Heat of Solution Formula: Definition, Equation, Examples & Sample Questions

Heat of solution formula is expressed as, ΔHwater = masswater × ΔTwater × specific heatwater. Heat of solution is the difference between the enthalpies in relation to the dissolving substance into a solvent. This happens at a constant pressure which leads to an infinite dilution. It is usually defined as the quantity of enthalpy that is evolved or observed in the solution during the process. The heat of solution is also known as enthalpy of solution. This article discusses the concept of the heat of solution along with its formula and solved questions.

Also check: Application of thermodynamics

Key terms: the heat of solution, the heat of solution formula, enthalpy of solution, solvation, enthalpy, enthalpy of dissolution.

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The Formula of Heat of Solution

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The formula of the heat of solution is described as,

In this formula,

ΔH = change occurred in the heat

mass water = sample of mass

ΔT = difference in the temperature

Specific heat of water is equal to 0.004184 kJ/g^oC.

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What is the Heat of Solution?

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The heat of solution is observed as the difference in the heat or enthalpy related to the dissolving substance into a solvent. Meanwhile, the pressure is constant which then leads to an infinite dilution. The unit of solution enthalpy is kJ/mol. Therefore, the heat of solution is the amount or quantity of heat energy that is released or absorbed when a solution is formed according to the concepts of Chemistry. 

There are a total of three steps in the process of dissolving the substance into a solvent.

1. The breaking of bonds between solute molecules, for example, the electrostatic attraction between the two ions. The heat of the solution is endothermic there.
2. The breaking of intermolecular attractions between solvent molecules, e.g., hydrogen bonds. The heat of the solution is endothermic there.
3. The formation of new solute-solvent attractive bonds. The heat of the solution is exothermic there.

Also read: Solutions

The absorption of Energy happens during the first two steps, and the same energy is released during the third and the last step. On the basis of the relative amounts of energy required to break bonds initially, as well as, the amount of energy that is released upon solute-solvent bond formation. The overall heat of solution can be calculated as either endothermic or exothermic. The heat solution is measured in terms of a calorimeter.

The addition of some solutes to a solvent would raise the temperature of the solution, while others may lower the temperature, and still, others would have no noticeable effect. This behavior depends on the heat of solution of the solute in the given solvent. The heat of solution, i.e, The amount of heat given off or absorbed during the process of solution, is equal to the difference between the energy that must be supplied to break up the crystals of the solute and the energy that is released when the solute particles are taken into solution by the solvent. If the heat of solution is negative (i.e., more energy is required to break up the crystal than is released in forming the solution), then the temperature would decrease; if the heat of solution is positive, the temperature would increase.

Read more: Heat Formula

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Examples of the Heat of Solution

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• Dissolution of sodium chloride 

Dissolution of sodium chloride (table salt) in water (liquid) is endothermic in chemistry. Due to the amount of energy that is required to break apart the hydrogen bonding interactions between water molecules. Along with the energy used to break apart the electrostatic attractions between sodium and chloride ions, is greater than the amount of energy released when new solute-solvent attractions are constructed between water molecules and aqueous ions in solution.

• Dissolving potassium hydroxide 

Dissolving potassium hydroxide is exothermic. It happens because more energy is released upon the formation of solute-solvent bonds than was required to break apart the hydrogen bonds in water, as well as the ionic bonds in KOH.

• Chemical hot packs and cold packs 

Chemical hot packs and cold packs also work due to the heat or enthalpy of the solution of the chemicals inside these packs. When these bags are squeezed, an inner pouch bursts, which allows the chemical to dissolve in water. Heat is released in the hot pack and absorbed in the cold pack.

Also check: Enthalpy of Dilution

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Solved Examples

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Example 1: When hydrated salt (Na₂CO₃.10H₂O) is dissolved at 291K isothermally in a large quantity of water is 65 KJ per mole solute, the heat absorbed. Calculate the heat of crystallization of 1 kg Na₂CO₃.10H₂O.

Solution: Given,

m = 65 kJ per mol

ΔT = 291K

Specific heat = 0.004184 KJ/g^oC

Substitute in the given formula, we get

ΔHwater= masswater × ΔTwater × specific heatwater

= 65 × 291 × 0.004184

Therefore, ΔHwater = 79.140 kJ/Kg solute

Also check: Latent heat

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Things to Remember

• The heat of solution is the difference between the enthalpies in relation to the dissolving substance into a solvent.
• The symbol of the heat of the solution is kJ/mol.
• The heat of solution formula is: ΔHwater = mass water × ΔTwater × specific heat water
• The heat of the solution is not constant. It varies with the concentration of the components.
• Specific heat of water is equal to 0.004184 kJ/g^oC.

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