Transition Metal Reactions (Edexcel International A Level (IAL) Chemistry): Flashcards

The chromate-dichromate equilibrium is the reversible interconversion between yellow CrO₄^2- and orange Cr₂O₇^2- ions, controlled by adding acid or alkali. Both species have chromium in the +6 oxidation state.

False.

This is an acid-base reaction, not a redox reaction. Both CrO₄^2- and Cr₂O₇^2- have chromium in the +6 oxidation state, so no change in oxidation number occurs.

In the reduction of Cr₂O₇^2- to Cr³⁺ by zinc under acidic conditions, the oxidation state of chromium changes from +6 to +3.

Cr(OH)₃ is oxidised to CrO₄^2- using hydrogen peroxide (H₂O₂) under alkaline conditions, due to the presence of OH^- ions in the equation.

A grey-green precipitate of Cr(H₂O)₃(OH)₃ (s) is formed, as three hydroxide ions deprotonate three water ligands in the complex.

The precipitate dissolves to give a green solution of [Cr(OH)₆]^3- (aq). Further deprotonation by excess OH^- makes the hydroxide act as an acid, showing amphoteric behaviour.

True.

Cr₂O₇^2- has the most positive standard electrode potential (+1.33 V) in the series, making it the strongest oxidising agent.

Adding alkali to a dichromate solution shifts the equilibrium towards chromate ions, causing the colour to change from orange to yellow.

Cr³⁺ and Zn²⁺ form precipitates that dissolve in excess NaOH (aq), showing amphoteric behaviour.

The aqueous ion [Fe(H₂O)₆]²⁺ (aq) is green in colour, and with limited NaOH (aq) it forms a green precipitate.

A brown precipitate of Fe(OH)₃(H₂O)₃ (s) is formed. Fe³⁺ does not dissolve in excess NaOH.

False.

Cu²⁺ (aq) forms a blue precipitate with NaOH but shows no further change with excess NaOH. It is excess NH₃ (aq) that dissolves the precipitate to give a dark blue solution.

1. Cr³⁺: purple solution [Cr(NH₃)₆]³⁺

2. Co²⁺: yellow solution [Co(NH₃)₆]²⁺

3. Ni²⁺: dark blue solution [Ni(NH₃)₆]²⁺

4. Cu²⁺: dark blue solution [Cu(NH₃)₄(H₂O)₂]²⁺

Both act as bases in limited amounts, removing H⁺ ions from the water ligands in a deprotonation reaction. The result is the same metal hydroxide precipitate.

When excess NH₃ (aq) is added to [Cu(H₂O)₆]²⁺ (aq), four ammonia ligands replace four water molecules to form a dark blue solution.

A blue precipitate of Co(OH)₂(H₂O)₄ (s) is formed. It does not dissolve in excess NaOH (aq), but does dissolve in excess NH₃ (aq) to form a yellow solution.

True.

[Zn(H₂O)₆]²⁺ (aq) is colourless, and adding NaOH (aq) forms a white precipitate of Zn(OH)₂(H₂O)₄ (s).

A heterogeneous catalyst is a catalyst that is in a different physical state (phase) from the reactants, typically a solid acting on gaseous or dissolved reactants.

1. Adsorption: reactants attach to active sites on the catalyst surface

2. Reaction: bonds in adsorbed molecules weaken and products form

3. Desorption: products detach from the catalyst surface

False.

Tungsten adsorbs reactants too strongly, preventing products from desorbing. Effective catalysts such as Ni and Pt have an intermediate adsorption strength.

Vanadium(V) oxide (V₂O₅) is the catalyst. It is reduced from +5 to +4 when it oxidises SO₂, then re-oxidised back to +5 by oxygen.

In a catalytic converter, the transition metals used are platinum and rhodium supported on a ceramic base.

2NO (g) + 2CO (g) → N₂ (g) + 2CO₂ (g)

This converts two pollutants into less harmful nitrogen and carbon dioxide.

Impurities can adsorb onto active sites, blocking reactants from attaching, forming strong bonds to the surface that are unlikely to desorb, and preventing bond weakening in the reactants.

False.

Adsorption occurs only at the surface of a substance, whereas absorption involves a substance becoming distributed throughout another, like water in a sponge.

A support medium maximises the surface area of the catalyst while minimising cost, as less precious metal is needed. The catalyst is spread over a hollow matrix such as a honeycomb-like ceramic structure.

A homogeneous catalyst is a catalyst that is in the same physical state (phase) as the reactants, most commonly in aqueous solution. A key feature is the formation of an intermediate species with a specific formula.

Both S₂O₈^2- and I^- are negatively charged, so electrostatic repulsion between them reduces the frequency of successful collisions.

1. Fe²⁺ reduces S₂O₈^2- to SO₄^2-, forming Fe³⁺

2. Fe³⁺ then oxidises I^- to I₂, reforming Fe²⁺

The iron cycles between +2 and +3, acting as both reducing and oxidising agent.

In the first step of Fe²⁺ catalysis, S₂O₈^2- is reduced to SO₄^2- and the iron is oxidised to Fe³⁺.

True.

The formation of an intermediate with a defined formula is the key feature that distinguishes homogeneous catalysis from heterogeneous catalysis.

Adding starch to the reaction mixture, which forms a blue-black colour in the presence of iodine.

They can adopt more than one stable oxidation state, allowing them to accept and donate electrons, acting as both oxidising and reducing agents in a catalytic cycle.

True.

The reactants (S₂O₈^2-, I^-), the catalyst (Fe²⁺/Fe³⁺), and the products are all in aqueous solution, confirming this is homogeneous catalysis.

Autocatalysis is a process in which a product of the reaction acts as a catalyst, causing the reaction rate to increase as more product accumulates over time.

The gradient becomes steeper over time, meaning the rate speeds up rather than slowing down. This is because the product that acts as the catalyst accumulates, accelerating the reaction.

Mn²⁺ (manganese(II) ions) are the autocatalyst. They are produced as a product of the reaction and then speed up further reaction by cycling between the +2 and +3 oxidation states.

In autocatalysis with MnO₄^- and C₂O₄^2-, the Mn²⁺ formed initially reacts with MnO₄^- ions to form Mn³⁺, which then oxidises C₂O₄^2- to regenerate Mn²⁺.

False.

Mn²⁺ is not present at the beginning; it is generated as the reaction proceeds. Once formed, it accelerates the reaction as an autocatalyst.

A colorimeter can be used to track the rate at which the purple manganate(VII) colour is consumed, which accelerates over time as Mn²⁺ accumulates.

Mn²⁺ is oxidised to Mn³⁺ in the first step, then Mn³⁺ is reduced back to Mn²⁺ in the second step, cycling between the +2 and +3 oxidation states.