"Chemistry is in the News"
'HEALTH RISKS' OF MOWING THE LAWN
BBC News, June 7, 2001.
Since its introduction in the mid-1970s, the catalytic converter has been both a blessing and a curse to the environment. Designed to convert noxious gases produced by the combustion of hydrocarbons in gasoline to compounds less harmful to humans, the catalytic converters have also created byproducts guilty of contributing to the greenhouse effect.
During complete combustion, hydrocarbons react with oxygen to form carbon dioxide and water. However, car engines do not provide favorable environmental conditions to perform complete combustion. Instead, the hydrocarbons are broken down less efficiently into products like CO. Carbon monoxide is a colorless and odorless compound. In humans, it binds to hemoglobin in place of the oxygen, which is required for cell function. This results in carbon monoxide poisoning and will eventually cause death to those who inhale too much. If cars were to release large amounts of carbon monoxide directly into the atmosphere as they would in the absence of a catalytic converter, rush hour would be a dash home to death.
Catalytic converters use a metal catalyst, usually in the form of platinum, palladium or rhodium. The metal is plated on a honeycomb structure that takes advantage of high surface area to convert poisonous carbon monoxide into less harmful carbon dioxide. The carbon dioxide emitted in car exhaust becomes suspended in the atmosphere. As more of these molecules gather in the atmosphere, more of the sunŐs radiation is reflected back to earth, contributing to global warming effects.
Engine conditions pose problems for the atmosphere as well. Nitrogen and oxygen gases find their way into the engine where combustion is taking place. The high temperatures cause a reaction between these gases, creating nitrogen oxides. Though the catalytic converter is designed to reduce these compounds back to N2 and O2, there is evidence that some of the nitrous oxides escape to the atmosphere. Nitrous oxides are greenhouse gases. There is also evidence that catalytic converters over-reduce the nitrogen oxides to form ammonia NH3. NH3 results in the hazy pollutants seen in city skylines.
Today, gases harmful to our environment are still emitted to the atmosphere, though at much lower rates than twenty years ago. More efficient processes are needed to dispel all of the gases that may contribute to global warming. These gases are not removable, and many mechanisms are being developed to limit future damages. Currently, global efforts are underway in Kyoto, Japan, to reduce greenhouse gas emissions by encouraging research and development of these mechanisms.
Pertinent Text References
Wade Chapters 1, 3, & 8
Carey Chapters 1, 2, & 6
Draw the Lewis-Kekuhle structures of CO2, CO, NH3,
NO2? Name something special about each molecule.
What is the hybridization about the center atom in each?
Label formal charges.
Answer: See textbook.
Surface area is essential to cells. It enables them to perform thousands
of reactions within a very small space. Chemically, how does surface area
in catalytic converters play a role in reducing CO to
Answer: The honeycomb design of catalytic converters provides a large surface area for reactions to take place between CO, the metal catalyst and O2 into CO2.
How does the amount of O2 affect the combustion of
hydrocarbons? Consider the reaction between ethane and O2
in ample amounts, in limited amounts.
Answer: The amount of O2 present in the system determines the products. When ample O2 is present for the combustion of ethane then two moles of CO2 and three moles of water are produced from one mole of ethane and five halves of a mole of oxygen. When there is not enough oxygen present in the system, then carbon monoxide, elemental carbon, and hydrocarbons are also produced in the combustion reaction.
What are some other causes of the greenhouse effect besides exhaust
emissions from automobiles?
Answer: CO2 released through -deforestation -burning fossil fuels.
Consider the reaction of CO and O2 on a catalytic surface. Is
the reaction thermodynamically favorable? How does this relate to the
conditions inside the vehicle?
Answer: 2 CO + O2 ---> 2 CO2
The reaction is exothermic but requires a significant activation energy. The reaction actually works well within the combustion engine system because the heat produced by the combustion reaction heats the catalytic converter and provided the necessary energy to oxidize the CO.
Question 6: Read about the US and the Kyoto Protocol. Is President Bush right in sacrificing the integrity of our environment in order to maintain a competitive economy in the face of other developing nations (like China and India)?