Thermodynamics and Chemistry

CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES

11.2 THEADVANCEMENT ANDMOLARREACTIONQUANTITIES 313

vaporization of water we write

H 2 O.l/!H 2 O.g/

For the dissolution of sodium chloride in water, we write

NaCl.s/!NaC.aq/CCl.aq/

For the Haber synthesis of ammonia, the reaction equation can be written

N 2 .g/C3 H 2 .g/! 2 NH 3 .g/

The essential feature of a reaction equation is that equal amounts of each element and
equal net charges appear on both sides; the equation is said to bebalanced. Thus, matter
and charge are conserved during the process, and the process can take place in a closed
system. The species to the left of a single arrow are calledreactants, the species to the right
are calledproducts, and the arrow indicates theforwarddirection of the process.
A reaction equation is sometimes written with right and left arrows

N 2 .g/C3 H 2 .g/ï2 NH 3 .g/

to indicate that the process is at reaction equilibrium. It can also be written as astoichio-
metric equationwith an equal sign:

N 2 .g/C3 H 2 .g/D2 NH 3 .g/

A reaction equation shows stoichiometric relations among the reactants and products. It
is important to keep in mind that it specifies neither the initial and final states of a chemical
process, nor the change in the amount of a reactant or product during the process. For
example, the reaction equation N 2 + 3 H 2 !2 NH 3 does not imply that the system initially
contains only N 2 and H 2 , or that only NH 3 is present in the final state; and it does not mean
that the process consists of the conversion of exactly one mole of N 2 and three moles of H 2
to two moles of NH 3 (although this is a possibility). Instead, the reaction equation tells us
that a change in the amount of N 2 is accompanied by three times this change in the amount
of H 2 and by twice this change, with the opposite sign, in the amount of NH 3.

11.2.1 An example: ammonia synthesis

It is convenient to indicate the progress of a chemical process with a variable called the
advancement. The reaction equation N 2 + 3 H 2 !2 NH 3 for the synthesis of ammonia syn-
thesis will serve to illustrate this concept. Let the system be a gaseous mixture of N 2 , H 2 ,
and NH 3.
If the system isopenand the intensive properties remain uniform throughout the gas
mixture, there are five independent variables. We can choose them to beT,p, and the
amounts of the three substances. We can write the total differential of the enthalpy, for
instance, as

dHD



@H

@T



p;fnig

dTC



@H

@p



T;fnig

dp

CHN 2 dnN 2 CHH 2 dnH 2 CHNH 3 dnNH 3 (11.2.1)