CK-12-Chemistry Intermediate

17.4. Hess’s Law http://www.ck12.org

2CO 2 (g)+H 2 O(l)→C 2 H 2 (g)+

5

2

O 2 (g) ∆H= 1301 .1 kJ

Now, these three reactions can be summed together. Any substance that appears in equal quantities as a reactant in
one equation and a product in another equation, cancels out algebraically. The values for the enthalpy changes are
likewise added.

2C(s,graphite)+2O 2 (g)→2CO 2 (g) ∆H= 2 (− 393. 5 ) =− 787 .0 kJ

H 2 (g)+





(^1) 
2
O 2 (g)→H 2 O(l) ∆H=− 285 .8 kJ

2CO 2 (g)+H 2 O(l)→C 2 H 2 (g)+


(^5) 
2
O 2 (g) ∆H= 1301 .1 kJ
2C(s,graphite)+H 2 (g)→C 2 H 2 (g) ∆H= 228 .3 kJ
So the heat of reaction for the combination of carbon with hydrogen to produce acetylene is 228.3 kJ. When one
mole of acetylene is produced, 228.3 kJ of heat are absorbed, making the reaction endothermic.
Standard Heat of Formation
A relatively straightforward chemical reaction is one in which elements are combined to form a compound. For
example, sodium and chlorine react to form sodium chloride (as seen in the video below), and hydrogen and oxygen
combine to form water. Like other reactions, these are accompanied by either the absorption or release of heat.
Thestandard heat of formation(∆Hf°)is the enthalpy change associated with the formation of one mole of a
compound from its elements in their standard states. The standard conditions for thermochemistry are 25°C and
101.3 kPa. Therefore, the standard state of an element is its state at 25°C and 101.3 kPa. For example, iron is a
solid, bromine is a liquid, and oxygen is a gas under those conditions. The standard heat of formation of an element
in its standard state is by definition equal to zero.∆Hf° = 0 for H 2 (g), N 2 (g), O 2 (g), F 2 (g), Cl 2 (g), Br 2 (l), and I 2 (g),
because these elements are most commonly found in their pure form as diatomic molecules. The graphite form of
solid carbon is defined as its standard state, so it has a∆Hf° of 0. Because diamond is not the standard state of
carbon, it has a non-zero value for∆Hf°. Some standard heats of formation are listed below (Table17.3).
Elemental sodium (Na) and chlorine gas (Cl 2 ) react to form sodium chloride, releasing 411 kJ of heat for every mole
of NaCl produced. This spectacular reaction can be seen in the following video: http://www.youtube.com/watch
?v=Ftw7a5ccubs (0:42).

MEDIA

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TABLE17.3: Standard Heats of Formation of Selected Substances

Substance ∆Hf° (kJ/mol) Substance ∆Hf° (kJ/mol)

Al 2 O 3 (s) −1669.8 H 2 O 2 (l) −187.6

BaCl 2 (s) −860.1 KCl(s) −435.87