PHYSICAL CHEMISTRY IN BRIEF

CHAP. 5: THERMOCHEMISTRY [CONTENTS] 128

Solution

2 NaOH+H 2 SO 4 =Na 2 SO 4 + 2H 2 O

and

0 =Na 2 SO 4 + 2H 2 O− 2 NaOH−H 2 SO 4

5.1 Heat of reaction and thermodynamic quantities of reaction

D Definition
The heat of reactionQr of the reaction (5.1) is the heat exchanged between a system and its
surroundings and needed for the reaction ofa units of a pure substance A, b units of a pure
substance B,... to formr units of a pure substance R,sunits of a pure substance S,... at
constant temperature. The unit can be a molecule, mol or kmol. Whenamoles of substance A
react withbmoles of substance B... , the heat of reaction is related to one mole.

Note:Sometimes we may encounter the termsheat of neutralization,heat of combustion,

heat of hydrogenation, etc. All these heats represent special cases of the heat of reaction,

and their names only specify the type of chemical reaction taking place.

Anexothermic reactionis such during whichQr<0 (heat is evolved).

Anendothermic reactionis such during whichQr>0 (heat is absorbed).

If a reaction proceeds at constant pressure and no other but volume work is performed, the

heat of reaction is equal to thereaction enthalpy, i.e. to the difference between the enthalpies

of the products and reactants

Qr = ∆rH=

∑

i=R,S,···

Hi−

∑

i=A,B,···

Hi

= rHm,R+sHm,S+···−aHm,A−bHm,B−···=

∑n

i=1

νiHmi, [T, p], (5.3)