- 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); ΔHf = -94kcal/mole.
H2 (g) + ½ O2 → H2O (l); ΔHf = -68 kcal/mole.
where ΔHf is heat of formation. ΔHf 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);ΔHc =-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; ΔHn = 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; ΔHhyd = -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); ΔHsoln = -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); ΔHfus = 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); ΔHvap = 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.
ΔHvap + ΔHfus = ΔHsub
Types of heat of reactions (enthalpies)