2025 Chemistry Redox Titration Depth Study

Memorise the answers to the following questions

Why should a dilute concentration be used in the burette

Using a highly concentrated solution results in each drop of solution added from the burette corresponding to a greater number of moles of said solution, and thus the reaction taking place to a greater extent. This increases the likelihood of the equivalence point being overshot by the experiment, and thus increases the uncertainty and inaccuracy of the titres found.

Why is potassium permanganate placed in the burette as opposed to the conical flask

It is easier to observe a colour change from colourless to pink than a colour change from pink to colourless. Therefore the endpoint can be more accurately determined.

Why do we need to heat the titrand to 60 degrees celsius

By increasing the temperature, the reaction rate will increase. At room temperature permanganate ions (MnO₄⁻) are strong oxidizing agents, but the oxidation of oxalic acid proceeds sluggishly unless heated.

What is the rinsing procedure before a titration

1. Rinse all glassware three times with deionised water

2. Rinse the bulb pipette with the solution that goes in the conical flask three times

3. Rinse the burette with the solution that goes in it three times

Note: Rinsing with the solution gets rid of the residue water from the previous rinsing to prevent dilution when placing the solution in

An iron (II) sulfate solution was prepared by a student using the procedure. In their haste to complete the experiment, they accidentally forgot to rinse the sample vial, beaker and stirring rod with small amounts of demineralised water and wash into the volumetric flask.. Explain the effect of this error on the calculated concentration of the iron (II) solution.

When the solution is transferred into the volumetric flask, there would still be residue amounts of iron sulfate that is left over in the sample vial, beaker and stirring rod respectively. Without washing these contents into the volumetric flask, then the amount of Fe2+ within the volumetric flask would be lower than expected, and the actual [Fe2+] would thus be lower than what was expected.

The powdered sample of iron (II) sulfate is not pure and actually contains a variety of different iron (III) compounds as well. Explain the effect of the impurity on the calculated concentration of the iron (II) solution.

When creating the standard solution, the measured mass of the powdered iron(II) sulfate consists of both Fe2+ and Fe3+ ions. As a result, when measuring the concentration of the formed solution, it would consist of both Fe2+ and Fe3+ ions. By accounting for the mass of the Fe3+ ions as part of the concentration, this calculated concentration would be higher than the theoretical concentration of Fe2+ ions that are in the solution.

A sample of iron (II) sulfate powder was stored in a sample vial that had a faulty seal. Describe the changes in iron (II) and iron (III) content of this sample.

A faulty seal means that that oxygen gas will be present from the external environment and cause the Fe2+ iron content to oxidise and become Fe3+. This is seen by the redox reaction Fe²⁺(aq) + O₂(g) + 4H⁺(aq) → 4Fe³⁺(aq) + 2H₂O(l)

Define validity, reliability, accuracy

Validity refers to whether or not a scientific method addresses its aim and tests the hypothesis

Reliability refers to the reproducibility and consistency of the experimental results under identical circumstances

Accuracy refers to the closeness of the measured value to the theoretical value

Note: At ruse according to Mr Hutchingson, the accuracy is dependent on the validity and reliability (accuracy lies on the top of the pyramid)

How to improve validity

• Having an appropriate standard of comparison, known as a control, such that the effect of changing the independent variable on dependent variable can be analysed and confirmed

• Introducing appropriate controlled variables to minimise effect of external variables

• Ensuring there is only one independent variable being changed

• Ensuring that any assumptions made are applicable [Not as important as others]

• Reducing systematic and random errors [Not as important as others]

Define a control

A control is an appropriate standard of comparison such that the effect of changing the independent variable on the dependent variable can be analysed and confirmed

How to assess reliability

• Repeat the experiment and compare results

• Observe deviation of data points from line of best fit

How to improve reliability

• Improve experimental design to reduce random errors. This may involve introducing a simpler and more specific method which details how to perform each step

• Repeat experiment, remove outliers, then averaging the trials to decrease random error [USE ONLY IF DESPARATE AS SCHOOL DOES NOT LIKE]

• Take data points over both existing data range and a large range to remove outliers and construct a better line of best fit

How to improve accuracy

Improved solely by eliminating systematic errors (try find the source of systematic error in the given method)

Explain the meaning of these symbols

1. This is the symbol for oxidizing substances. These substances, such as oxidizing liquids, solids, and gases, can react with other materials, increasing the risk of fire or explosions. 

2. This is the symbol for a low level of toxicity. This may cause respiratory, skin, and eye irritation, skin sensitisers and chemicals harmful if swallowed, inhaled or in contact with skin.

3. This symbol means that it is hazardous to aquatic life and the environment.