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Thermodynamics I 23
4. Identify the data given in the question, and write any corre-
sponding formation reactions of the species from their component
elements:

(a) C(,) + 02(g) + C02(,) AH,"(C02,g)) = -393.51 kJ
(b) 2Hqg) + Oqg) + 2H20(1) AH,"(H20(1)) = (2 x -285.83) =
- 571.66 kJ mol-
(c) C(,) + 2Hqg) + CHqg) AH,"(CHqg)) = -74.81 kJ
5. Identify the unknown in the question.
CHqg) + 202(g) + CO2(g) + 2H20(1) AHRn = AH:
6. Examine each of the three equations in step 4, and re-arrange
them, such that the reactants and products required in step 5 are
on the same side as those in step 3. However, if the direction of a
reaction is changed, the sign of AH" also changes.

(a) C(,) + 02(g) -+ CO2,,) AH;(C02(gl) = -393.51 kJ
No problem here, as COz(,) is on the right-hand side.
(b) 2Hqg) + Oqg) + 2H20(1) AH;(H20(1)) = -571.66 kJ
Neither is there a problem here, as H,O(,) is also on the right-
hand side, and the stoichiometry is correct (i.e. 2), so AH" does
not have to be changed.
(c) C(,> + 2Hqg) + CHq,) AH,"(CHqg)) = -74.81 kJ
In this equation, CH,,,) should be on the left-hand side. Hence, if
the direction of the reaction is changed, the sign of AH; also
changes.
Hence: (c)CHqg) -+ C(s) + 2H2,,) AH;(CH4(g)) = +74.81 kJ
7. Now add the three reactions, and find the sum of their
enthalpies:
(a) $(,) + 02(g) + C02(,) AH,"(CO2(g)) = -393.51 kJ
(b) 2$fqg) + Oqg) + 2H200) AH;(H20(,)) = -571.66 kJ
(c) CH4(g) + $(s) + 2g2(g) AH;(cH,g)) = +74.81 kJ

(4 CHqg) + 202(g) + C02(g) + 2H20(1)
AH:x, = AH: = (-393.51) + (-571.83) + (+74.81) =
- 890.36 kJ mol- '.
The value of AH: means that when methane is burned in a
stream of oxygen gas, 890.36 kJ mol-' of heat is given out. This
is an example of an exothermic reaction, since AH;xn is negative.
This will be discussed further in Chapter 3.