Enthalpy of Crystallisation
Thermodynamic terms can be difficult for sixth formers to understand, especially when they are not directly measurable. This experiment does not provide an accurate method for determining the enthalpy of crystallisation of sugar, but does offer an (edible) illustration of it.
Boiled sweets are essentially amorphous sugar and very soluble in water with an accompanying increase in temperature. Granulated sugar, being crystalline, requires that enthalpy of crystallisation is added to the system to enable dissolution. Consequently, granulated sugar dissolves endothermically in water.
Method
· Using a measuring cylinder, measure 100cm3 of water and leave it to reach an equilibrium temperature.
· Crush about three Fox’s Glacier mints in a mortar and pestle. Take care as large pieces take longer to dissolve and too much crushing may compact the mint into the base of the mortar.
· Weigh an empty polystyrene cup.
· Add the crushed mint to the cup.
· Reweigh the polystyrene cup and obtain the mass of the mint used.
· Take the temperature of the water using a 0 to 50°C thermometer (easily readable to 0.2°C).
· Add the water to the mint, in the polystyrene cup, and stir vigorously to dissolve the mint. Take care not to let any solution splash out of the cup.
· Record the highest temperature reached.
· Repeat the experiment using granulated sugar.
· For the purpose of approximation, assume that both materials are chemically identical and the mint is both anhydrous and completely amorphous.
Questions
1. Explain the observed difference in the change of temperature.
2. If left exposed to the atmosphere, the mint becomes sticky on the surface, but granulated sugar does not. Explain this observation.
3. Of the two forms of sucrose, which has the highest energy level?
4. Make an estimation of the difference in energy of the two forms of sugar.
5. You may make very precise measurements in this experiment, but it cannot be accurate. Explain the meaning of the terms accuracy and precision in this context.
6. Briefly discuss the accuracy of this experiment.
Assume specific heat capacity, s, of the solution is 4.3 J g-1 °C-1.
Thermal energy change = ms(T2-T1),
Where m = mass of solution and (T2-T1) = Temperature change of the system.