Why is Enthalpy Important?
The concept of enthalpy is vital for temperature and pressure that any chemical reaction requires. We also need to know the enthalpy to calculate the total amount of heating and cooling that a commercial production of any substance requires.
For example, in the mass production of any compound such as ammonia, calcium carbonate, oxygen etc. we need to know the enthalpy in the reaction. So, how do we calculate the change in enthalpy for any reaction?
Enthalpy change is the standard enthalpy of formation. We have determined it for a vast number of substances. In any given chemical reaction, the reactants undergo chemical changes and combine to form different products.
For any such reaction, we represent the enthalpy change as ΔrH. We term it as the reaction enthalpy. We can calculate the reaction enthalpy by subtracting the sum of enthalpies of all the reactants from that of the products. Mathematically,
ΔtH = Sum of enthalpies of the product – Sum of the enthalpies of the reactants
This is a brief outline of the concept of enthalpy. However, our main aim in this chapter is to earn about the enthalpies of different reaction. Let us now look at these.
Types of Reactions and Their Enthalpies
Below, we will look at the various types of reactions and their associated enthalpies.
Enthalpy of Formation
We can define it as “The quantity of heat evolved or absorbed when one mole of the compound forms from its elements”. We can express it as , e.g.
It is important to remember that we should balance the thermochemical equation in such a way that it represents the formation of a single mole of the substance only. Standard heat of formation is the value of the heat of formation at 298 K and 1 atm pressure. We take it as zero in case of the free state of elements.
Enthalpy of Combustion
We can define it as, “The quantity of heat evolved when one mole of the substance completely oxidises”. Eg: always exothermic
The heat of combustion finds its use in the calculation of the heat of formation which looks difficult in certain cases, like in the calculation of the calorific value of fuels. The heat of combustion is also useful in elucidating or explaining the structure of various organic compounds.
Enthalpy of Solution
We may define it as, “The quantity of heat evolved or absorbed when one mole of a solute totally dissolves in a large excess of water, so that further dilution of solution does not produce any heat change”. E.g.
Enthalpy of Neutralisation
It is nothing but, “the quantity of heat evolved when one equivalent (or equivalent mass) of an acid is completely neutralised by one equivalent (or equivalent If mass) of a base in dilute solution”. E.g.
We take the heat of neutralisation of strong acid and a strong base as 13.7 kcal. or 57kJ. We can use the electrolytic dissociation theory to explain that this heat of neutralisation is merely the heat of formation of water from of an acid and of a base.
The heat of neutralisation for some weak acids or weak bases is a little lesser than 13.7 kcal. This is because some energy is used up in dissociating the weak electrolyte. We can calculate the dissociation energy of the weak acid or base by calculating the difference in the values.
Here, 1.8 kcal of heat is used in the dissociation of both the weak electrolytes. We can measure the heat of neutralisation in the lab by using pothythene or polystyrene bottles.
Enthalpy of Dissociation
We can define this term as, “The quantity of heat absorbed when one mole of a substance completely dissociates into its ions”. E.g.
Enthalpy of Dilution
We can define it as, “The quantity of heat evolved or absorbed when a solution containing one mole of a solution dilutes from one concentration to another”.
Heat of dilution =
Enthalpy of Precipitation
We can define it as, “The quantity of heat given out in the precipitation of one mole of a sparingly soluble substance on mixing dilute solutions of suitable electrolytes”. E.g.
Enthalpy of Hydration
We may define it as, “The quantity of heat evolved or absorbed? when one mole of an anhydrous or a partially hydrated salt is combined with the required number of moles of water to form a specific hydrated substance”. E.g.
Enthalpy of Fusion
We can define it as, “It is the change in enthalpy during conversion of 1 mole of a substance from solid to liquid state at its melting point (mostly endothermic)”.
Enthalpy of Vaporisation
We can define it as the change in enthalpy when 1 mole of a substance converts from liquid to gaseous state at its boiling point.
Solved Example For You
Question: State the enthalpy of sublimation.
Answer: It is the change in enthalpy when I mole of a substance is converted directly from solid to Vapour (gaseous) state.