Enthalpy of Dissolution of Copper Sulphate or Potassium Nitrate

Molar heat of solution of a substance is the heat absorbed or released when the molecules of that substance get dissolved in water. It is denoted by ΔH. The overall heat of the solution can be either endothermic or exothermic, depending on the relative amount of energy needed to break bonds initially, as well as how much is released upon the formation of solute-solvent bonds.

Table of Content

• Aim
• Theory
• Materials Required
• Apparatus setup
• Procedure
• Observation
• Result and Discussions
• Precautions
• FAQs

Aim:

To determine the enthalpy of dissolution of given Copper Sulphate or Potassium Nitrate at room temperature using water as a reaction medium.

Theory:

During thermochemical measurements, aqueous solutions are mixed in water as the medium. Change in temperature during the reaction or dissolution is measured using which molar enthalpy can be calculated.

The sum of enthalpy changes occurring in the calorimeter due to either loss or energy gain, must be zero, according to the law of conservation of energy. We can, therefore, write the following equation,

ΔH₁ + ΔH₂ + ΔH₃ + ΔH₄ = 0

Solution formation often goes hand in hand with heat changes. Solution enthalpy is the amount of heat released or absorbed when one mole of a solute(solid/liquid) is dissolved in such a large amount of solvent (usually water) that further dilution does not change heat.

Materials Required:

1. Beakers
2. Thermometer
3. Glass rod
4. Weight box
5. Physical balance
6. Stirrer
7. Measuring cylinder
8. Hydrated copper sulfate
9. Potassium nitrate
10. Distilled water
11. Small wooden block
12. Cotton wool
13. Small piece of cardboard
14. Filler

Apparatus Setup:

Determining the Enthalpy of Dissolution of Given Solid Copper Sulphate in Water at Room Temperature

Procedure:

1. Enthalpy of Dissolution of Copper Sulphate:

1. Take 100 mL of distilled water in the beaker with a constant calorimeter and place it on a wooden block in a larger 500 mL beaker.
2. Pack the empty space with cotton wool between the bigger and the smaller beaker and cover with cardboard.
3. Record temperature of water. Let it be T₁°C.
4. Transfer copper sulphate powder which is already weighed. See that all the added solid completely dissolves.
5. Keep the thermometer and stirrer intact on the foam case mouth.
6. Note down the solution’s maximum temperature after copper sulphate is dissolved. Let it be T₂°C.

2. Enthalpy of Dissolution of Potassium Nitrate:

1. Take 100 mL of distilled water in the beaker with a constant calorimeter and place it on a wooden block in a larger 500 mL beaker.
2. Pack the empty space with cotton wool between the bigger and the smaller beaker and cover with cardboard.
3. Record the temperature of water. Let it be T₁°C.
4. Transfer potassium nitrate powder which is already weighed. See that all the added solid completely dissolves.
5. Keep the thermometer and stirrer intact on the foam case mouth.
6. Note down the solution’s maximum temperature after potassium nitrate is dissolved. Let it be T₂°C.

Observation and Inference:

Assuming density and specific heat of the solution to be the same as that of water heat evolved or absorbed for dissolution of w g of the solute.

Calculations:

For w/M moles of the solute dissolved heat change =

For 1 mole of the solute dissolved heat change =

Enthalpy of dissolution =

Where

M = formula mass of the solute

w = mass of the solute

W = water equivalent of the calorimeter

Δt = change in temperature

Enthalpy of dissolution ΔH is positive if heat is absorbed and negative if heat is evolved.

Similarly we can find the enthalpy of dissolution of potassium nitrate. For that dissolve 5.5g of KNO₃ in 200ml of water. Here the mole ratio of solute and solvent is 1:200

Results and Discussions:

1. The heat of the solution of potassium nitrate KNO₃ is __________J/mole.
2. The heat of solution of copper sulfate solution CuSO₄.5H₂O is ________J/mole.
3. % Error for KNO₃ =
   \(\begin{array}{l}\frac{Heat\ of\ solution\ determined}{Experimental\ value}\times 100 \end{array} \)
4. % Error for CuSO₄.5H₂O =
   \(\begin{array}{l}\frac{Heat\ of\ solution\ determined}{Experimental\ value}\times 100 \end{array} \)

Precautions:

1. Record the temperature of hot water just before mixing when the calorimeter constant is determined.
2. To dissolve the solid and record the temperature, stir the solution well. Avoid too much stirring, because friction can produce heat.
3. Carefully weigh copper sulphate as it is naturally hygroscopic.
4. The potassium nitrate should be taken in a small amount that is 2.27g to be dissolved in 100ml of water.
5. In order to create isolation between the two beakers, use cotton wool.
6. Use a 0.1°C thermometer to record the water temperature.