Enthalpy of Dissolution of Copper Sulphate

Molar heat of a solution or enthalpy of solution is defined as the amount of heat taken in or thrown out while per mole of a solution is being dissolved in any solvent, mostly water. In popular terms and academics, this molar heat is denoted by ΔH and measured in kJ/mol. 

The reaction ensuing while being dissolved may be exothermic or endothermic. If heat is generated while the solute dissolves, then reaction is endothermic. On the other hand, if heat is absorbed while the solute dissolves, the reaction is called endothermic. The exchange of heat is determined by the amount of energy required to break down intermolecular bonds and also, heat released when new solute-solvent bonds are being made.

This process mentioned above is also called as heat of dissolution.

Aim of Experiment

To calculate value of enthalpy of dissolution of copper sulphate or potassium nitrate with help of water as a solvent and potassium nitrate (KNO3) or copper sulphate(CuSO4) as solute. 

Theory behind Experiment

For all experiments such as this, aqueous solutions are used as a solvent. And among all aqueous solutions, water is mostly used due to the ease with which compounds are dissolved in water. Also, water shows much accurate changes in temperature when solutes are being dissolved in it. 

Going by law of conservation of energy proposed in thermodynamics, sum of all enthalpy exchanges must amount to zero. Therefore, the below equation is followed while any reaction, which involves heat, takes place.

ΔH1 + ΔH2 + … + ΔHn = 0

While this heat exchange is taking place, the solution is also being formed simultaneously. Usually, a solution is declared as final when mixing any more solute inside the solvent does not result in any more heat exchange. 

Amount of exchange of heat is measured with the help of a calorimeter.

Equipment Needed for Enthalpy of Dissolution of Copper Sulphate or Potassium Nitrate

1. Glass rod

2. Thermometer

3. Suitable beakers

4. Weight box

5. Copper sulphate in water

6. Measuring cylinder

7. Stirrer

8. Potassium sulphate water

9. Potassium nitrate

10. Water as a solvent

11. Block of wood

12. Cotton wool

13. Block of cardboard

14. Filler

How to Setup the Apparatus?

A crucial aspect while conducting this experiment, follow this image below to understand how to setup the equipments.

(image will be uploaded soon)

Procedure of Whole Experiment

This experiment, when carried on inside laboratories, is divided into three parts. First, we measure calorimeter constant to be used further. Then we first dissolve CuSO4 in water and find its heat of dissolution using the calorimeter constant value. Separately, we again dissolve potassium nitrate in water and find its heat of dissolution value. 

Calculation of Calorimeter Constant

1. Take a polythene bottle and fix the thermometer and stirrer inside it as shown in the figure below. 

2. Pour 100 ml of water (distilled) in the bottle. 

3. Take down the temperature at which the water stands. Let this temperature be t1°C. 

4. Take some water in a beaker. Place this beaker on a heater and heat it not more than 30°C above room temperature. 

5. Keep aside 100 ml of this heated water and keep it aside in another beaker. 

6. Take down the temperature at which this water stands. Let this temperature be t2°C. 

7. Instantly, add this warm water into the polythene bottle. Do not waste any time. Otherwise, the temperature reading will be invalid. 

8. Stir the mixed contents vigorously. 

9. After mixing, take down the temperature at which the mixture stands and name it t3°C.

Pop Quiz 1

1. What material should be used to fill in the space between the two beakers?

1. Cotton wool (Answer)

2. Iron

3. Water

4. Should be kept empty

Calculation of Enthalpy of Dissolution of Copper Sulphate

1. Take a small beaker with a fixed calorimeter and put 100 ml of water inside it. 

2. Place this smaller beaker inside a larger one of volume, say 500 ml.  

3. Place the whole system on a block of wood. 

4. Put the smaller beaker inside the larger beaker and fill the space with cotton wool. 

5. Also, cover the entire system with a piece of card board. 

6. Take down the temperature at which the water stands, say T1°C.

7. Take a fixed amount of copper sulphate powder and put it inside the water. 

8. Let the copper sulphate dissolve and remove the excess solution. 

9. Make sure your stirrer and thermometer are in place. 

10. After the whole copper sulphate is dissolved, take down the temperature of the solution, say T2°C. 

Calculation of Potassium Nitrate Enthalpy of Dissolution

1. Take a small beaker with a fixed calorimeter and put 100 ml of water inside it. 

2. Place this smaller beaker inside a larger one of volume, say 500 ml.  

3. Place the whole system on a block of wood. 

4. Put the smaller beaker inside the larger beaker and stuff the empty space with cotton wool. 

5. Also, cover the entire system with a piece of cardboard.

6. Take down the temperature at which the water stands, say T1°C.

7. Take a fixed amount of potassium nitrate powder and put it inside the water. 

8. Let the potassium nitrate dissolve and remove the excess solution. 

9. Make sure the thermometer and stirrer are in place. 

10. After the whole potassium nitrate is dissolved, take down the temperature of the solution, say T2°C. 

Inference and Observation

Calculations

If we assume water density to be constant and the specific heat of the solution equal to that of water, then the amount of heat absorbed or released can be given by the equation

Q = (W + 200) x (t1 – t2) cals

The above equation, when transformed to give result in joules, converts to

Q = (W + 200) x (t1 – t2) x 4.2 J

So, for w/M moles of solute in solvent (here, water), 

Q = (W + 200) x (t1 – t2) x 4.2 J

Hence, for 1 mole of solute dissolved in solvent (here, water), 

Q = (W + 200) x (t1 – t2) x 4.2 x M/w joules

So, the final formula for enthalpy of dissolution, 

ΔH = (W + 200) x (t1 – t2) x 4.2 x M/w joules

The symbols in the above formula can be expressed as follows. 

M = Formula mass of solute,

w = Mass of solute,

W = Equivalent weight of water calorimeter.

Did You Know?

ΔH is positive if the reaction is exothermic and heat is released during solution formation, and negative if the reaction is endothermic and heat is absorbed during solution formation. 

Activity

Perform the above experiment in your school laboratory and mention what is the enthalpy of dissolution of both these compounds. 

Precautions to be Taken

1. In the first step of the experiment, transfer the hot water immediately into the cold water such that the temperature reading does not change. 

2. Mix the solute inside the solvent well enough. Do not stir too fast, or else the heat of the solution will increase due to friction. 

3. Copper sulphate, being hygroscopic, should be measured well to take the initial reading of the powder. 

4. For potassium nitrate, a small amount, lesser than 3 gm, has to be dissolved in 100 ml of water. 

5. The space between the small beaker and the large beaker should be stuffed with cotton wool. Cotton wool is an insulator and does not let heat pass through it. 

6. Use a well-calibrated thermometer to measure the solution temperature. The thermometer should be accurate till 0.1°C such that minor changes in temperature can be noticed. 

This was all about the enthalpy of copper sulphate or potassium nitrate, when dissolved in water. If you are curious to read more about enthalpy of solution and other physio-chemical processes, check out our reference notes, sample papers and free study material, available on our Vedantu app.