Page 152 - Modern physical chemistry

144 Relationships among Reactants

In writing the equation for a reaction, one needs to specify the initial and final states
of each participant. Abbreviation g indicates the gaseous state, 1 indicates the liquid state,
and s indicates the solid state. One may further distinguish a solid as cr for crystalline,
am for amorphous, vit for vitreous. Where more than one crystalline form exists, the
form employed may be indicated with a number or letter following s or cr. The temper-
ature may appear as subscript on the thermodynamic variable employed. A superscript
zero indicates that standard states were involved. Where necessary, the pressure may be
added.

ExampleZl

Interpret the following:
MI~98.15 = 49.04 kJ.
The chemical equation represents 6 moles solid carbon and 3 moles gaseous hydro-
gen reacting to form 1 mole liquid benzene. When each of these is in its standard state
at 298.15 K, the enthalpy increase is 49.04 kilojoules.

ExampleZ2
The standard enthalpy offormation (a) of H 20 (g) is -241.83 kJ moI-l , (b) of H20 2 (g)
is -136,11 kJ moI-l , at 298.15 K. Determine Mfl 298 . 15 for the reaction

at 1 bar pressure.
It is given that

MI&8.15 = -241.83 kJ.

MI&8.15 = -136.11 kJ.
Reverse the first equation of this pair and multiply by 2:

2H20 (g) ~ 2H2 (g) + O2 (g), MI&8.15 = 483.66 kJ.
Then add to the second equation to get

MI~98.15 = 347.55 kJ.

Z3 Measuring Reaction Heats

The heat evolved in a rapid exothermic reaction of a small amount of material can
be readily determined. The heat -q for other reactions may be obtained by combining
these as in example 7.2.
For a determination, the chosen reaction is carried out in a small gas-tight container
immersed in water in an insulated vessel. The initial temperature TI and the final tem-
perature T2 are measured. The overall process involves

A (11) + calorimeter (11) ~ B (T 2 ) + calorimeter (T 2 ),q = O. [7.1]
Here A symbolizes the reactants and B the products for the reaction.