This question is about ethanoic acid (HOOCCH3) and the ethanoate ion (−OOCCH3) - AQA - A-Level Chemistry - Question 1 - 2021 - Paper 3

Polyesters are biodegradable because they can be broken down by microorganisms due to the presence of polar functional groups, such as esters, which are more susceptible to hydrolysis. In contrast, polyalkenes are made up of non-polar hydrocarbon chains, which resist microbial attack and are generally more stable in the environment, making them non-biodegradable.

First, calculate the moles of sodium ethanoate:

1. Number of moles of Na2C2O4:

a. Moles = mass / molar mass

a. Moles = 162 g / 134 g/mol = 1.209 mol

2. Concentration of Na2C2O4 solution:

Concentration = moles / volume

Volume = 0.250 dm³

Concentration = 1.209 mol / 0.250 dm³ = 4.836 mol/dm³

3. From the balanced equation, 2 moles of MnO4− react with 5 moles of C2O4^2−. Therefore:

Moles of MnO4− used = (2/5) * moles of C2O4^2−

4.836 mol/dm³ * 0.025 dm³ = 0.1209 mol

Thus, moles of MnO4− = (2/5) * 0.1209 mol = 0.04836 mol

4. Volume of MnO4− used in the titration:

Volume = 23.85 cm³ = 0.02385 dm³

5. Concentration of MnO4− solution:

Concentration = moles / volume = 0.04836 mol / 0.02385 dm³ = 2.023 mol/dm³.

When drawing the meniscus in Figure 1, ensure that the bottom of the curve touches the graduation mark and does not cross it.

1. Ensure to wear gloves and goggles to protect skin and eyes from harmful chemicals.
2. Always use a funnel when pouring the solution to prevent spills and minimize exposure.

1. Always wear gloves to avoid skin contact with the corrosive solid.
2. Work under a fume hood to avoid inhalation of dust.

The colour change observed at the end point of the titration of potassium manganate(VII) is from deep purple to pale pink.

1. Ensure the burette is filled with the solution without any air bubbles.
2. Allow the tap to fill the space below the tap.

The replacement of water ligands by ethanoate ions is favourable due to the following reasons:

1. Enthalpy change: The formation of more stable bonds with ethanoate ions releases energy, contributing to a negative enthalpy change.
2. Entropy change: The displacement of six water molecules by a more complex ligand like ethanoate increases the disorder (entropy) of the system.
3. The overall free-energy change of the reaction can be expressed as:

$\Delta G = \Delta H - T \Delta S$

Where a negative enthalpy change and a positive entropy change will lead to a negative free energy change, indicating a spontaneous reaction.