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
Na2EDTA(aq) → [Al(H2O)6]3+(aq) → Na2CO3(aq)
B(s)
White precip... 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 Al2(CO3)3·xH2O.
Observation: When Na2CO3(aq) is added to the solution, bubbles or effervescence may be observed due to the release of carbon dioxide gas.
Reaction Equation:
ightarrow ext{Al}_2( ext{CO}_3)_3 (s) + 6 ext{H}_2 ext{O}$$
Step 2
Give the formula of the complex ion C.
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Answer
The formula of the complex ion C is [Al(OH)4]−.
Condition: One condition needed for the formation of this complex ion is the presence of excess NaOH.
Reaction Equation:
ightarrow [Al(OH)_4]^− + 6 ext{H}_2 ext{O}$$
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 [Al(H2O)6]3+ complex ion has a tendency to donate protons (H+) to the solution, leading to an acidic environment. This is due to the high charge density of aluminum, which weakens the O-H bonds in the water ligands.
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