Problem 3 One important source of emissions from gasoline-powered automobile eng

Question

Problem 3

One important source of emissions from gasoline-powered automobile engines that cause smog is the nitrogen oxides NO and NO2 . They are formed whether combustion is complete or not as follows. At the high temperatures that occur in an internal combustion engine during the burning process, oxygen and nitrogen combine to form nitric oxide (NO). The higher the peak temperatures and the more oxygen available, the more No is formed. There is insufficient time for the NO to decompose back to O2 and N2 because the burned gases cool too rapidly during the expansion and exhaust cycles in the engine. Although both NO and nitrogen dioxide (NO2) are significant air pollutants (together termed NOx), the NO2 is formed in the atmosphere as NO is oxidized.

Suppose that you collect a sample of a NO-NO2 mixture (after having removed the other combustion gas products including N2, O2 and H2O by various separations procedures) in a 100-cm3 standard cell at 300C. Certainly some of the NO will have been oxidized to NO2


2NO + O2 2NO2

during the collection, storage and processing of the combustion gases, so that measurement of NO alone will be misleading. If the standard cell contains 0.291 g of NO2 plus NO and the pressure measured in the cell is 170 kPa, what percent of the NO + NO2 is in the form of NO?

Explanation / Answer

In atmospheric chemistry, air pollution, and related fields, nitrogen oxides refers specifically to NOx (NO and NO2).[1][2] Only the first three of these compounds can be isolated at room temperature. N2O3, N2O4, and N2O5 all decompose rapidly at room temperature. NO3, N4O, and N(NO2)3 are very reactive. N2O is stable and rather unreactive at room temperature, while NO and NO2 are quite reactive but nevertheless quite stable when isolated. NOx (often written NOx) refers to NO and NO2. They are produced during combustion, especially at high temperature. These two chemicals are important trace species in Earth's atmosphere. In the troposphere, during daylight, NO reacts with partly oxidized organic species (or the peroxy radical) to form NO2, which is then photolyzed by sunlight to reform NO: NO + CH3O2 ? NO2 + CH3O NO2 + sunlight ? NO + O The oxygen atom formed in the second reaction then goes on to form ozone; this series of reactions is the main source of tropospheric ozone. CH3O2 is just one example of many partly oxidized organic molecules that can react with NO to form NO2. These reactions are rather fast so NO and NO2 cycle, but the sum of their concentration ([NO] + [NO2]) tends to remain fairly constant. Because of this cycling, it is convenient to think of the two chemicals as a group; hence the term NOx. In addition to acting as a main precursor for tropospheric ozone, NOx is also harmful to human health in its own right. NOx may react with water to make nitric acid, which may end up in the soil, where it makes nitrate, which is of use to growing plants. [edit]Derivatives See also: Birkeland–Eyde process Oxidized (cationic) and reduced (anionic) derivatives of many of these oxides exist: nitrite (NO- 2), nitrate (NO- 3), nitronium (NO+ 2), and nitrosonium (NO+). NO2 is intermediate between nitrite and nitronium: NO+ 2 + e- ? NO2 NO2 + e- ? NO- 2

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