Page 152 - Physical chemistry understanding our chemical world

INDIRECT MEASUREMENT OF ENTHALPY 119

(We have numbered these three (1) to (3) simply to avoid the necessity of rewriting
the equations.)

To obtain the enthalpy of forming ethanol, we first draw a cycle. It is usual to start
by writing the reaction of interest along the top, and the elements parallel, along the
bottom. Remember, the value of H r is our ultimate goal.

DH r
CH 2 CH 2 + H 2 O CH 3 CH 2 OH Reaction of interest
Step 1
1
2C + 3H 2 + O 2 Elements
2
The next three stages are inserting the three enthalpies H f (1) to (3). Starting on
the left-hand side, we insert H f(1) :

DH r
CH 2 CH 2 + H 2 O CH 3 CH 2 OH

Step 2
∆H f(1)
1
2C + 3H 2 + O 2
2
We then put in the enthalpy of forming water, H f(3) :

DH r
CH 2 CH 2 + H 2 O CH 3 CH 2 OH
Step 3
D H f(1) D H f(3)
1
2C + 3H 2 + O 2
2
And finally we position the enthalpy of forming the product ethanol, H f(2) :

DH r
CH 2 CH 2 + H 2 O CH 3 CH 2 OH

Step 4
D H f(1) D H f(3) D H f(2)
1
2C + 3H 2 + O 2
2
We can now determine the value of H r . Notice how we only draw one arrow per
reaction. The rules are as follows: