Nitrogen is one of the few elements that readily forms strong multiple bonds. There are strong triple bonds between the nitrogen atoms in the nitrogen molecule. The N=N bond energy of azide is less than half of the triple bond energy, and the N-N bond is very weak. The N-O single bond is much weaker than the N-H bond. N-X bond energies in kJ/mol are summarized below. Due to the exceptional strength of the bond in N2, the molecule is neither chemically nor photochemically reactive.
Ammonia and other volatile organic amines are released by the decomposition of plant and animal materials. They oxidize easily, so most of the nitrogen compounds in the atmosphere, apart from molecular nitrogen, are nitrogen oxides. The oxidation state of nitrogen varies from I to V among oxides.
- （A）Yes2Europa, or nitric oxide, and some NO, or nitrous oxide, are produced by soil and ocean bacteria. Nitric oxide is relatively inactive in the troposphere and is the main source of nitric acid and other nitrates in the stratosphere. This is important in the chemistry associated with stratospheric ozone depletionozone layerNitrous oxide is relatively non-toxic and is used as a propellant in anesthetics and canned cream.
(b) nitrous oxide (Yes) results from the combination of O2i N2In a lightning strike. It is a radical and therefore very active in the atmosphere. Its unpaired electrons can be removed by oxidation to NO+Or the molecule can be reduced to NO-In biology, NO is important as a signaling agent. NO reacts with oxygen to form nitrogen dioxide.
(c) nitrogen dioxide (Yes2) is the brown gas responsible for the color of photochemical smog. This is caused by the burning of fossil fuels and biomass and by the oxidative decomposition of ammonia in the atmosphere. It is a free radical and therefore reactive in the atmosphere. Nitrogen dioxide is in equilibrium with its colorless dimer,Yes2Europa4.
(d) Dinitrogen tetroksid (
(e) Other neutral nitrogen oxides includeYes2Europa5iYes2Europa3.
(f) Dinitrogen trioxide (Yes3) is a free radical with an unpaired electron on one of the oxygen atoms. Its reactivity is similar to that of the hydroxyl radical (HO).
(g) In addition to inert nitrogen oxide (NNO), neutral nitrogen oxides are generally classified asYesx.They are poisonous to humans and othersGround-level ozone and photochemical smog.
No repliesxRadicalThree of these nitrogen oxides are free radicals. no and no2It is a nitrogen-centered free radical.
More reactive NO3, like OH, has an unpaired electron on oxygen.
NO usually reacts with organic peroxy radicals in the form of NO2and an alkoxy group. Not2It can be added to alkyl peroxy radicals. In the example below, acetaldehyde is oxidized and the resulting peroxyacetyl group reacts with NO2This produces nitric anhydride of peracetic acid. PAN is a highly irritating molecule, toxic to humans, animals and plants.
Nitric oxide is an important oxidant at night. It is photolyzed by sunlight. Below is a summary of some of the more important types of reactions involving NO3.
thermodynamicsMost nitrogen oxides are unstable with respect to molecular nitrogen and molecular oxygen and have positive values for the change in Gibbs free energy of formation. The table above gives the standard heat of formation of the elements (H0The relation of free energy (G) to enthalpy (H) and entropy (S) is given by the expression:
for the formation of NO from N2and Europe2, the entropy becomes smaller and positive.
The change in Gibbs free energy is related to the equilibrium constant K. R is the gas constant 8.314 J/K, and the temperature T is measured in Kelvin.
In equilibrium, i.e. under thermodynamic control, the concentration of NO in the atmosphere should be 10-15.5atm, but the actual concentration of NO is significantly higher than this value (approximately 10-10ATM).
Therefore, although thermodynamics shows that NO is unstable with respect to N2and Europe2, does not spontaneously break down into those elements. There is a high activation barrier between reactants and products that must be overcome. Nitric oxide is kinetically stable, but thermodynamically unstable.
Nitrogen dioxide is a brown gas in rapid equilibrium with its colorless dimer. This is a thermodynamically controlled reaction because the activation barrier between products and reactants is small. As shown in the figure below, the equilibrium depends on the temperature (see: University of MichiganWeb pageMore information about the demonstrations. )
at low temperatureGibbs free energyis negative, andbalanceIn favor of the product, dimer. At higher temperatures, the Gibbs free energy is positive and favors equilibrium, favoring the reactants.
In summer, the temperature is higher. The concentration of NO is higher2It can be photolyzed to form ozone. Lower winter temperatures retain some nitrogen dioxide due to reduced dimer and ozone production.
dynamicsThe reaction rate usually depends onactivation energyThe slowest step in a reaction, the concentration, temperature, and pressure of the reactants that combine during that step.
In the troposphere, the concentration of nitrogen oxides is very low. We can talk about gas concentration in the atmosphere in terms of partial pressure. Near the Earth's surface, the pressure is 1 atmosphere. The partial pressure of molecular nitrogen in dry air is 0.78 atm, O20.21 atmospheres. Everything else is the sum of 0.1 atmosphere. Gas concentrations can be expressed in ppm or parts per million (1 part in 10-6atmosphere). If the concentration of NO in a place is 10-4ppm, its concentration in the atmosphere = 10-4desetmilijunti gave-6atmosfera/ppm = 10-10ATM.
|Consider the decomposition of NO into elemental nitrogen and oxygen.|
As written, the reaction is favored because the Gibbs free energy change is negative. The equilibrium constant favors the product and the reaction should continue until the NO concentration falls to 10-15.5ATM.
However, the reaction did not go as planned. This is because the activation energy is high, higher than that available at ambient temperature, and because the NO concentration is already low. The probability of two NO molecules colliding with enough energy and in the correct orientation to form molecular nitrogen and oxygen is very small.
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|Now consider the dimerization of NO2 to N2O4. In this case, the reaction enthalpy is small and negative, and the Gibbs free energy is strongly influenced by entropy and temperature. Entropy favors reactants (2 molecules, less ordered state) over reactants.|
At low temperature, TThe S term is not very important, a negative enthalpy term means that the reaction proceeds as written. Here the activation energy is very small. This is a typical free radical coupling reaction.
At higher temperatures, the Gibbs free energy is dominated by the entropy term, favoring the reverse reaction. The activation energy for this reaction is also low because the N-N bond broken during this step is very weak.