Chemistry in Context, 9th Edition A Project of the American Chemical Society

Chapter 2 The Air We Breathe What is air? What are the components that make up the air we breathe? How does the composition of air change from place to place? What are the impurities in air and how did they get there? Are there harmful components in the air you breathe indoors? Are there ways we can prevent or limit contaminants from polluting our atmosphere?

The Troposphere 75% of our air, by mass, is in the troposphere, the lowest region of the atmosphere in which we live.

The Composition of Air It’s a mixture – a physical combination of two or more substances present in variable amounts.

What’s in a Breath? Substance Inhaled Air (%) Exhaled Air (%) Nitrogen (N2) 78.0 Oxygen (O2) 21.0 16.0 Argon (Ar) 0.9 Carbon dioxide (CO2) 0.04 4.0 Water Variable

What Else Is In a Breath? These images show Beijing, China from the same vantage point on different days. In addition to nitrogen, oxygen, and other colorless gases, there are harmful nitrogen oxides and particulate matter that contribute to air pollution.

Air Inversions Cooler air can be trapped beneath warmer air due to weather conditions. Pollutants often accumulate in the cooler air of an inversion layer. This situation is worsened when air flow is limited, such as in cities surrounded by mountains.

Visualizing the Molecules in Air A molecule is a fixed number of atoms held together by chemical bonds in a certain spatial arrangement. The chemical formula symbolically represents the type and number of each element present. Chemists use three viewpoints to study and understand matter, macroscopic, symbolic, and particulate.

Molecular Structures The particulate view of matter shows the 3-D molecular structure, with atoms color-coded.

Naming Binary Compounds Prefixes are used to designate the number of each type of element: Number Prefix 1 mono 2 di 3 tri 4 tetra 5 penta 6 hexa 7 hepta 8 octa 9 nona 10 deca

Naming Binary Compounds of Nonmetals Prefixes are used to designate the number of each type of element: N2O = dinitrogen monoxide (also known as nitrous oxide, or laughing gas) P2O5 = diphosphorus pentoxide

Naming Hydrocarbons Hydrocarbons are compounds containing only carbon and hydrogen atoms Chemical Formula Number of Carbon Atoms Compound Name CH4 1 Methane C2H6 2 Ethane C3H8 3 Propane C4H10 4 Butane C5H12 5 Pentane C6H14 6 Hexane C7H16 7 Heptane C8H18 8 Octane C9H20 9 Nonane C10H22 10 Decane

Air Pollutants: Risk Assessment Risk Assessment – evaluating scientific data and making predictions in an organized manner about the probabilities of an occurrence. Toxicity – intrinsic health hazard of a substance. Exposure – the amount of the substance encountered. Evaluate the risk of breathing the amount of ozone in the air between the hours of 4:00 p.m. and 6:00 p.m. where you live. Note: You may have to go online to access information from the EPA to help in your calculation.

Concentration Terms Parts per hundred (percent) Atmosphere is 21% oxygen = 21 oxygen molecules per 100 molecules and atoms in air Parts per million (ppm) Midday ozone levels reach about 0.4 ppm: 0.4 ozone molecules 1  106 molecules and atoms in air Parts per billion (ppb) Sulfur dioxide in the air should not exceed 30 ppb: 30 sulfur dioxide molecules 1  109 molecules and atoms in air

Concentration Conversions 21% means 21 parts per hundred… means 210 parts per thousand means 2,100 parts per ten thousand means 21,000 parts per hundred thousand means 210,000 parts per million The difference between % and ppm is a factor of 10,000.

The Harmful Few… These gases represent the most harmful for our health and the health of our planet: Carbon monoxide Ozone Sulfur dioxide (SO2) and nitrogen dioxide (NO2) Particulate matter (PM)

U.S. National Ambient Air Quality Standards Pollutant Standard (ppm) Approximate Equivalent Concentration (mg/m3) Carbon monoxide 1-hour average 35 40,000 8-hour average 9 10,000 Nitrogen dioxide 0.100 200 Annual average 0.053 100 Ozone 0.070 140 Particulates PM10, 24-hour average N/A 150 PM2.5, 24-hour average PM2.5, annual average 15 Sulfur dioxide 0.075 210 3-hour average 0.50 1,300

Air Quality Monitoring and Reporting The average concentration of air pollutants in the United States have decreased dramatically since 2000.

The Number of Unhealthy Days Per Year Metropolitan Area Ozone PM2.5 Boston 8 Chicago 10 Cleveland 1 Houston 21 Los Angeles 43 Phoenix 11 4 Pittsburgh 14 Sacramento 35 13 Seattle 2 Washington, DC The Number of Unhealthy Days Per Year Although air quality has improved in recent years, on average, people in some metropolitan areas breathe air containing unhealthy levels of pollutants.

EPA’s Air Quality Index

Example: Air Quality Index Values for Phoenix, AZ Variations reflect those in the local weather patterns. Regional events such as forest fires and volcanic eruptions can influence air quality.

Chemical Reactions Chemical reactions are characterized by the rearrangement of atoms when reactants are transformed into products. This is an example of a combustion reaction. The number of atoms on each side of the arrow must be equal (Law of Conservation of Mass).

Balancing Equations Tips for balancing equations: If an element is present in just one compound on each side, balance it first Balance anything that exists as a free element last Balance polyatomic ions as a unit Check when done – same number of atoms, and same total charge (if any) on both sides

Balancing Equations: An Example As an example, consider the combustion of methane (CH4) to generate carbon dioxide (CO2) and water (H2O): CH4 + O2 →CO2 + H2O (unbalanced) CH4 + 2 O2 → CO2 + 2 H2O (balanced) When balanced, there is 1 carbon atom, 4 oxygen atoms, and 4 hydrogen atoms on either side of the equation.

“Incomplete Combustion” If the amount of oxygen is altered, the hydrocarbon can burn incompletely: 2 C8H18 + 25 O2 →16 CO2 + 18 H2O (complete combustion) 2 C8H18 + 17 O2 →16 CO + 18 H2O (incomplete combustion) Verify that both of these equations are properly balanced!

Vehicle Emissions This U.S. auto emissions report shows the amount of CO generated from the exhaust, which can tell if the vehicle is operating properly or exhibits incomplete combustion products.

Direct Source of Sulfur Trioxide S + O2 →SO2 2 SO2 + O2 →2 SO3 Sulfur trioxide then reacts with water to produce sulfuric acid, a contributor to acid rain. The good news: Since 1985, we have seen a 55% reduction in SO2 emissions in the U.S.

The Complexities of Nitrogen Dioxide Formation NO2 is a brown gas, giving smog its characteristic brown tinge. At high temperatures: i) N2 + O2 → 2 NO ii) 2 NO + O2 →2 NO2 NO2 is also produced from the reaction with ground-level ozone: NO + O3 → NO2 + O2 Volatile Organic Compounds (VOCs) emitted from vehicle exhausts are also involved in NO2 production: VOC + ⋅OH →A A + O2 →A’ A’ + NO →A’’ + NO2

Vehicle Catalytic Converters Catalytic converters reduce the amount of carbon monoxide (CO) in the exhaust due to catalyzing the combustion of carbon monoxide to carbon dioxide (CO2). Newer designs also limit the release of nitrogen oxides by reducing them to nitrogen and oxygen gases.

AIRNOW Website: Local Air Quality Conditions and Forecasts The EPA AirNow website provides information regarding particulate matter and ozone concentrations for local regions in the U.S. This shows that the ozone concentrations in southern California peak around mid-afternoon and drop overnight. Sunlight is needed to produce ozone from VOCs and nitrogen oxides.

Ozone (O3): A Secondary Pollutant Unlike nitrogen and sulfur oxides that are direct pollutants, ozone is a “secondary pollutant”. It is produced from one or more other pollutants (VOCs and NO2): i) NO2 Sunlight NO + O ii) O + O2 → O3

Indoor Air Pollutants? Do you think of harmful pollutants when you light your incense candle or want to begin painting a room in the house? Why do you think these are considered indoor air pollutants?

What is “Green Chemistry”? The EPA Design for the Environment Program initiated green chemistry programs. Green chemistry reduces pollution through the design or redesign of chemical processes. “Green” processes use less energy, create less waste, use fewer resources, and use renewable resources. Some examples include: Plastics synthesized from renewable sources instead of typical fossil-fuel derived precursors. Paints that contain fewer volatile organic compounds (VOCs) Cheaper and less wasteful ways to produce consumer products. Limiting or eliminating the use of organic solvents. Removing arsenic from the touchscreens of portable electronic devices.