Suggest one reason why the enthalpy change for this reaction cannot be determined directly by calorimetry - AQA - A-Level Chemistry - Question 1 - 2017 - Paper 3
Question 1
Suggest one reason why the enthalpy change for this reaction cannot be determined directly by calorimetry.
Some enthalpies of solution are shown in Table 1.
Table ... show full transcript
Worked Solution & Example Answer:Suggest one reason why the enthalpy change for this reaction cannot be determined directly by calorimetry - AQA - A-Level Chemistry - Question 1 - 2017 - Paper 3
Step 1
Suggest one reason why the enthalpy change for this reaction cannot be determined directly by calorimetry.
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Answer
It is not possible to prevent some dissolving of anhydrous magnesium chloride, which could affect the measurement of energy changes observed.
Step 2
Calculate the enthalpy change for the absorption of water by MgCl₂(s) to form MgCl₂·4H₂O(s)
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Answer
To calculate the enthalpy change, use the equation:
Describe how you would carry out an experiment to determine the enthalpy of solution of anhydrous magnesium chloride.
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Answer
Measure a known volume of distilled water (e.g., 100 mL) in a calorimeter.
Weigh 0.8 g of anhydrous magnesium chloride.
Add the weighed magnesium chloride to the calorimeter containing water.
Stir the solution gently and record the temperature change until a stable temperature is reached.
Calculate the enthalpy of solution using the formula:
extq=mcextΔT
where m is the mass of the water, c is the specific heat capacity, and ΔT is the temperature change.
Step 4
Explain how your results could be used to calculate the enthalpy of solution.
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Answer
The enthalpy of solution can be calculated by dividing the heat absorbed or released by the amount of solute dissolved. This calculation provides the enthalpy change associated with dissolving 1 mole of anhydrous magnesium chloride in water.
Step 5
Calculate the gradient of the line on your graph and hence calculate the entropy change, ΔS, for the formation of anhydrous magnesium chloride.
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Answer
Plot the values of ΔG against temperature to create a straight line. The gradient (m) of the line represents
the negative entropy change divided by the temperature:
