Science Notes: Chemical Formations

Formation of Ammonia (NH3) as Example
Let us look at a more complicated example of the formation of a molecule. Just as the energy released by water falling can be captured, we can find ways to capture chemical energy. Let us look at a more complicated reaction: the formation of ammonia. Nitrogen gas (N2) and hydrogen gas (H2) can be made to combine to form NH3, or ammonia gas. As we proceed, look for the answers to these questions: Is the reaction exothermic? How many kilograms of hydrogen are needed to produce one kilogram of ammonia?

The reaction is N2 + 3H2 2NH3

This equation says that one mole of N2 requires three moles of H2 for a complete reaction, and this would then yield two moles of NH3. Note that we can also say that one molecule of N2 reacts with three molecules of H2 to yield two molecules of ammonia (NH3). But the table gives energies in units of kcal/mole, which is why it is easier to work with moles. The energy involved in the reaction involving just one or two molecules of ammonia is too small.

The following table contains the variety of ways in which you can write the reaction that forms ammonia. All the descriptions below are equivalent. The atomic masses are rounded off values from the Periodic Table.

Nitrogen
+
Hydrogen
Ammonia
N2
+
3H2
2NH3
+
3 H-H
2NH3
1 mole of N2
+
3 moles of H2
2 moles of ammonia
28 g of N
+
6 g of H
34 g ammonia

What are the energies of the reactions? In order to get the molecular structure of ammonia, we have to break one NN bond and three H - H bonds. This requires that we supply energy to break the bonds. From Table 8, we get the bond energies:

NN
225 kcal/mole
H - H
104 kcal/mole
N - H
94 kcal/mole

To form each molecule of NH3, we break one bond of N2 and three of H2, and then three N - H bonds form to make NH3. To calculate energy released (or absorbed) in the reaction, we have to calculate the energy needed to break the bonds of N2 and H2, and the energy released when the atoms rearrange to form NH3.

Energy required to break
the bonds of N2, 3H2
Energy released forming
the bond 2NH3
N2
:
1 mole x 225 kcal/mole
=
225
2NH3
:
2 moles x (3 x 93 kcal/mole) =
2 moles x 279 kcal/mole =
558
3H2
:
3 moles x 104 kcal/mole
=
312
total energy released
=
558 kcal
total energy absorbed
=
537 kcal
 
558 kcal - 537 kcal = 21 net kcal released for 2 moles of NH2 formed, so the net energy released is 10.5 kcal/mole of ammonia formed.

More energy is released than absorbed in the formation of ammonia, so this reaction is exothermic. We can also say that 10.5 kcal are released when 17g of ammonia are formed. The energy level diagram here is more complex than that for the H + H = H2 reaction because of the steps involved.

Figure 18: Energy Level Diagram for the formation of Ammonia (NH3).

For this reaction, we had to put in some energy to "activate" the reaction, which was the energy required to break the N2 and H2 bonds. N2 and H2 brought together with no addition of energy would not spontaneously react. This is analogous to our striking a match to start the burning of coal. The energy required to start the reaction is called activation energy. Then, left to themselves, the N and H form bonds to release 568 calories.

What is a corresponding example with the gravitational force?

Formation of Water (H2O) as Example
Hydrogen and oxygen combine to form water. Which is more stable -- hydrogen and oxygen gas separately, or in combination as water?

Write equation and balance:

2H2 + O2 2H2O

amounts element   element   compound
molecules
2 molecules hydrogen
+
1 molecule
oxygen
2 molecules water
moles
2 moles hydrogen
+
1 mole
oxygen
2 moles of water
grams
4 g hydrogen
+
32 g oxygen
36g water
kilograms
4kg H2
+
32 kg O2
36 kg water

1. Is the reaction exothermic or endothermic?

2. Which is more stable, hydrogen gas and oxygen gas separately or combined chemically as water? Explain.

Structural formula:

2 H-H + O=O 2 H-O-H
(drawn as linear although molecule is not)

Bonds: break old (spend energy), recombine to form new bonds (release energy).

Break 2 H-H, break O=O and in recombination, 4 H-O bonds are made and release energy. Using the bond energies of H-H 104 kcal/mole, O-O 119 kcal/mole, and H-O 111kcal/mole from Table 8, we can see how much energy is released when bonds are broken and formed.

BONDS BROKEN
 
BONDS FORMED
Bonds
# of bonds
Energy Required
Bonds
# of bonds
Energy Released
H-H
2
208 kcal
H-O
4

444 kcal

O=O
1
119 kcal
Total

327 kcal required
Total

444 kcal released

 

Figure 19: Energy level diagram for formation of water.

Energy released is greater than energy required to break bonds so there is a net energy release. The reaction is exothermic. The amount of energy released is 444 kcal minus 327 kcal which equals 117 kcal per mole of oxygen burnt or:

444 kcal - 327 kcal = 117 kcal per mole of oxygen burnt
58.5 kcal/mole of hydrogen burnt or 58.5 kcal/mole water formed.
117 kcal for 36g of oxygen; OR
58.5 kcal per 2 g hydrogen; OR
58.5 kcal released when 18 g water formed

58.5 x 1000 = 5,850 kcal of energy released when 18kg water formed

5,850/18 = 325 kcal per kg water formed. The energy released (or absorbed) per mole of the product is called "the heat of reaction". Thus the heat of reaction of water is above 58.5 kcal/mole

(All of the above statements are equivalent.)

The energy released per unit mass may also be calculated in Joules. When we talk about released energy as the output of a power plant, Joules are the customary units; therefore it is often necessary to convert the energy from kcal to Joules.

Kilocalories are units of heat. 4,190 kilojoules make 1 kilocalorie. So, multiply kilocalories by 4,190 to convert to kilojoules. This is necessary to convert heat (or chemical energy) units to work (or mechanical energy) units. Recall that these two systems of units evolved separately. Heat units were used by chemists and chemical engineers and mechanical units by physicists and mechanical engineers.

325 x 4,190 = 1,361,750 kJ of energy is released per kg water formed. This reaction can theoretically do 1,361,750 kJ of work. We usually get less useful work because of Second Law of Thermodynamics--some energy is lost as heat.

 


Exercise:
In each of the following reactions:
  1. Name the product.
  2. Balance the reaction.
  3. Write the moles of each component.
  4. Say whether the reaction is endothermic or exothermic.
  5. Say whether the system is stable before or after the reaction.
 Reaction
Heat of Reaction
(kcal/mole)
1. H2 + O2   H2
-58.5
2. N2 + O2   NO2  
+21.6
3. C + H2    C2H6    
-20.2
4. C + H2   C3H8 
-24.8
5. N2 + H2   NH3  
-11.0
The ( - ) sign means that there is a net energy release when the compound is formed; that is, the final product is lower in energy or the reaction is exothermic.

 

 

 

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