Chemistry 9701 · AS & A Level · The Periodic Table: chemical periodicity

The Periodic Table: chemical periodicity — practice question

The reducing agent $\text{LiAlH}_4$ may be made by allowing aluminium chloride to react with lithium hydride, $\text{LiH}$.
(a(i))[2]

At $200\,^{\circ}\text{C}$, aluminium chloride is present as $\text{Al}_2\text{Cl}_6(g)$. Draw the structure of $\text{Al}_2\text{Cl}_6(g)$ and show all coordinate (dative covalent) bonds in the molecule in full.

(a(ii))[1]

At $1000\,^{\circ}\text{C}$, aluminium chloride is found as $\text{AlCl}_3(g)$. State the bond angle in $\text{AlCl}_3(g)$.

(a(iii))[1]

Lithium hydride contains the ions $\text{Li}^+$ and $\text{H}^-$. State the electronic configuration of each ion.

(a(iv))[1]

$\text{LiAlH}_4$ decomposes slowly to form $\text{LiAl}(s)$ and $\text{H}_2(g)$. $\text{LiAlH}_4(s) \rightarrow \text{LiAl}(s) + 2\text{H}_2(g)$ $\text{LiAl}(s)$ has metallic bonding. Describe metallic bonding.

(b)[1]

$\text{LiAlH}_4$ is unsuitable in aqueous solution because it reacts with water to form $\text{LiOH}(aq)$, $\text{H}_2(g)$ and a white precipitate that dissolves in excess sodium hydroxide. Identify the white precipitate.

(c(i))[2]

State the reagents and conditions needed for reaction 1.

(c(ii))[2]

Only one of the students makes 2-hydroxybutanoic acid successfully. Identify which of $Q$ or $R$ is 2-hydroxybutanoic acid and explain how reactions 2 and 3 differ.

(d(iii))[4]

Draw the mechanism for the reaction of propanal with the mixture of $\text{NaCN}$ and $\text{HCN}$ to form $S$. Identify the ion that reacts with propanal. Draw the structure of the reaction intermediate. Include all charges, partial charges, lone pairs and curly arrows.

(d(iv))[1]

Fill in the equation for the reaction in step 2, when $S$ is heated under reflux with $\text{HCl}(aq)$. $\text{C}_2\text{H}_5\text{CH(OH)CN} + \ldots \rightarrow \text{C}_2\text{H}_5\text{CH(OH)COOH} + \ldots$

(iii)[4]

Draw the mechanism for the reaction of propanal with the mixture of NaCN and HCN to form $S$. • State which ion attacks propanal. • Show the intermediate formed in the reaction. • Include all charges, partial charges, lone pairs and curly arrows.

(iv)[1]

Finish the equation for the reaction in step 2, when $S$ is heated under reflux with $\text{HCl(aq)}$. $\text{C}_2\text{H}_5\text{CH(OH)CN} + \ldots \rightarrow \text{C}_2\text{H}_5\text{CH(OH)COOH} + \ldots$

(v)[2]

The infrared spectrum of an organic compound is shown. The organic compound is either $S$ or 2-hydroxybutanoic acid. Deduce the identity of the compound. Give two reasons for your answer. In your answer, identify any relevant absorptions above $1500\,\text{cm}^{-1}$ in the spectrum and the bonds that correspond to these absorptions.

Worked solution & mark scheme

This 22-mark question has a full step-by-step worked solution and mark scheme. One marking point: Accurate dot-and-cross or line diagram of $\mathrm{Al_2Cl_6}$

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