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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
Step 2
Explain the shape of the graph in terms of the energy transfers taking place.
Answer
Initially, the temperature decreases as the citric acid is added since the reaction with sodium hydrogencarbonate is endothermic, absorbing energy from the solution. As more citric acid is added, the temperature stabilizes when all the sodium hydrogencarbonate has reacted. Once the reaction completes, if excess citric acid is present, the temperature may slightly increase as the solution equilibrates.
Step 3
Sketch a line on Figure 6 to show the second student’s results until 1.00 g of citric acid has been added. The starting temperature of the solution was the same.
Answer
The line should be less steep, starting at 16.8 °C and reaching a plateau at a higher temperature than the first student’s graph, indicating that the metal container absorbed more heat from the surroundings than the polystyrene cup.
Step 4
Describe how the student would complete the titration.
Answer
The student would add the citric acid dropwise from the burette to the conical flask while swirling the solution until a permanent color change indicates the endpoint of the titration. The final volume in the burette would then be recorded.
Step 5
Give two reasons why a burette is used for the citric acid solution.
Answer
- The burette allows for precise measurement of the volume of citric acid added due to its scale, enabling accurate titration.
- It allows for controlled addition of the acid drop by drop, which is essential for identifying the endpoint of the titration accurately.
Step 6
Calculate the concentration of the sodium hydroxide solution in mol/dm³.
Answer
From the neutralization reaction, using the formula Moles = Concentration × Volume, the moles of citric acid used is:
ext{Moles of citric acid} = 0.0500 imes rac{13.3}{1000} = 0.000665
ext{From the stoichiometry of the reaction,}\
3 ext{ moles of NaOH react with 1 mole of citric acid, so:}
ext{The concentration of NaOH is then: }\ ext{Concentration} = rac{0.001995}{ rac{25.0}{1000}} = 0.0798 ext{ mol/dm³}.$$
The concentration of the sodium hydroxide solution is approximately 0.080 mol/dm³.