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)63++3CO32−→Al(OH)3(s)+3H2O+3CO2(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−→[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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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++H2O⇌[Al(H2O)5(OH)]2++H+
The presence of H+ ions lowers the pH, contributing to its acidic nature.
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