Enthalpy – Hess Cycles

Aim

The purpose of this experiment is to determine the enthalpy change for the reaction:

MgSO_{4 (s)} + 7H₂O_{(l) →} MgSO₄•7H₂O_(s)

Introduction

It is impossible to measure the enthalpy change for this reaction directly because the process cannot be controlled. However, you can calculate this enthalpy change by measuring the enthalpy change of solution for the two solids:

MgSO_{4 (s)} + H₂O _(l) → MgSO_{4 (aq)}

MgSO₄•7H₂O_(s) + H₂O_(l) → MgSO_{4 (aq)}

Requirements

• safety glasses
• 2 weighing boats
• spatula
• magnesium sulphate (anhydrous), MgSO_{4 (s)}
• balance, 2dp
• 2 polystyrene cups and lids
• distilled water
• teat pipette
• thermometer (0° to 50°C) or Data logger temperature probe.
• magnesium sulphate heptahydrate, MgSO₄•7H₂O_(s)

Procedure

Heat of solution of MgSO_{4 (s)}

1. Weigh 3.01 g of MgSO_{4 (s)} to the nearest 0.01 g into a clean, dry weighing boat. Record the exact mass is a copy of the results table.
2. Using a measuring cylinder add 50 cm³ of H₂O into a polystyrene cup.
3. Put the thermometer through the hole in the lid and measure the temperature of the water. Record the initial temperature in your table,
4. Carefully transfer the MgSO₄ into the water, stir gently with the thermometer, and record the maximum temperature.

Heat of solution of MgSO₄•7H₂O_(s)

1. Weigh 6.16 g of MgSO₄•7H₂O_(s) to the nearest 0.01 g into a clean, dry weighing boat.
2. Using a measuring cylinder add 50cm³ of H₂O into a polystyrene cup.
3. Measure and record the temperature change associated with dissolving the MgSO₄•7H₂O_(s)

Calculations

1. From the data in your results table, calculate the enthalpy change of solution for one mole of MgSO_4 (s).
2. Calculate the enthalpy change of solution for one mole of MgSO₄•7H₂O_(s)
3. Use a Hess cycle to calculate the enthalpy change for the reaction:

MgSO_{4 (s)} + 7H₂O_{(I) →} MgSO₄•7H₂O_(s)

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Energy transferred: Q=mcΔT

• m=mass of water (g)
• c= Specific heat capacity of water (4 .18 J g^-1 K^-1)
• ΔT= Change in temperature (K)

Enthalpy change ΔH=Q/n

• n=number of moles

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Questions.

1. Construct a Hess cycle for this experiment. (Answer)

2. Why is it not necessary to plot a temperature/time graph and extrapolate to find the change in temperature?
3. Compare your result with the accepted value of – 104 kJ mol^-1. Suggest three reasons for any difference.