TITLE.PM5

500 ENGINEERING THERMODYNAMICS

dharm

\M-therm\Th11-1.pm5

HHRR 01 − = mh hii i

R

∑ () 01 − ... on mass basis ...(11.21)

HHRR 01 − = mc Ti pi T

R

∑ ()^01 −

= (T 0 – T 1 ) mci pi

R

∑ ... in terms of mean specific heats

Products :HHPP 20 − = nh hii i

P

∑ () 20 − ... on mole basis ...(11.22)

HHPP 20 − = mh hii i

P

∑ () 20 − ... on mass basis ...(11.23)

HHPP 20 − = mc Ti pi T

P

∑ ()^20 −

= (T 2 – T 0 ) mci pi

P

∑ ... in terms of mean specific heats ...[11.23 (a)]
It may be noted that niCpi = micpi
From the definition of the enthalpy of a perfect gas
H = U + pV = U + nR 0 T
So if we are concerned only with gaseous mixtures in the reaction then for products and
reactants
HP 0 = UP 0 + nPR 0 T 0
and HR 0 = UR 0 + nRR 0 T 0
where nP and nR are the moles of products and reactants respectively and the temperature is the
reference temperature T 0.
Thus, using eqns. (11.13) and (11.19), we have
∆H 0 = ∆U 0 + (nP – nR)R 0 T 0 ...(11.24)
If there is no change in the number of moles during the reaction or if the reference tem-
perature is absolute zero, then ∆H 0 and ∆U 0 will be equal.

11.17. ENTHALPY OF FORMATION (∆∆∆∆∆Hf)
A combustion reaction is a particular kind of chemical reaction in which products are
formed from reactants with the release or absorption of energy as heat is transferred to as from
the surroundings. In some substances like hydrocarbon fuels which are many in number and
complex in structure the heat of reaction or combustion may be calculated on the basis of known
values of the enthalpy of formation, ∆Hf of the constituent of the reactants and products at the
temperature T 0 (reference temperature). The enthalpy of formation (∆Hf) is the increase in enthalpy
when a compound is formed from its constituent elements in their natural form and in a stand-
ard state. The standard state is 25°C, and 1 atm. pressure, but it must be borne in mind that not
all substances can exist in natural form, e.g. H 2 O cannot be a vapour at 1 atm. and 25°C.
The expression of a particular reaction, for calculation purposes, may be given as :
∆H 0 = nHi f
P

∑ ∆ i – nHi f

R

∑ ∆ i ...(11.25)