A smoggy sunset in Shenzhen, China. The reaction of $$O_2$$ with $$N_2$$ at high temperature in an internal combustion engine produces small amounts of $$NO$$, which reacts with atmospheric $$O_2$$ to form $$NO_2$$, an important component of smog. The reddish-brown color of $$NO_2$$ is responsible for the characteristic color of smog, as shown in this true-color photo.
In this chapter, we describe the methods chemists use to quantitatively describe the composition of chemical systems at equilibrium, and we discuss how factors such as temperature and pressure influence the equilibrium composition. As you study these concepts, you will also learn how urban smog forms and how reaction conditions can be altered to produce $$H_2$$ rather than the combustion products $$CO_2$$ and $$H_2O$$ from the methane in natural gas. You will discover how to control the composition of the gases emitted in automobile exhaust and how synthetic polymers such as the polyacrylonitrile used in sweaters and carpets are produced on an industrial scale.