How to Calculate the Enthalpy of Formation of a Molecule – Part 2

The standard enthalpy of formation of a compound (ΔfHѲ) is the change of enthalpy (heat) during the formation of one mole of the compound/substance from its constituent elements in their reference/standard states.


The enthalpy/heat/energy/calorie/Joule value of a substance, usually a fuel/food, is the amount of heat released during the combustion/burning of a given amount of it.

The enthalpy of combustion always has a negative value (exothermic reaction) but that of formation may have a negative or a positive value (exothermic or endothermic reaction respectively).

One could write the balanced chemical equations for all the given combustion reactions, and find out the enthalpy of formation of the target reaction by using the enthalpies of combustion of the given reactions, but there exists a shortcut!

Question:

Calculate the standard enthalpy of formation of CS2(l). The standard enthalpies of combustion of C(s), S(s) & CS2(l) are ─393.3, ─293.72 & ─1108.76 kJ/mol respectively.

Solution:



Click to enlarge the image

ΔcHѲ = Standard enthalpy of combustion

Mystery of thermochemistry solved❓ Let Chemaficionado know at mychemistryhomework@gmail.com❗

References:

(1)    Wikipedia Contributors. Standard enthalpy of formation. Wikipedia. https://en.wikipedia.org/wiki/Standard_enthalpy_of_formation.

(2)    Wikipedia Contributors. Heat of combustion. Wikipedia. https://en.wikipedia.org/wiki/Heat_of_combustion.

‌(3)    Ncert. Chemistry : Textbook for Class XI - Part.I; National Council Of Educational Research And Training: New Delhi.

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