The amount of heat evolved or absorbed when one mole of a substance is formed from its constituent elements is called heat of formation.
H2(g) + Cl2(g) ® 2HCl(g); DH = – 44 kcal
Thus, enthalpy of formation of HCl is – 22 kcal
Standard Enthalpy of Formation (H°f):
It is enthalpy change of a reaction by which a compound is formed from its constituent elements, the reactant and products all being in a given, standard state (i.e., at 298 K and 1 atm pressure). For example,
S(s) + O2(g) ® SO2(g); DH°f = – 296.9 kJ
2Al(s) + 3/2O2(g) ® Al2O3(s); DH°f = – 1669.8 kJ
Let us consider a reaction, C(s) + O2(g) ® CO2(g); DH°f = – 393.5 kJ
DH°f = H°P – H°R = – 393.5 kJ
Enthalpies of free elements in their standard states are assumed to be zero.
So, DH°f = DH°P = – 393.5 kJ
The compounds having positive enthalpies of formation are called endothermic compounds and are less stable than the reactants and those having negative enthalpies of formation are known as exothermic compounds and are more stable than the reactants.
We are the leading online assignment help provider. Find answers to all of your doubts regarding the Thermodynamics in chemistry. We at assignmenthelp.net provide homework, assignment help to the school, college or university level students. Our expert online tutors are available to help you in Thermodynamics. Our service is focused on: time delivery, superior quality, creativity and originality.
Chemistry Assignment Help | Chemistry tutor | Science tutor | Chemistry help | Chemistry homework help | Organic chemistry help | Help with chemistry | Online chemistry help | Free chemistry help | High school chemistry help | College chemistry help | General chemistry help | AP chemistry help | Help in chemistry | Help with chemistry homework | Chemistry experiments | homework help | Homework assignment help | Online tutoring services | Online Tutoring