Some reactions of the [Al(H2O)6]3+(aq) ion are shown. Colourless solution containing a complex ion A Na2.EDTA(aq) → [Al(H2O)6]3+(aq) → Na2CO3(aq) → B(s) White precipitate that reacts to form a colourless solution containing complex ion C. NaOH(aq) 1. Give the formula of the white precipitate B. State one other observation when Na2CO3(aq) is added to a solution containing [Al(H2O)6]3+(aq) ions. Give an equation for this reaction. 2. Give the formula of the complex ion C. State one condition needed for the formation of C from [Al(H2O)6]3+(aq) and NaOH(aq). Give an equation for this reaction. 3. Deduce the formula of the complex ion A. 4. Explain, with the use of an equation, why a solution containing [Al(H2O)6]3+ has a pH < 7.

A-Level AQA Chemistry Chemical Equilibria, Le Chateliers Principle & Kc: Some reactions of the [Al(H2O)6]

Some reactions of the [Al(H2O)6]3+(aq) ion are shown - AQA - A-Level Chemistry - Question 5 - 2020 - Paper 1

Question 5

Some reactions of the [Al(H2O)6]3+(aq) ion are shown. Colourless solution containing a complex ion A Na2.EDTA(aq) → [Al(H2O)6]3+(aq) → Na2CO3(aq) → B(s) White pre... show full transcript

Worked Solution & Example Answer:Some reactions of the [Al(H2O)6]3+(aq) ion are shown - AQA - A-Level Chemistry - Question 5 - 2020 - Paper 1

Step 1

Give the formula of the white precipitate B.

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Answer

The formula of the white precipitate B is Al(H2O)3(OH)3.

One observation when Na2CO3(aq) is added is the formation of bubbles or effervescence due to the release of CO2 gas.

The equation for this reaction is:

$Al(H2O)_6^{3+} + 3CO_3^{2-} \rightarrow Al(OH)_3 (s) + 3H_2O + 3CO_2 (g)$

Step 2

Give the formula of the complex ion C.

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Answer

The formula of the complex ion C is [Al(OH)4]-.

One condition needed for the formation of C is the use of excess NaOH.

The equation for this reaction is:

$Al(OH)_3 (s) + OH^- \rightarrow [Al(OH)_4]^-$

Step 3

Deduce the formula of the complex ion A.

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Answer

The formula of the complex ion A is [Al(H2O)6]3+.

Step 4

Explain, with the use of an equation, why a solution containing [Al(H2O)6]3+ has a pH < 7.

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Answer

The solution containing [Al(H2O)6]3+ has a pH < 7 because it is a weak acid that partially releases H+ ions into the solution. This occurs due to the hydrolysis of the complex ion:

$[Al(H2O)_6]^{3+} + H_2O \rightleftharpoons [Al(H2O)_5(OH)]^{2+} + H^+$

The presence of H+ ions lowers the pH, contributing to its acidic nature.

Some reactions of the [Al(H2O)6]3+(aq) ion are shown - AQA - A-Level Chemistry - Question 5 - 2020 - Paper 1

Worked Solution & Example Answer:Some reactions of the [Al(H2O)6]3+(aq) ion are shown - AQA - A-Level Chemistry - Question 5 - 2020 - Paper 1

Give the formula of the white precipitate B.

Give the formula of the complex ion C.

Deduce the formula of the complex ion A.

Explain, with the use of an equation, why a solution containing [Al(H2O)6]3+ has a pH < 7.

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