PHYSICAL CHEMISTRY IN BRIEF

CHAP. 5: THERMOCHEMISTRY [CONTENTS] 132

Note:It follows from the definition that the standard enthalpy of formation of elements

in their most stable phases and molecular forms are zero.

5.2.2 Standard enthalpy of combustion ∆cH◦.

Definition:The standard enthalpy of combustion is the standard reaction enthalpy of a reaction
during which one mole of a substance reacts with oxygen while producing defined waste products
of the reaction.

Note: If a substance contains elements C, H, N, Cl, S, the defined waste products are

gaseous CO 2 , N 2 , HCl, SO 2 , and water in their most stable state at a given temperature,

i.e. either gaseous or liquid.

If we know the enthalpies of combustion of all substances present in the general reaction
(5.1), then for the reaction enthalpy of this reaction we write

∆rH◦ = a∆cHA◦+b∆cHB◦+ ··· −r∆cHR◦−s∆cHS◦− ···

= −

∑n

i=1

νi∆cHi◦. (5.13)

This equation is the consequence of Hess’s law [compare with equation (5.12)].

Example

The reaction enthalpy of the reaction (5.11) is the standard enthalpy of combustion of carbon,

the reaction enthalpy of the reaction (5.10) is not.

Note:The termheating valueis often used in practice, indicating the change in enthalpy

during the combustion of a defined amount of fuel to gaseous (water, carbon dioxide,... )

and solid products (ash).