To Determine the Enthalpy of Dissolution of Copper Sulphate
Blue crystals stirred into water in a polystyrene cup, and the thermometer goes down, not up. Dissolving is not always a warming business.
01Aim
To determine the enthalpy of dissolution of copper sulphate pentahydrate, CuSO4·5H2O, in water using a simple polystyrene cup calorimeter.
02Requirements
| No. | Material | Specification / purpose |
|---|---|---|
| 1 | Polystyrene cup with a lid | The calorimeter; it loses very little heat |
| 2 | Beaker (250 ml) | To hold the cup steady |
| 3 | Thermometer | Reading to 0.1 °C |
| 4 | Stirrer | A glass or plastic rod |
| 5 | Measuring cylinders | 50 ml and 25 ml |
| 6 | Chemical balance | Reading to 0.01 g |
| 7 | Copper sulphate pentahydrate | About 5 g, finely powdered |
| 8 | Distilled water | 100 ml |
03Principle
The enthalpy of dissolution is the heat change when one mole of a substance dissolves in so much solvent that further dilution produces no further heat change.
Two opposing energies decide the sign. Breaking the crystal lattice absorbs energy, the lattice enthalpy; hydrating the separated ions releases energy, the hydration enthalpy. The sum is what the thermometer shows:
For the anhydrous salt the hydration enthalpy wins and the process is exothermic. For the pentahydrate the ions are already hydrated, so there is little hydration energy left to release, the lattice term wins, and dissolving is endothermic.
The heat taken from the solution is found from its mass, its specific heat and the fall in temperature:
Dividing by the number of moles gives the molar value:
04The calorimeter
A polystyrene cup inside a beaker, with a thermometer and a stirrer through the lid. The whole temperature change is read off in one or two minutes.
05Procedure
- Weigh the empty polystyrene cup and record its mass.
- Measure 100 ml of distilled water into the cup and stand it in the beaker.
- Fit the lid, put in the thermometer and the stirrer, and leave it for two or three minutes.
- Stir gently and record the steady initial temperature to 0.1 °C.
- Weigh out about 5 g of finely powdered CuSO4·5H2O accurately.
- Add it to the water all at once, replace the lid and stir continuously.
- Watch the thermometer and record the lowest steady temperature reached.
- Repeat the whole determination and take the mean.
06Observations
| Quantity | Symbol | Reading |
|---|---|---|
| Mass of CuSO4·5H2O taken | w | 5.00 g |
| Volume of water | V | 100 ml |
| Mass of the solution | m | 105.0 g |
| Initial temperature | T1 | 28.0 °C |
| Final temperature | T2 | 26.4 °C |
| Fall in temperature | ΔT | 1.6 °C |
| Molar mass of CuSO4·5H2O | M | 249.7 g mol−1 |
- The crystals dissolved to give a clear pale blue solution.
- The thermometer fell steadily for about a minute and then held.
- The outside of the cup felt slightly cold to the touch.
- The anhydrous salt, tried separately, gave a rise in temperature instead.
07Calculation
08Result
09Precautions
- Use a polystyrene cup, not glass. Glass absorbs a good deal of heat itself and the answer comes out low.
- Keep the cup covered and stir gently and continuously.
- Read the thermometer to 0.1 °C, with the bulb fully immersed and not touching the wall.
- Note the highest or lowest steady temperature reached, not the first reading after mixing.
- Work quickly; the longer the mixture stands the more heat is exchanged with the room.
- Both solutions must be at the same starting temperature.
- Powder the crystals finely so they dissolve quickly, and add them all at once.
10Viva voce
Q1Define the enthalpy of solution.
ANSThe heat change when one mole of a substance dissolves in so much solvent that further dilution causes no further heat change.
Q2Why is dissolving the pentahydrate endothermic while the anhydrous salt is exothermic?
ANSIn the pentahydrate the ions are already hydrated, so very little hydration energy is released and the lattice energy absorbed dominates. The anhydrous salt still has its full hydration energy to give out.
Q3Why is a polystyrene cup used instead of a glass beaker?
ANSPolystyrene is a poor conductor and has a very small heat capacity, so almost no heat is lost to the calorimeter itself.
Q4What is the specific heat capacity of water?
ANS4.18 J per gram per kelvin.
Q5What is lattice enthalpy?
ANSThe energy released when one mole of an ionic solid is formed from its gaseous ions; the same magnitude of energy is needed to break the lattice apart.
Q6Why must the solution be stirred?
ANSTo keep the temperature uniform, so that the thermometer reads the temperature of the whole solution and not of one layer.