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.

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.

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:

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: Name the product. Balance the reaction. Write the moles of each component. Say whether the reaction is endothermic or exothermic. Say whether the system is stable before or after the reaction.  Reaction Heat of Reaction (kcal/mole) 1. H2 + O2   H2O  -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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