Types of heat of reactions (enthalpies)

• When the reactants of a chemical reaction are completely reacted to form products, then the heat evolved or absorbed is called the heat of reactions.
• Heat of reactions are of various types depending upon the nature of reaction.
• It must be defined for one mole.
• The various types of heat of reactions are as follows:

A) Heat of formation

• It is the amount of heat evolved or absorbed when one mole of the compound is formed from its elements.
• When all the substances are taken in their standard state i.e. 298K and 1 atm pressure, it is called standard heat of formation.

C (s) + O₂ (g) → CO₂ (g); ΔH_f  = -94kcal/mole.

H₂ (g) + ½ O₂ → H₂O (l); ΔH_f = -68 kcal/mole.

where ΔH_f  is heat of formation. ΔH_f of elements in free state ( i.e. C, H₂, O₂, N₂, etc) is zero.

B) Heat of combustion

• It is amount of heat evolved when one mole of substance is completely burnt or oxidized.
• Heat of combustion is always negative as it involves exothermic reaction.
• It is used to calculate calorific value of fuels.
• Calorific value of fuel is the amount of heat in calories produced from the complete combustion of one gram of the fuel.

CH₃OH (l) + 3O₂ (g) → 2CO₂ (g) + 2H₂O (l);ΔH_c =-726 KJ/mole.

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C) Heat of neutralization

• It is the amount of heat evolved when one gram equivalent of the acid (base) is completely neutralized by one gram equivalent base (acid) in fairly dilute solution.
• It is always exothermic.
• Heat of neutralization of strong acid by strong base is always constant and equal to 57.1 KJ/mole or 13.7 kcal.
• It is because in dilute solutions all strong acids and bases ionize completely the heat of formation of water from H⁺ and OH^–
• If one or both of them are weak heat of neutralization is less as part of the heat is used up for the dissociation of weak acid or weak base.

HCN + NaOH→ NaCN + H₂O; ΔH_n = 45.3 KJ/mole.

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D) Heat of hydration

• It is the amount of heat evolved or absorbed when one mole of an anhydrous salt combines with required number of water molecules to form the hydrated salt.

CuSO₄ (s) + 5H₂O (l) → CuSO₄.5H₂O; ΔH_hyd = -78.2KJ.

E) Heat of solution

• It is the amount of heat evolved or absorbed when one mole of the substance is dissolved in excess of the solvent.
• Hot packs and cold packs use this property.
• Many hot packs use calcium chloride, which releases heat when it dissolves according to the equation below.

CaCl₂ (s) →Ca²⁺ (aq) + 2Cl^– (aq); ΔH_soln = -82.8KJ/mole.

F) Heat of fusion

• It is the heat required to convert one mole of solid substance into liquid at melting point.

H₂O (s) → H₂O (l); ΔH_fus = 6.01 KJ/mole.

G) Heat of vapourization

• It is the heat required to convert one mole of the liquid into vapour at the boiling point.

H₂O (l) → H₂O (g); ΔH_vap = 40.67 KJ/mole.

H) Heat of sublimation

• It is the heat required to convert one mole of solid directly to vapour below the boiling point.
• It is equal to the heat of fusion + heat of vapourization.

ΔH_vap + ΔH_fus = ΔH_sub

References: 

i) https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/07%3A_Thermochemistry/7.6%3A_Heats_of_Reactions_-_U_and_H

ii) https://www.britannica.com/science/heat-of-reaction

Types of heat of reactions (enthalpies)