Transition Metals (AQA A Level Chemistry): Flashcards

CO is a stronger ligand than O₂ and binds irreversibly to the Fe(II) centre in haemoglobin, forming carboxyhaemoglobin. This prevents oxygen from binding and being transported to cells.

The chelate effect is driven by a large positive entropy change (ΔS). Replacing monodentate ligands with bidentate or multidentate ligands produces a net increase in the number of particles, increasing entropy. The resulting negative ΔG makes chelation spontaneous.

[Co(H₂O)₆]²⁺ (aq) + EDTA^4- (aq) → [CoEDTA]^2- (aq) + 6H₂O (l)

The number of particles goes from 2 to 7, giving a positive entropy change.

True.

The ligand exchange is reversible. Water molecules displace the larger chloride ligands when excess water is added, restoring the blue hexaaqua complex.

False.

Only cisplatin (the cis isomer) can cross-link DNA in cancer cells because of its ligand arrangement. The trans isomer (transplatin) cannot bind to DNA in the same way and has no therapeutic use.

The complex must have no plane of symmetry — the structure and its mirror image must be non-superimposable.

This most commonly occurs in octahedral complexes with bidentate ligands (e.g., [Co(en)₃]³⁺), but can also arise with an appropriate mix of monodentate ligands.

The two optical isomers rotate the plane of polarised light in opposite directions.

Both geometries are rigid — the positions of ligands around the central metal ion are fixed and cannot interchange without breaking bonds.

This means when two or more different ligands are present, the arrangement is locked: the same ligands can sit either adjacent (cis, 90°) or opposite (trans, 180°), giving two distinct, non-interconvertible isomers.

2 coordinate bonds: shape = linear (example: [Ag(NH₃)₂]⁺)

4 coordinate bonds (large ligands): shape = tetrahedral (example: [CoCl₄]^2-)

6 coordinate bonds: shape = octahedral (example: [Fe(H₂O)₆]²⁺)

True.

The diamminesilver(I) ion [Ag(NH₃)₂]⁺ is a linear complex with a coordination number of 2 and bond angle 180°. It is the complex present in Tollens' reagent used to test for aldehydes.

The coordination number of a complex ion is the total number of coordinate bonds formed between the central metal ion and its ligands. Common coordination numbers are 2 (linear), 4 (tetrahedral or square planar), and 6 (octahedral).

Cisplatin's cis arrangement places the two Cl ligands adjacent (90° apart), allowing it to cross-link guanine bases on the same DNA strand.

The trans isomer places the Cl ligands opposite (180° apart), which prevents this cross-linking — so it cannot bind to DNA in the same way and has no therapeutic use.

Ligands split the 3d orbitals into two sets of different energy. An electron absorbs light of frequency ν = ΔE/h and is promoted to the higher set. The colour observed is the complementary colour of the wavelength absorbed.

In an isolated transition metal ion, the five 3d orbitals are degenerate (equal in energy).

When ligands bond to the metal, the orbitals split into two sets.

The energy difference is called ΔE.

An electron is promoted when it absorbs energy equal to ΔE (= hν).

True.

Different ligands cause different degrees of d-orbital splitting (ΔE), so the wavelength of light absorbed changes. For example, [Cu(H₂O)₆]²⁺ is light blue but [Cu(NH₃)₄(H₂O)₂]²⁺ is deep blue, both with Cu in the +2 state.

A colorimeter passes light of a selected wavelength (the complementary colour to the solution, chosen with a filter) through the sample. Absorbance is measured and compared to a calibration curve of absorbance vs. known concentration.

A blue solution absorbs light from the red region of the spectrum. The red filter selects the wavelength most strongly absorbed, giving maximum absorbance and the most accurate concentration measurement.

Stronger-field ligand: ΔE increases, wavelength absorbed decreases (shorter), colour changes

Higher oxidation state: ΔE increases, colour changes to absorb higher energy light

True.

At low concentrations, the Beer-Lambert law holds and a plot of absorbance vs. concentration is linear. This straight-line graph is used to determine the concentration of unknown solutions.

A d-d transition occurs when an electron in a transition metal complex absorbs a photon of visible light and is promoted from a lower-energy 3d orbital to a higher-energy 3d orbital. The energy absorbed equals ΔE (the splitting energy), and the complementary colour of the absorbed wavelength is observed.

The successive ionisation energies of transition metals are relatively close together for the first few electrons. This means removing additional electrons beyond the 4s requires little extra energy, allowing multiple stable oxidation states.

VO₂⁺ (+5): yellow

VO²⁺ (+4): blue

V³⁺ (+3): green

V²⁺ (+2): violet/purple

False.

When transition metals form ions, they lose 4s electrons first, then 3d electrons. This is because in the presence of other electrons, the 4s orbital is at higher energy than the 3d, giving a common +2 oxidation state.

In acidic conditions MnO₄^- is reduced to Mn²⁺ (+2 state, colourless). In neutral conditions, the product is MnO₂ (+4 state, brown solid) because fewer H⁺ ions are available. The E^ꝋ value also changes with pH.

Tollens' reagent contains [Ag(NH₃)₂]⁺ (diamminesilver(I)). An aldehyde reduces Ag⁺ to Ag (s), producing a silver mirror. The equation is:

[Ag(NH₃)₂]⁺ + e^- → Ag (s) + 2NH₃ (aq)

True.

Different ligands cause different degrees of d-orbital splitting. Ammonia binds more strongly to Ni²⁺ than water, stabilising the Ni²⁺ ion and making it harder to reduce. This is reflected in a more negative standard electrode potential for the ammonia complex compared to the aqua complex.

Autocatalysis is when a product of a reaction catalyses the same reaction. In the MnO₄^- / C₂O₄^2- titration, Mn²⁺ ions formed as a product catalyse the further reduction of MnO₄^-, causing the reaction to initially speed up.

Autocatalysis is when a product of a reaction acts as a catalyst for that same reaction, causing the reaction rate to increase as the product accumulates. A classic example is the MnO₄^- / C₂O₄^2- titration, where Mn²⁺ ions produced in the reaction catalyse the further reduction of manganate(VII).

A redox titration involves titrating an oxidising agent against a reducing agent. Many transition metal ions used in redox titrations (e.g. MnO₄^-) are self-indicating: they are intensely coloured in one oxidation state and colourless or a different colour in another.

MnO₄^- (aq) + 8 H⁺ (aq) + 5 e^- → Mn²⁺ (aq) + 4 H₂O (l)

Colour change: purple to colourless (very pale pink)

True.

KMnO₄ is placed in the burette. The solution remains colourless as it reacts with the reducing agent. At the end point MnO₄^- is in slight excess, giving a permanent pink/purple colour.

moles KMnO₄ = 0.00265

moles Fe²⁺ = 0.01325 (ratio MnO₄^- : Fe²⁺ = 1:5)

mass Fe = 0.740 g

% Fe = 32.9%

The reaction between MnO₄^- and C₂O₄^2- is slow at room temperature. Heating to ~60 °C provides enough activation energy for the reaction to proceed at a reasonable rate (the Mn²⁺ produced then autocatalyses further reaction).

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

True.

The oxidation half-equation is Fe²⁺ → Fe³⁺ + e^-. The reduction half-equation is MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O. Five Fe²⁺ ions are oxidised per MnO₄^- reduced.

A self-indicating reagent is one that shows a visible colour change at the end point of a titration without the need for a separate indicator. Transition metal ions such as manganate(VII) (MnO₄^-) are self-indicating because they have a strong colour in one oxidation state (purple) that disappears when reduced.

A cascade reaction is a series of consecutive reactions in which the product of one step immediately becomes the reactant for the next, allowing complex molecules to be built up efficiently.

True.

Transition metals have variable oxidation states, which allows them to gain and lose electrons readily during a reaction, providing alternative lower-energy reaction pathways.

In the Contact process, the catalyst V₂O₅ converts SO₂ to SO₃, being reduced from V(V) to V(IV) in the process, then regenerated by reaction with O₂ (g).

A chiral centre is a carbon atom bonded to four different groups. A molecule containing a chiral centre exists as two non-superimposable mirror-image forms called enantiomers.

True.

Repulsion between the two negative ions reduces the frequency of successful collisions. Fe²⁺ acts as an intermediate, reacting with each ion in turn and avoiding this repulsion.

Autocatalysis is when a product of the reaction acts as a catalyst. In the reaction between MnO₄^- and C₂O₄^2-, the autocatalyst is Mn²⁺ (aq).

The gradient becomes steeper during the reaction — the rate increases over time as the autocatalyst product accumulates — rather than slowing as reactants are consumed.

To maximise surface area while minimising the amount of (often expensive) catalyst used. More surface area means more active sites and a faster rate.