What chemical compounds might be present in drinking water?

# 2.1.1 Natural nitrogen fixation

Despite a vast reservoir of nitrogen being readily available in the air, green plants cannot use it because of the high dissociation energy of the dinitrogen bond.

What is the bond order in dinitrogen, N2?

## Answer

Three as dinitrogen contains a triple bond, N≡N.

Green plants depend on nitrogen fixation - the combination of atmospheric nitrogen with hydrogen and oxygen to form ammonium compounds or nitrates. On the roots of peas, beans and other members of the legume family, there are nodules (Figure 5), inside which live nitrogen-fixing bacteria of the rhizobia group. These bacteria fix about 108 tonnes of nitrogen per year worldwide, which is approximately 60% of all nitrogen fixed. This is a symbiotic relationship, with the bacteria providing the plant with nitrogen compounds, and the plant supplying nutrients to the bacteria.

Figure 5 Root nodules on a pea plant containing nitrogen-fixing bacteria.
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To bring about cleavage of the nitrogen molecule requires a high energy input. Indeed, two molecules of the energy-transfer agent adenosine triphosphate (ATP; Section 3.3.2) are needed to bring about transfer of each electron. The reaction is catalysed by the enzyme nitrogenase, a complex molecule that incorporates molybdenum, iron and sulfur.

How many electrons are transferred when converting nitrogen to nitrate?

## Answer

The oxidation number change is 0 (N2) to +5 (NO3−) suggesting the transfer of five electrons.

So how many molecules of ATP are needed to provide the energy for this conversion?

## Answer

Each electron transfer requires two molecules of ATP, so ten ATP molecules in total.

Nitrogen compounds are produced in several other processes, the most obvious being for fertiliser, either farmyard manure or synthetic, such as ammonium nitrate (NH4NO3). Atmospheric lightning also causes some combination of oxygen and nitrogen (Equation 7) which forms nitrogen monoxide, which after further reactions leads to the passage of nitrogen into the soil as nitrates dissolved in rainwater.

The formation of gaseous oxides of nitrogen, NOx, from coal burning and internal combustion engines, is a process more commonly associated with detrimental environmental effects. So it should also be included.

N2(g) + O2(g) = 2NO(g)

The nitrogen cycle is completed by plant death, decay and bacterial denitrification, which returns nitrogen to the seas and the atmosphere. Originally fertilisers with a high nitrogen and phosphorus content originated from guano or bird excrement. For many years, naturally occurring sodium nitrate, NaNO3, from Chile was the main raw material for producing fertilisers.

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