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This question is about citric acid (C6H8O7) - AQA - GCSE Chemistry - Question 9 - 2020 - Paper 1
Question 9
This question is about citric acid (C6H8O7). Citric acid is a solid. A student investigated the temperature change during the reaction between citric acid and sodi... show full transcript
Worked Solution & Example Answer:This question is about citric acid (C6H8O7) - AQA - GCSE Chemistry - Question 9 - 2020 - Paper 1
Step 1
Suggest one reason for the anomalous point.
Answer
The anomalous point at 0.60 g of citric acid could be due to insufficient stirring of the solution, which would result in an uneven temperature distribution. This might have caused the thermometer reading to reflect a temperature before the solution had fully reacted and stabilized.
Step 2
Explain the shape of the graph in terms of the energy transfers taking place.
Answer
The graph exhibits a decreasing temperature trend as more citric acid is added, indicating an endothermic reaction where heat is absorbed from the solution. Initially, the temperature decreases rapidly due to the reaction of citric acid with sodium hydrogencarbonate, using energy from the surroundings. As the reaction reaches its endpoint, the addition of further citric acid leads to minimal temperature change as the reagents become limited.
Step 3
Sketch a line on Figure 6 to show the second student's results until 1.00 g of citric acid had been added. The starting temperature of the solution was the same.
Answer
The line should show a less steep slope starting at the initial temperature of the solution. As the metal container conducts heat better than the polystyrene cup, the temperature should decrease less dramatically compared to the original experiment until 1.00 g of citric acid is added.
Step 4
Describe how the student would complete the titration.
Answer
The student would add the citric acid solution from the burette into the conical flask containing the sodium hydroxide solution until a permanent color change is observed in the indicator. It’s important to add the citric acid slowly while swirling the flask and to measure the final burette reading accurately.
Step 5
Give two reasons why a burette is used for the citric acid solution.
Answer
- A burette allows for precise measurement of the volume of citric acid added, which is essential for accurate titration results.
- The burette enables the citric acid to be added drop by drop, providing better control and allowing the student to indicate the endpoint more accurately.
Step 6
Calculate the concentration of the sodium hydroxide solution in mol/dm³.
Answer
To find the concentration of NaOH, first calculate the moles of citric acid:
- Moles of citric acid = 13.3 cm³ × 0.0500 mol/dm³ / 1000 = 0.000665 mol.
- The stoichiometry of the reaction indicates 3 moles of NaOH react with 1 mole of citric acid:
Moles of NaOH = 3 × 0.000665 = 0.001995 mol. 3. Finally, using the volume of NaOH solution (25.0 cm³), the concentration is:
Concentration of NaOH = moles / volume = 0.001995 / (25.0 / 1000) = 0.0798 mol/dm³.