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Enthalpy of Dissolution (molar heat of a solution) of a substance can be defined as the heat absorbed or evolved when one of its molecules dissolves in the solvent. In most cases, the solvent is water. The molar head can be symbolized as ΔH and is measured in kJ/mol. Based on how much energy is required to break bonds originally and the amount of heat released when solute-solvent bonds are formed, the overall heat of the solution is determined to be endothermic or exothermic. This process is also called heat of dissolution. On the other hand, enthalpy, in the thermodynamic system means the measurement of energy.
| Table of Contents |
Key Terms: Enthalpy, Endothermic Reactions, Exothermic Reactions, Energy Conservation Law, Joules, Heat of Dissolution, Specific Heat, Density
Aim
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To estimate the enthalpy of dissolution of given copper sulphate or potassium nitrate with water as reaction solvent at room temperature.
Materials Required
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- Weight box
- Physical balance
- Beakers
- Stirrer
- Measuring cylinder
- Thermometer
- Distilled water
- Small wooden block
- Cotton wool
- Small piece of cardboard
- Filler
- Glass rod
- Potassium nitrate
- Hydrated copper sulphate
| Topics Related to Thermochemistry | ||
|---|---|---|
| Ammonium Nitrite Formula | Heat Capacity Formula | Enthalpy of Dilution |
| Potassium Iodate | Cadmium Nitrate Formula | Potassium Chloride |
Theory
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Aqueous solutions are used as a solvent for almost all experiments. When compared to other solvents, water easily dissolves other compounds. So, water is the commonly used solvent. Apart from that, water dissolves solutes and exhibits exact changes in temperature.
According to the energy conservation law, the sum of enthalpy changes happening in the calorimeter, whether energy loss or gain, must be zero. As a result, we may write the following equation:
ΔH1 + ΔH2 + ΔH3 +…. ΔHn = 0
Heat changes are frequently associated with the formation of solutions. The quantity of heat given out or absorbed when one mole of a solvent, either solid or liquid is dissolved in such a vast amount of solvent which is typically water, that further dilution has no effect on the heat is referred to as solution enthalpy.
Read More: Conservation of Energy Formula & Solved Examples
Procedure
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The procedure of the experiment is:
Enthalpy of Dissolution of Potassium Nitrate
- Place a wooden block in a 500 mL beaker containing another beaker with 100 mL of distilled water and a constant calorimeter
- Between the big and smaller beakers, fill the space with cotton wool and cover with cardboard.
- Keep track of the temperature of the water and set the temperature to T1°C.
- Transfer the weighed potassium nitrate salt. Remove the extra solution after all of the potassium nitrate solute is dissolved.
- Keep the stirrer on the foam case mouth, and the thermometer undisturbed.
- After adding the solute, record the maximum temperature of the solution and set the temperature to T2°C.
Enthalpy of Dissolution of Copper Sulphate
- Place a wooden block in a 500 mL beaker containing another beaker with 100 mL of distilled water and a constant calorimeter
- Between the big and smaller beakers, fill the space with cotton wool and cover with cardboard.
- Keep track of the temperature of the water and set the temperature to T1°C.
- Transfer the weighed copper sulphate salt. Remove the extra solution after all of the potassium nitrate has dissolved.
- Keep the stirrer on the foam case mouth, and the thermometer undisturbed.
- After adding the salt, record the maximum temperature of the solution. Set the temperature to T2°C.
Diagram of a Calorimeter
Observations
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The observation of the experiment is:
| Enthalpy of Dissolution of Copper Sulphate | Enthalpy of Dissolution of Potassium Nitrate | ||
|---|---|---|---|
| Hydrated copper sulphate dissolved- weight. | w g | Hydrated potassium nitrate dissolved- weight. | w g |
| Volume of water taken into the bottle. | 200 ml 200 g (Supposing sp density=1) | Volume of water taken into the bottle. | 200 ml 200 g (Supposing sp density=1) |
| Temperature of water. | T1oC | Temperature of water. | T1oC |
| Temperature of water after dissolving hydrated copper sulfate. | T2oC | Temperature of water after dissolving potassium nitrate. | T2oC |
| Water equivalent of the polythene bottle. | Given (W g) | Water equivalent of the polythene bottle. | Given (W g) |
If we suppose the solution’s density and specific heat to be the same as water, to know
the heat absorbed or evolved for the solute’s dissolution, we can arrive at the equation,
Q = (W + 200) x (t1 – t2) cals
Calculations
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By using this equation, for w/M moles of solute in water,
Q = (W + 200) x (t1 – t2) x 4.2 J
Therefore, for 1 mole of solute dissolved in water,
Q = (W + 200) x (t1 – t2) x 4.2 x M/w Joules
Thus, the formula for enthalpy of dissolution is given as
ΔH = (W + 200) x (t1 – t2) x 4.2 x M/w joules
Where,
- M = Formula mass of solute,
- W = Equivalent weight of water calorimeter,
- w = Mass of solute
- ΔH = Change in temperature
Results
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- Heat of solution
- Potassium nitrate (KNO3) = __________J/mole.
- Copper sulphate solution CuSO4.5H2O = ________J/mole.
- % Error by = Heat of solution determined/Experimental value x 100
- KNO3 =
- CuSO4.5H2O =
Precautions
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- Stir the solution thoroughly to dissolve the solute and note the temperature. Keep in mind that stirring too much can result in heat production due to friction.
- When the calorimeter constant is found, record the temperature of hot water right prior to mixing.
- Cotton wool can be used to create separation between the two beakers.
- To measure the temperature of the water, use a 0.1°C thermometer.
- Copper sulphate is naturally hygroscopic, therefore weigh it cautiously.
- Potassium nitrate should be consumed in a little amount of 2.27g, which should be dissolved in 100ml of water.
Things to Remember
- The molar heat of a solution of a substance can be defined as the heat absorbed or evolved when one of its molecules dissolves in the solvent.
- Water dissolves substances very easily and exhibits exact changes in temperature.
- If the reaction is exothermic, i.e., heat is released during solution formation, the ΔH is negative.
- If the reaction is endothermic, i.e., heat is absorbed during solution formation, the ΔH is positive.
- Heat changes are frequently associated with the formation of solutions.
- Endothermic reactions store energy in the form of some chemical reaction bonds, on the other hand, an exothermic reaction radiates heat energy during a reaction process.
Previous Year Questions
- Why is the heat evolved in neutralization of HF is highest… [UPSEE 2018]
- The enthalpies of combustion of carbon and carbon monoxide are… [JCECE 2008]
- The enthalpy of sublimation of aluminium is 330 KJ/mol… [BITSAT 2014]
- If the enthalpy of vaporization of water is 186.5 KJ/mol… [BITSAT 2014]
- 6g of graphite is burnt in a bomb calorimeter at… [AP EAPCET 2019]
- The bond dissociation energies of X2, Y2 and XY are in the ratio… [NEET 2018]
Sample Questions
Ques. What is the molar heat solution of a substance? (3 Marks)
Ans. The molar solution heat of a substance can be defined as the heat absorbed or evolved when one of its molecules dissolves in the solvent. In most cases, the solvent is water. The molar head can be symbolised as ΔH and is measured in kJ/mol. The molar heat solution of a substance is also known as the enthalpy of dissolution.
Ques. What is the energy conservation law equation followed for enthalpy of dissolution of a substance? (3 Marks)
Ans. With respect to the energy conservation law, the sum of enthalpy changes happening in the calorimeter which can either lose or gain energy is supposed to be zero. As a result, we may write the following equation for enthalpy of dissolution of a substance:
ΔH1 + ΔH2 + ΔH3 + ΔH4…. ΔHn = 0
Ques. Define specific heat capacity. (3 Marks)
Ans. Specific heat capacity is a thermodynamic parameter that indicates the amount of heat necessary for a single unit of mass of a substance to be heated by one degree of temperature. The Specific Heat Capacity of a system can be denoted by c.
c=C/M = 1/M. dQ/dT
Where, dQ is the amount of heat required to regularly increase the temperature of the sample substance by an infinitesimally small increment in dT, temperature.
Ques. How to calculate the %Error of any given solute? (1 Marks)
Ans. The %Error can be found by the following equation,
Heat of the solution determined/Experimental value x 100
Ques. What is the enthalpy of neutralization? (3 Marks)
Ans. Enthalpy of neutralization can be defined as the enthalpy change that occurs if an acid equivalent and a base equivalent undergo a neutralization reaction for the formation of water and salt. Therefore, the resultant neutralization enthalpy will be constant. In most cases, the enthalpy of the neutralization after the complete ionization of the salt is 57.1 kJ.
Ques. What is the equivalent weight of copper sulphate? (1 Mark)
Ans. Copper sulphate’s equivalent weight is about 80 g/mol.
Ques. What is the procedure of Enthalpy of Dissolution of a salt? (3 Marks)
Ans. The procedure of enthalpy of dissolution of salt is as follows:
- Place a wooden block in a 500 mL beaker containing another beaker with 100 mL of distilled water and a constant calorimeter
- Fill the space with cotton wool between the beakers and cover with cardboard.
- Note the temperature of the water, and set it to T1°C.
- Transfer the weighed salt. Remove the extra solution after complete dissolution.
- Keep the stirrer on the foam case mouth, and the thermometer undisturbed.
- After adding the solute, record the maximum temperature of the solution (T2°C).
Ques. What is the association between entropy and enthalpy? (3 Marks)
Ans. The association between the formula and the association between the enthalpy and entropy is that the enthalpy is observed as the central factor in thermodynamics; however, on the other hand, the entropy is the measurement of the thermal energy with respect to thermodynamics.
Ques. What is a calorie? (1 Mark)
Ans. A gram calorie is the quantity of heat energy needed to raise one gram of water temperature by one degree Celsius
Ques. What is the change in enthalpy when work is done by the system? Explain the situation if work is done on the system. (3 Marks)
Ans. When work is done by the system, energy will be released from the system or absorbed from the system. So, it will lead to a decrease in the internal energy and an increase in the energy respectively. If the energy is released, a decrease in the enthalpy of the system can be seen. If the energy is absorbed, an increase in the enthalpy can be observed.
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