How is the chelate effect explained in terms of entropy?

The substitution reaction increases the number of moles of particles in the system (more products than reactants), leading to a positive entropy change (ΔS > 0).

Why is the enthalpy change (ΔH) for the chelate effect often close to zero?

Because the number and type of coordinate bonds (e.g., N to metal) being broken and formed are similar, resulting in minimal energy change.

What is the formula for the complex formed between copper(II) and EDTA4-?

[Cu(EDTA)]2-

Why is carbon monoxide (CO) toxic to humans?

CO forms a stronger coordinate bond with the iron in haemoglobin than oxygen does, effectively replacing the oxygen and preventing its transport in the blood.

What is the coordination number of [Cr(NH2CH2CH2NH2)3]3+?

6 (each of the 3 bidentate ligands forms 2 coordinate bonds).

What is the geometry of the [CuCl4]2- complex?

Tetrahedral.

What is the typical stoichiometry of an EDTA-metal ion titration?

1:1 ratio.

What is the colour of the [CuCl4]2- complex?

Yellow/green.

What is the colour of the [CoCl4]2- complex?

Blue.

What happens when solid copper chloride is dissolved in water?

It forms the aqueous [Cu(H2O)6]2+ complex, not the chloride complex.

What is the primary application of EDTA in shampoos?

It removes calcium ions from hard water, which helps the shampoo lather effectively.

How do you calculate the moles of EDTA4- in a titration?

Moles = concentration × volume

What is the typical coordination geometry for square planar complexes?

Square planar, such as in cisplatin.

What type of complex geometry do transition metal ions typically form with larger ligands like Cl-?

Tetrahedral complexes.

What type of complex geometry do transition metal ions typically form with small ligands like H2O and NH3?

Octahedral complexes.

What geometry does the Ag+ ion commonly form?

Linear complexes, such as [Ag(NH3)2]+.

What are the two types of stereoisomerism found in complex ions?

Cis-trans isomerism and optical isomerism.

What is cis-trans isomerism in complex ions a special case of?

E-Z isomerism.

What condition is required for a complex to exhibit optical isomerism?

The presence of 3 bidentate ligands, forming non-superimposable mirror images.

What three factors cause colour changes in transition metal complexes?

Changes in oxidation state, coordination number, or ligands.

Why do transition metal complexes appear coloured?

Ligands cause d-orbital splitting; visible light is absorbed to promote electrons to higher energy states, and the remaining transmitted light gives the substance its colour.

What is the formula relating energy difference (ΔE) to frequency (v) and Planck's constant (h)?

ΔE = hv

What is the formula relating energy difference (ΔE) to wavelength (λ), speed of light (c), and Planck's constant (h)?

ΔE = hc/λ

Why are Sc3+ ions colourless?

They have no d-electrons, so no d-d electron transitions are possible.

Why are Zn2+ and Cu+ ions colourless?

They have a full 3d10 subshell, leaving no space for d-d electron transitions.

What is the purpose of adding a ligand in spectrophotometry?

To intensify the colour of the solution to allow for more accurate absorption measurements.

In spectrophotometry, how is the concentration of a coloured ion determined?

By measuring absorption and comparing it to a calibration graph of absorption vs. known concentrations.

When transition metals form ions, which electrons are lost first?

The 4s electrons are lost before the 3d electrons.

What factors influence the redox potential of a transition metal ion changing oxidation states?

pH and the nature of the ligands present.

What is the general trend for the stability of the +2 oxidation state relative to the +3 state across the transition period?

The relative stability of the +2 state increases across the period.

What are the four main oxidation states of Vanadium and their corresponding colours?

+5 (yellow), +4 (blue), +3 (green), +2 (violet).

What is the role of Zinc metal in acidic solution regarding transition metal ions?

It acts as a strong reducing agent, reducing ions to their lowest stable oxidation state.

What is the chemical use of Tollens' reagent [Ag(NH3)2]+?

It is used to distinguish between aldehydes and ketones; aldehydes reduce the silver ions to metallic silver.

Why is the titration between Fe2+ and MnO4- considered 'self-indicating'?

Because the MnO4- reactant is purple and the Mn2+ product is colourless, the disappearance of the purple colour marks the end point.

What is the balanced equation for the reaction between MnO4- and Fe2+ in acidic conditions?

MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 4H2O + 5Fe3+

If MnO4- is in the burette during a titration, what indicates the end point?

The first appearance of a permanent pink colour.

Why is a manganate titration considered 'self-indicating'?

Because there is a significant, visible colour change from the reactant (purple MnO4-) to the product (colourless Mn2+).

What is the end point of a manganate titration if the manganate is in the burette?

The first permanent pink colour.

Which acid is recommended for manganate titrations and why?

Dilute sulfuric acid, as it provides the necessary H+ ions without interfering with the redox reaction.

What happens if insufficient sulfuric acid is used in a manganate titration?

MnO2 (brown solid) is produced instead of Mn2+, which masks the colour change and leads to inaccurate, higher volume readings.

Why is hydrochloric acid unsuitable for manganate titrations?

Cl- ions are oxidised to poisonous Cl2 gas by MnO4-, leading to inaccurate readings and safety hazards.

Why is nitric acid unsuitable for manganate titrations?

It is an oxidising agent that oxidises Fe2+ to Fe3+, leading to a smaller volume of manganate being used.

Why is ethanoic acid unsuitable for manganate titrations?

It is a weak acid and cannot supply the large amount of hydrogen ions (8H+) required for the reaction.

What is the overall ionic equation for the reaction between manganate(VII) and iron(II)?

MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) -> Mn2+(aq) + 4H2O(l) + 5Fe3+(aq)

What is the overall ionic equation for the reaction between manganate(VII) and hydrogen peroxide?

2MnO4-(aq) + 6H+(aq) + 5H2O2 -> 5O2(g) + 2Mn2+(aq) + 8H2O(l)

What is the overall ionic equation for the reaction between manganate(VII) and ethanedioate?

2MnO4-(aq) + 16H+(aq) + 5C2O4 2-(aq) -> 10CO2(g) + 2Mn2+(aq) + 8H2O(l)

Why is the reaction between manganate(VII) and ethanedioate often heated to 60°C?

The reaction between two negative ions is slow at room temperature; heating speeds up the initial reaction.

What is the stoichiometric ratio of MnO4- to FeC2O4 in their titration reaction?

3 moles of MnO4- react with 5 moles of FeC2O4.

Define a catalyst.

A substance that increases the reaction rate without being consumed, by providing an alternative reaction route with a lower activation energy.

What is the difference between a heterogeneous and a homogeneous catalyst?

A heterogeneous catalyst is in a different phase from the reactants, while a homogeneous catalyst is in the same phase.

Where does the reaction occur in heterogeneous catalysis?

At the surface of the catalyst, specifically at active sites.

What are the four steps of heterogeneous catalysis?

1. Adsorption of reactants, 2. Weakening/breaking of reactant bonds, 3. Formation of new bonds between reactants, 4. Desorption of products.

How does surface area affect heterogeneous catalyst effectiveness?

Increasing the surface area increases the number of active sites, improving the effectiveness of the catalyst.

Why is a support medium used for heterogeneous catalysts?

To maximise the surface area while minimizing the cost of the catalyst material.

What determines the effectiveness of adsorption in heterogeneous catalysis?

The strength of the adsorption; if too strong, products cannot desorb; if too weak, reactants do not adsorb in sufficient concentration.

What catalyst is used in the Contact process and what is the overall equation?

V2O5 (Vanadium(V) oxide); 2SO2 + O2 -> 2SO3.

What is the role of V2O5 in the Contact process steps?

It acts as a catalyst: SO2 + V2O5 -> SO3 + V2O4, followed by 2V2O4 + O2 -> 2V2O5.

What catalyst is used in the Haber process?

Iron (Fe).

What is the equation for the Haber process?

N2 + 3H2 -> 2NH3.

Which catalyst is used in the manufacture of methanol from CO and H2?

Chromium(III) oxide (Cr2O3).

What does it mean for a catalyst to be 'poisoned'?

The catalyst's efficiency is reduced due to impurities binding to its active sites.

What is catalyst poisoning?

A process where impurities adsorb onto the surface of a catalyst, reducing its efficiency and potentially requiring its replacement.

Why is leaded petrol unsuitable for cars with catalytic converters?

Lead strongly adsorbs onto the surface of the catalyst, poisoning it and rendering it ineffective.

What defines homogeneous catalysis?

A process where the catalyst and reactants are in the same phase, typically involving an intermediate species.

Why are transition metals effective homogeneous catalysts?

They can form various oxidation states, allowing them to donate and receive electrons due to their partially filled d-subshells.

What is the requirement for a substance to act as a homogeneous catalyst in a redox reaction?

Its electrode potential must lie between the electrode potentials of the two reactants.

Why is the uncatalysed reaction between iodide and persulfate ions slow?

It requires a collision between two negative ions, leading to high electrostatic repulsion and a high activation energy.

What is autocatalysis?

A phenomenon where one of the products of a reaction acts as a catalyst for that same reaction.

In the titration of C2O4 2- with MnO4-, which species acts as an autocatalyst?

Mn2+ ions.

How can the rate of the reaction between ethanedioate and manganate ions be monitored?

By titrating samples at set times or by using a spectrometer to measure the intensity of the purple colour.

What is the primary advantage of using a spectrometer to monitor reaction rates?

It does not disrupt the reaction mixture and allows for a quicker determination of concentration.

Why is silver not considered a transition metal by definition?

It has a full 4d subshell (4d10) in both its atom and ion, meaning it lacks a partially filled d-subshell.

What are two key differences between silver and typical transition metals?

Silver does not form coloured compounds and does not exhibit variable oxidation states.

In the complex Co(NH3)6Cl3, how many chloride ions are free to react with silver nitrate?

Three, as they are all outside the complex.

If a compound reacts with silver nitrate in a 1:1 mole ratio, what does this imply about the chloride ions?

Only one chloride ion is free (outside the complex), while the others are ligands.

What happens to the oxidation state of a transition metal catalyst during a reaction?

It changes to form an intermediate and then returns to its original state at the end of the reaction.

What is the role of d-electrons in transition metal catalysis?

They allow the metal to easily lose or gain electrons, facilitating oxidation and reduction steps.

Does a favorable electrode potential guarantee a faster reaction rate?

No, it only indicates that catalysis is thermodynamically possible, not that the rate will necessarily increase.

What is the formula for the complex formed when silver chloride dissolves in ammonia?

[Ag(NH3)2]+

Why do transition metals form coloured compounds?

Because they have partially filled d-orbitals that allow for electron transitions between d-orbitals.