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A student investigated the temperature change in the reaction between dilute sulfuric acid and potassium hydroxide solution - AQA - GCSE Chemistry - Question 9 - 2019 - Paper 1
Question 9
A student investigated the temperature change in the reaction between dilute sulfuric acid and potassium hydroxide solution. This is the method used. 1. Measure 25... show full transcript
Worked Solution & Example Answer:A student investigated the temperature change in the reaction between dilute sulfuric acid and potassium hydroxide solution - AQA - GCSE Chemistry - Question 9 - 2019 - Paper 1
Step 1
Suggest why the student used a polystyrene cup rather than a glass beaker for the reaction.
Answer
Polystyrene is a better thermal insulator compared to glass. This quality helps to minimize energy exchange with the surroundings, which means that the heat generated during the exothermic reaction is retained within the reaction mixture. Thus, this choice ensures more accurate temperature measurements and observations.
Step 2
Complete Figure 11: plot the data from Table 6.
Answer
To accurately plot the data from Table 6 on Figure 11, each volume of dilute sulfuric acid added (in cm³) should be marked along the x-axis and the corresponding temperatures measured (in °C) should be plotted along the y-axis. Connect these points to visualize the trend and then draw a line of best fit through the plotted points.
Step 3
Determine the volume of dilute sulfuric acid needed to react completely with 25.0 cm³ of the potassium hydroxide solution.
Answer
From Figure 11, extend the lines of best fit for both the curves representing the temperature change and the volume of acid. The intersection point signifies the volume of dilute sulfuric acid needed to completely react with 25.0 cm³ of potassium hydroxide. This value is found to be approximately 11 cm³.
Step 4
Determine the overall temperature change when the reaction is complete.
Answer
To find the overall temperature change, refer to Figure 11. Measure the temperature at 0 cm³ of sulfuric acid (18.9 °C) and at 11 cm³ when the reaction is complete (27.1 °C). Thus, the temperature change is calculated as follows:
Overall temperature change = 27.1 °C - 18.9 °C = 8.2 °C.
Step 5
Calculate the concentration of the potassium hydroxide solution in mol/dm³ and in g/dm³.
Answer
Using the reaction stoichiometry, we find:
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Calculate moles of H₂SO₄:
- Volume = 15.5 cm³ = 0.0155 dm³
- Concentration = 0.500 mol/dm³
- Moles H₂SO₄ = Volume × Concentration = 0.0155 × 0.5 = 0.00775 moles.
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The reaction shows that 2 moles of KOH react with 1 mole of H₂SO₄, therefore:
- Moles KOH = 2 × Moles H₂SO₄ = 2 × 0.00775 = 0.0155 moles.
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Concentration of KOH:
- Concentration (mol/dm³) = Moles KOH / Volume of KOH solution = 0.0155 / 0.025 = 0.62 mol/dm³.
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Now, to find the concentration in g/dm³:
- Molar mass of KOH = 56 g/mol,
- Concentration (g/dm³) = Moles KOH × Molar mass = 0.62 × 56 = 34.72 g/dm³.