Page 155 - Physical chemistry understanding our chemical world

122 ENERGY AND THE FIRST LAW OF THERMODYNAMICS

top, so we have drawn it on the left as H r .

1
CO + O 2 CO 2 Compounds
2
Step 1
DH r
Constituent elements
C + O 2
Next we insert the enthalpies for the reactions of interest; we first insert H c(1) :

1
CO + O 2 CO 2
2
Step 2
DH r DH c(1)

C + O 2

and finally, we insert H c(2) :

D H c(2)
1
CO + O 2 CO 2
2
Step 3
D H r D H c(1)

C + O 2

We want to calculate a value of H r . Employing the same laws as before, we see
that we can either go along the arrow for H r directly, or along H c(1) in the same
direction as the arrow (so we do not change its sign), then along the arrow H c(2) .
Concerning this last arrow, we go in the opposite direction to the arrow, so we
multiply its value by −1.
The value of H r is given as

H r = H c(1) + (−1 × H c(2) )

Inserting values:

−1
H r =−393.5kJ mol −1 + (+283 kJ mol )
so
H r =−110.5kJ mol −1

SAQ 3.10 Calcite and aragonite are both forms of calcium carbonate,
CaCO 3 . Calcite converts to form aragonite. If H O =−1206.92 kJ
f (calcite)