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This question is about ethanoic acid (HOOCCH3) and the ethanedioate ion (COOCCO−) - AQA - A-Level Chemistry - Question 1 - 2021 - Paper 3

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This question is about ethanoic acid (HOOCCH3) and the ethanedioate ion (COOCCO−). 1. Ethanoic acid reacts with propane-1,3-diol (HOCH2CH(CH3)OH) to form a polyeste... show full transcript

Worked Solution & Example Answer:This question is about ethanoic acid (HOOCCH3) and the ethanedioate ion (COOCCO−) - AQA - A-Level Chemistry - Question 1 - 2021 - Paper 3

Step 1

Draw the repeating unit of this polyester.

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Answer

The repeating unit of the polyester formed by the reaction of ethanoic acid and propane-1,3-diol can be represented as follows:

  O   O
  ||  ||
HOOC-CH2-CH-CH2-C-O
     |   |
     CH3  CH3

This structure includes the required ester linkages formed from the reaction.

Step 2

Explain why polyesters are biodegradable but polyalkenes are not biodegradable.

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Answer

Polyesters are biodegradable because they can be broken down by hydrolysis, especially in the presence of water and biological enzymes, leading to their decomposition into their monomeric units. This process allows them to be assimilated into the environment more effectively.

In contrast, polyalkenes are not biodegradable since they are composed of strong carbon-carbon bonds that resist enzymatic and chemical degradation processes. These polymers are stable and do not readily absorb water or undergo the hydrolysis necessary for environmental breakdown.

Step 3

Calculate the concentration, in mol dm−3, of the potassium manganate(VII) solution.

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Answer

To find the concentration of the potassium manganate(VII) solution, first find the moles of sodium ethanedioate:

  1. Calculate moles of sodium ethanedioate used:

    Moles of Na2C2O4=massMr=162134=1.206 moles\text{Moles of } Na2C2O4 = \frac{\text{mass}}{\text{Mr}} = \frac{162}{134} = 1.206 \text{ moles}

  2. The volume of sodium ethanedioate solution used in titration is 25.0 cm³ (0.025 dm³).

  3. Since the reaction ratio is 5:2 (5 moles of C2O4^{2−} react with 2 moles of MnO4^{-}), find moles of C2O4^{2−}:

    Moles of C2O42−=1.206250×25=0.1206 moles\text{Moles of } C2O4^{2−} = \frac{1.206}{250} \times 25 = 0.1206 \text{ moles}

  4. Therefore, moles of MnO4^{-} is:

    Moles of MnO4−=25×0.1206=0.04824 moles\text{Moles of } MnO4^{-} = \frac{2}{5} \times 0.1206 = 0.04824 \text{ moles}

  5. The mean titre is 23.85 cm³ (0.02385 dm³), so:

    Concentration of MnO4−=0.048240.02385=2.023 mol dm−3\text{Concentration of } MnO4^{-} = \frac{0.04824}{0.02385} = 2.023 \text{ mol dm}^{-3}.

Step 4

On Figure 1, draw the meniscus of the solution when the pipette is ready to transfer 25.0 cm³ of the sodium ethanedioate solution.

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Answer

The meniscus should be drawn at the graduation mark of the pipette, with the bottom of the curve aligning with the mark. It is important to ensure no parallax error occurs when reading the meniscus, by ensuring that the eyeline is at the same level as the bottom of the meniscus.

Step 5

Suggest safety precautions, other than eye protection, that should be taken when filling the burette with potassium manganate(VII) solution.

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Answer

  1. Ensure that the burette is filled below eye level to avoid spillage.
  2. Use gloves while handling potassium manganate(VII) solution to prevent skin contact.

Step 6

Suggest safety precautions, other than eye protection, that should be taken when dissolving the solid sodium ethanedioate in water.

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Answer

  1. Work in a fume cupboard to avoid inhalation of dust.
  2. Handle the solid with a spatula and avoid direct contact with skin.

Step 7

State the colour change seen at the end point of each titration.

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Answer

At the end point of the titration, the solution changes from purple (due to potassium manganate(VII)) to a pale pink, indicating the endpoint has been reached.

Step 8

Give two practical steps needed before recording the initial burette reading.

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Answer

  1. Ensure the burette is free of air bubbles by running a small amount of solution through the tap.
  2. Allow the tap to fill space below the tap but not overfilling.

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