Transition metals: coloured ions, variable oxidation states, catalysis ：練習問題

In heterogeneous catalysis involving a transition metal, which step is typically necessary for the reaction to occur efficiently on the catalyst surface?

Vanadium is a transition metal that can form ions with multiple oxidation states. If a solution containing $$VO_2^+$$ ions (vanadium(V)) which are yellow, is reduced to $$V^{2+}$$ ions (vanadium(II)), which are violet, what would be the observable colour change?

Titanium (Ti) exhibits variable oxidation states. Solid zinc metal is used to reduce titanium ions in aqueous solution according to the following scheme:
\(\text{Ti}^{4+}(\text{aq}) \xrightarrow{\text{Zn}(\text{s})} \text{Ti}^{3+}(\text{aq}) \xrightarrow{\text{Excess Zn}(\text{s})} \text{Ti}^{2+}(\text{aq})\)
(\(\text{Ti}^{4+}\) is colourless, \(\text{Ti}^{3+}\) is purple, and \(\text{Ti}^{2+}\) is violet).
Which conclusion regarding the relative stabilities or reactivity of the titanium ions is best supported by these observations?

Which of the following statements correctly describes a common characteristic of transition metals?

Which of the following statements about the role of vanadium(V) oxide (\(V_2O_5\)) as a catalyst in the Contact process is correct?

Transition metals exhibit a wide range of interesting chemical properties, including the formation of coloured ions, variable oxidation states, and catalytic activity.

(a) Explain why most aqueous ions of transition metals are coloured, while most ions of main group elements are colourless. Your explanation should refer to the electronic structure of transition metal ions and the interaction with visible light.

(b) Acidified potassium dichromate(VI) ($$\text{Cr}_2\text{O}_7^{2-}\text{(aq)}$$) is a common oxidising agent. When an aqueous solution of acidified potassium dichromate(VI) is reacted with ethanol ($$\text{CH}_3\text{CH}_2\text{OH}\text{(aq)}$$), a distinct colour change is observed and ethanoic acid ($$\text{CH}_3\text{COOH}\text{(aq)}$$) is formed.

(c) Transition metal compounds are widely used as catalysts in industrial processes, such as vanadium(V) oxide in the Contact process. Explain how the variable oxidation states of a transition metal enable it to act as a catalyst.