Heat Of Hydration Formula: Definition & Solved Example

The heat of hydration in chemistry is defined as the amount of energy released when one mole of ions undergoes hydration. It is a type of dissolution energy, and the solvent used is water. The process through which water hardens concrete is known as hydration. 

• The enthalpy of a hydrated salt is the heat change when 1 mole of anhydrous substance reacts with water molecules to form a hydrate.
• The heat of hydration can be easily found when the heat of the solutions of anhydrous salt and the hydrated forms are known.
• The ion's size and charge determine the extent of hydration.
• The heat of hydration formula is represented as Heat of hydration = (ΔH _solution – ΔH _{lattice energy})

Key Terms: Compound, Ions, Covalent Bonds, Heat Of Hydration Formula, Thermodynamics, Dehydration, Exothermic Reaction, Chemical Bond

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What Is Heat Of Hydration?

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The heat of hydration is a reaction of cement with water that is exothermic, releasing a significant amount of heat. The heat release is further termed the heat of hydration.

• Hydration is a chemical reaction that occurs when the cement's major ingredients form chemical bonds with water molecules, producing hydrates or hydration products.
• The heat is provided by the hydration of one mole of particles under constant pressure.
• The more hydrated the particle, the more heat is given.
• Dehydration occurs when your body does not have enough water to function correctly.
• The amount of heat released during hydration depends on the size and charge of ions and the strength of their bonds with water molecules.
• Heat of hydration affects crystal formation, salt dissolution in water, and body temperature regulation in living organisms.

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Heat Of Hydration Formula

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The heat of hydration formula describes how much energy is delivered when one mole of particles is hydrated. The formula for the heat of hydration is the difference between the enthalpy change of solution (ΔH_solution) and the enthalpy change of lattice energy (ΔH_{lattice energy}). 

• ΔH_solution = Amount of heat absorbed or released when a substance dissolves in water,
• ΔH_{lattice energy} = Amount of energy required to break the bonds between ions in a crystalline solid

The heat of hydration can be calculated using this formula, which takes into account both the energy released by the formation of new bonds between water molecules and ions, as well as the energy required to break the existing bonds in the anhydrous salt. Alternatively, the heat of hydration can also be calculated using the following formula:

ΔH_hydration = q / n

• Where q is the heat absorbed or released during the reaction
• n is the number of moles of water involved in the hydration process.

This formula can be useful in experimental settings where the heat released or absorbed can be measured directly, such as with calorimetry.

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Hydration Reaction

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When hydrogen and hydroxyl ions are bound to a carbon in a carbon double bond, a hydration process occurs. Most of the time, one reactant (usually an alkene or an alkyne) reacts with water to generate ethanol, isopropanol, or 2-butanol (alcohols). 

The general chemical reaction of hydration is stated as follows:

RRC = CH₂ + H₂O/acid → RRC – CH₂ – OH

In the industrial sector, ethanol is produced by the hydration of ethene.

CH₂ = CH₂ (g) + H₂O(g) → CH₃CH₂OH (g)

Varying substances release different amounts of heat at different rates. The addition of retarders to cement modifies the quick-setting qualities of C₃A. The fineness of the cement also has an effect on the pace of heat buildup. 

On average, normal cement produces 89-90 calories/gm in 7 days and 90-100 calories/gm in 28 days. As a result, the process involved in heat development is quicker in the beginning and continues eternally at a diminishing pace.

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Heat Of Hydration In Cement

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When cement and other additional chemicals are mixed as one, an exothermic process involving cement and water occurs (hydration). Following that, the heating impact is addressed. At lower temperatures, it takes longer to establish a concrete hydration reaction. Rising temperatures lead to a rapid creation of heat.

The formula is 

• H = heat of hydration of ignited cement, kJ/kg,
• H₁ = heat of solution of dry cement,
• H₂ = heat of solution of a partially hydrated sample, and 
• th = final calorimeter temperature at end of determination on the partially hydrated sample, °C.

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Previous Year Questions

1. Half Mole Of An Ideal Monoatomic Gas Is Heated At… [JEE Main 2019]
2. Heat Treatment Of Muscular Pain Involves Radiation…  [JEE Main 2019]
3. A solution of acetone in ethanol… [NEET 2006]
4. 1-phenyl ethanol can be prepared by the reaction of benzaldehyde with… [NEET 1997]
5. 2 Moles Of An Ideal Monatomic Gas Is Carried From… [JEE Main2017]
6. Unit of entropy is… [NEET 2002]
7. The efficiency of an ideal heat engine working between the freezing point and boiling point of water, is…. [NEET 2018]
8. The efficiency of a fuel cell is given by… [NEET 2007]
9. The correct thermodynamic conditions for the spontaneous reaction at all temperatures is… [NEET 2016]
10. In which case change in entropy is negative? [NEET 2019]

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Things To Remember

• The heat of hydration formula calculates the energy change that occurs when water molecules combine with an ionic substance to form a hydrated salt.
• The formula takes into account the enthalpy change of solution and the enthalpy change of lattice energy.
• The enthalpy change of solution refers to the heat absorbed or released when a substance dissolves in water.
• The enthalpy change of lattice energy refers to the amount of energy required to break the bonds between ions in a crystalline solid.
• The heat of hydration formula is expressed as ΔH_hydration = ΔH_solution – ΔH _{lattice energy.}
• The heat of hydration can be either exothermic or endothermic, depending on whether the formation of the hydrated salt releases or absorbs heat energy.

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