Chemical Reactions and the Environment

1. Chemical Reactions and the Environment Ch 8

2. Key Concepts in Chapter 8 • How different factors change the rates of chemical reactions • How chemicals play a role in our daily lives • Why chemicals are important to the Canadian economy • How the products and by-products of chemical industries affect our environment

3. Factors affecting chemical reactions • The amount of time required for a given product to form or for given amounts of reactants to react is called the rate of reaction. • Time Rates refer to the amount of time required for an event to occur.

4. Factors affecting rates of reaction • Concentration of reactants • Surface area of reactants • Temperature (thermal energy) of reactants • Catalysts

5. Concentration of reactants • Concentration refers to the amount of substance in a given volume. • In chemistry, we commonly use the molarity to represent the concentration of a solution. The molarity refers to the number of moles of solute per litre of solution. A 5.0 M(molar) solution of sulfuric acid is highly concentrated.

6. Concentration of reactants • If the concentration of reactants is greater, then there is a greater probability that collisions among the particles will occur and the rate of reaction will increase. Conversely, a lower concentration will likely mean fewer collisions and a lower rate. As an example, consider your reaction of magnesium ribbon and HCl. Hydrogen gas was produced and you heard a pop when the splint was lowered into the test tube. If you used a higher concentration of HCl, then more hydrogen would be produced and you would hear a louder pop!

7. Surface area and reactants • When lighting a fire, it is much easier to use kindling or small pieces of wood to start the fire instead of trying to ignite a log by itself. The increased surface area from the kindling enables a greater exposure to oxygen and increases the rate at which the reaction proceeds. So, increasing the surface area of the reactants increases the rate of reaction.

8. Temperature and Reaction Rates • Perishable foods are often refrigerated to slow down the natural reactions in food spoiling. Lowering the temperature reduces the thermal energy of the reactants and the rate of reaction. Conversely, inceasing the temperature increases the thermal energy of the particles of the reactants. This in turn increases the likelihood of collisions among the reactants thereby increasing the rate of the reaction.

9. Catalysts are used to speed up reaction rates • A catalyst is a substance which speeds up the rate of a reaction by lowering the energy needed to break the bonds in the reactants. Catalysts themselves are not consumed by the reaction. • Catalytic converters on cars change most of the CO and nitrogen oxides to CO2 and N2 and O2 respectively. • Biological catalysts called enzymes, are proteins which accelerate cellular reactions such as breaking down starch, digesting carbohydrates, and producing fats for storage.

10. Chemicals for Consumers • Antacids • Soaps • Detergents • Household Cleaners

11. Antacids • HCl is the stomach acid produced to assist in the digestion of foods. If we overeat, excess acid is produced and we experience “heartburn” or “acid indigestion” . This is just a burning sensation we feel as the stomach contents is pushed into the lower esophagus. To relieve this feeling, we take an antacid to neutralize the excess acid. Some of these are Tums, Rolaids, Enos etc.

12. Soaps • Sweat and oils are produced by glands in our skin in order to maintain body temperature and to keep our skin soft and flexible. Bacteria and other microorganisms feed on the particles trapped by sweat and oils interacting with our environment. An unpleasant side effect of this interaction is “b.o.” i.e. body odour 

13. Soaps • Look at the diagram of the water molecule and note its “polarity” It has two charged poles - +

14. Soaps • A polar substance such as water usually dissolves other polar molecules as well as ionic salts. Think about the meaning of polarity • It does not dissolve non polar substances such as oils, fats and grease. They are immiscible i.e. oil and water do not mix. • The chemical structure and composition of soap allows it to dissolve oils and grease and to work with water.

15. Soaps • Soap is an example of an organic salt i.e. it has an ionic head and a non-polar tail. See transparency and diagram in your text p.256. • The tail mixes with the oil or grease and the head interacts with water. • Soap is made by reacting a fat in either solid or liquid phase with a strong base such as NaOH or KOH. The word equations are • Fatty Acids + Glycerol = Fat • Fatty Acid + strong base = soap • Glycerol is a skin softener

16. Detergents and Soaps Differ • Soap reacts with Ca and Mg ions in hard water and leaves a soap scum. This is just a solid precipitate from the reaction. Many households in HRM and in NS have water softener which use reactions to remove the Ca and Mg ions. • Detergents are a partial solution to the hard water problem and help to minimize the soap scum problem. • Detergents have a different ionic head than soaps. Again refer to your text p. 258. Detergents are completely soluble in hard water. • They are used for laundry, shampoos, dishwashing.

17. Household Cleaners • All purpose cleaners • Toilet bowl cleaners • Drain cleaners • Some specific examples are: Comet, Windex, Mr. Clean, Lysol, Javex • You remember that many of these are strong bases (i.e corrosive) and that they produce exothermic reactions

18. Chemicals and Our Environment • We depend on chemicals in our daily lives • Chemical and related industries such as the pharmaceutical industry employ over 250 000 people in Canada and are an important sector of the Canadian economy (see text p. 260)

19. Chemicals and Our Environment • The rate at which we are consuming the raw materials needed to produce the chemical products we use is increasing dramatically. There is a finite amount of resources available on our planet. • Chemical reactions normally produce waste or by-products. • These two illustrate conservation of matter and energy. • Unwanted by products pollute our environment • New technologies, government legislation, greener thinking will help to minimize the adverse effects of pollution.

20. Sulfuric and Nitric Acid Production • Sulfuric Acid is one of the most important industrial chemicals. See text p. 262. • S (s) + O2 (g) → SO2 (g) • SO2 (g) + O2 (g) → SO3 (g) (catalyst) • SO3 (g) + H2O (l) → H2SO4 (l) • These exothermic reactions generate energy which can be recycled.

21. Sulfuric and Nitric Acid Production • NH3 (g) + O2 (g) → NO2 (g) (Catalyst Pt) • NO2 (g) + H2O (l) → HNO3 (l) but today  • H2SO4 (l) + 2NaNO3→Na2SO4 + 2HNO3

22. Consequences of Using Sulfuric and Nitric Acid • When fossil fuels are burned in air, sulfur oxides and nitrogen oxides are formed • These may be represented as SOx and NOx • SO2 (g) + H2O (g) → H2SO3 (aq) sulfurous • SO3 (g) + H2O (l) → H2SO4 (aq) sulfuric • 2NO2 (g) + H2O (l) → HNO2 (aq) + HNO3 (aq) i.e. nitrous and nitric • When sulfurous, sulfuric, nitrous, and nitric acids are added to our atmosphere, together with the carbonic acid already present, they further acidify our precipitation to perhaps between a pH of 4 to 5

23. Consequences of Using Sulfuric and Nitric Acid • Increasingly acidic precipitation has a cascading effect on the environment • Acid precip is soluble in water which results in acidified lakes, rivers, brooks, ponds and streams. • Aquatic insects and organisms begin to die around a pH of 6. • Aquatic plants and plant-like micro-organisms die as the pH tends to 5. This further depletes the food supply of other aquatic organisms • Below a pH of 5 all lake/pond life disappears and the water appears crystal clear. • Birds and mammals relying on organisms in the aquatic ecosystem must either move or starve • Acid precip also dissolves metals (e.g. Al, Hg, Cu) which may leach into ground water and end up in lakes, ponds, streams etc. These metals are toxic for most organisms including humans

24. Acid Precipitation in Eastern Canada • Levels of acid precip are higher in Eastern Canada (east of Manitoba) • Where are most of the smelters and power plants in Canada and the US located? • In what direction do the winds blow across Canada and the US northeast? • Atlantic and Eastern Canada received a greater amount of acid precip. The soils of these areas tends to be more rocky and have less acid-neutralizing compounds

25. Using Chemistry to Control Acid Effects • Neutralization reactions cancel the effects of acids • Liming acidified lakes or soils may be a partial solution • Brooks and streams may continue to feed acidified water to limed lakes • Liming may kill plant and animals sensitive to calcium levels • Ecologically, it makes more sense to reduce sulfur and nitrogen oxides by reducing source emissions rather than trying to use chemistry to mitigate against the effects of acid precipitation.

26. Using Chemistry to Control Harmful Emissions • Since 1970, emissions of SOx and NOx have been reduced thanks to mandated catalytic converters in cars (see text p.248) • Coal-fired industrial plants have installed “scrubbers” on their smokestacks. • Scrubbers are anti-pollution devices which remove gases such as SO2 from smokestacks (up to 95%) • They work by adding calcium carbonate to the coal before combustion, then adding calcium oxide to the sulfur dioxide produced to give calcium sulfite. • Another way to reduce emissions is to remove sulfur from fuel by reacting hydrogen sulfide in fuels with oxygen then reacting the sulfur dioxide produced with more hydrogen sulfide to give water and sulfur. See text p. 267