Chapter 6 - Neutralizing the Threat of Acid Rain
Why is rain naturally acidic?
Carbon dioxide in the atmosphere dissolves to a slight extent in water and reacts with it to produce a slightly acidic solution of carbonic acid: - The carbonic acid dissociates slightly leading to rain with a pH around 5.3
VII. Reactive nitrogen species
These reactive nitrogen species are biologically active, chemically active, or active with light in our atmosphere
Some consequences of acid rain:
▪ #1... acidification of freshwater - if acid rain falls for a longer period, then freshwater can become acidic, causing significant reduction in fish population ▪ it can also lead to deforestation, because it can kill many trees ▪ acid rain can also damage infra-structures ▪ and finally, it can have devastating effects on human health
Catalytic converter reaction:
2 NO -> N2 + O2
Obvious effect of acid deposition:
Obvious effect of acid deposition which can be observed is haze, especially in summer. Smoke and particulate matter in air such as soil dust and soot create haze.
How do the oxides of sulfur and nitrogen, sox and nox, behave as acids?
They are acid anhydrides, meaning "acids without water." When an anhydride is added to water, an acid is generated:
What about the NOx?
▪ 4 NO2(g) + 2 H2O(l) + O2(g) →4 HNO3(aq) - nitric acid ▪ Like sulfuric acid, nitric acid also dissociates to release the H+ ion: HNO3 (aq) → H+(aq) + NO3-(aq)
Essential background knowledge
▪ Acid has a capacity to release H+ ion in solution, and it can dissolve a solid to a certain extent, and it tastes sour. The more H+ ions an acid releases, the stronger the acid is. ▪ Base has a capacity to release OH- ion in solution, and it is slippery and tastes bitter. The more OH- ions a base releases, the stronger the base is. ▪ The strength of an acid or base is indicated by pH value. The pH values range from 1 to 14. Solutions with pH less than 7 are acidic and with pH greater than 7 are basic. A solution of pH equal to 7, the mid-point between the range of 1 and 14, is neutral. Distilled water is neutral. ▪ The lower the pH value of an acid, the stronger the acid is. The higher the pH of a base, the stronger the base is. ▪ Natural water such as rain and river/lake water has pH slightly less than 7. This is because natural water contains dissolved CO2 which actually causes the water acidic by the following reaction: ▪ CO2(g) + H2O(l) → H+(aq) + HCO3-(aq) ▪ Acidic solution can be neutralized (which means a solution that has a pH less than 7 can be a new pH of 7) by adding a base. Thus, bases can be called antacids.
Threats from acid rain
▪ Acid rain may pose even greater threat to human welfare than carbon emission causing global warming. ▪ The list of threats posed by acid rain include acidification of fresh water that causes: (1)significant reduction in fish population, (2) death of trees leading to deforestation, (3) damage of infra-structures such as dissolution of sculptures, and corrosion of bridge structures, and (4) deteriorating human health conditions.
What is acid rain and what threats does it pose?
▪ Acid rain refers to the precipitations which are much more acidic than normal rain. ▪ Acid rain greatly accelerates the rusting process: 4Fe(s) + 2O2(g) + 8H+(aq) → 4Fe2+(aq) + 4H2O(l) ▪ To protect solid iron from getting rusty the surface of the iron is coated with a thin layer of a second metal such as chromium or zinc. Iron coated with zinc is called galvanized iron. But galvanized iron is still susceptible to the presence of acid rain. ▪ Marble limestone slowly dissolves in presence of acid rain.
VIII. Nitrogen cycles in the biosphere
▪ Earlier we examined the link between energy production and the acidic emissions of SO2 and NOx. Here we will explore another link between food production and NOx emissions. ▪ Though the most abundant among all the nitrogen species, nitrogen gas (N2 ) is not reactive. But one reaction involving the nitrogen molecule (N2 ) is of utmost important: biological nitrogen fixation: N2 → NH3 → NH4+ ▪ In the first step in the above reactions (i.e., N2 → NH3), nitrogen-fixing bacteria remove nitrogen from the air and convert it to ammonia. ▪ The other form of reactive nitrogen that plants can absorb is the nitrate ion: NH4+ → NO2- → NO3- ▪ In both the first step (i.e., NH4+ → NO2-) and second step (i.e., NO2- → NO3-) in the above reactions, bacteria in the soil mediate the reactions. This process of converting ammonia in the soil to the nitrate ion is called nitrification. ▪ Finally, to come full circle, bacteria in the soil mediate the following reactions: NO3- → NO → N2O → N2 ▪ This process of converting nitrate eventually to nitrogen element is called denitrification. ▪ All of these pathways (i.e., nitrogen fixation, nitrification, and denitrification )are part of the nitrogen cycle which is a set of chemical pathways whereby nitrogen moves through the biosphere.
Lakes & Streams
▪ Effects of acid rain: damage to lakes & streams ▪ in the previous slide, we saw that the range for acid rain is roughly from 3-5 in this slide, we see the effects of acid rain and how it can damage lakes and streams ▪ as you can see on this scale, if lakes receive acid rains with a pH around 4, then lakes will biologically be dead ▪ at acid rain with a pH of 5-6, most aquatic life including fish , would disappear ▪ the normal pH range for aquatic life is about 6.5-9.5, at that range
VIV. How does the nitrogen cycle relate to acid rain?
▪ For plant growth reactive forms of nitrogen are needed. Since bacteria in the soil cannot supply enough amounts of ammonia, ammonium ion, or nitrate ion for plant growth, farmers use fertilizers. ▪ Fertilizers are obtained in large quantities from the following synthesis: N2(g) + 3H2(g) → 2NH3(g) ▪ As a fertilizer, this synthesized ammonia can be directly applied to the soil. This ammonia production in massive scale results in the large increase in reactive nitrogen. ▪ We now have the increases in reactive nitrogen from the two distinct man-made processes: from burning fossil fuels (energy production) and from fertilization (food production).
Haze
▪ Haze is most pronounced in summer when there is more sunlight to accelerate the photochemical reactions leading to sulfuric acid. ▪ Once inhaled, the acidic droplets attack sensitive lung tissue. Those most susceptible include the elderly, the ill, and those with asthma, emphysema, and cardiovascular disease. ▪ Clearly there is a connection between burning fossil fuels, acidic precipitation, and human health.
IV. Sulfur dioxide and acid rain
▪ Most of the sulfur dioxide emissions can be traced to coal-burning electrical utility plants. Coal contains sulfur. When coal burns sulfur reacts with oxygen and becomes SO2: S(s) + O2(g) → SO2(g) ▪ Since the sulfur content of coal varies, burning coal produces sulfur dioxide in varying amount. Once in the atmosphere, SO2 can react with oxygen to form sulfur trioxide (SO3): 2SO2(g) + O2(g) → 2SO3(g) ▪ This reaction is fairly slow, but is accelerated by the presence of finely divided solid particle. Once SO3 is formed, it reacts rapidly with water vapor in the atmosphere to form sulfuric acid (H2SO4): SO3(g) + H2O(l) → H2SO4(aq)
V. Nitrogen oxides and acid rain
▪ NOx emission can be traced not only to coal burning electrical power plants but also to heavy automobile traffic in urban settings. Since gasoline contains very little sulfur, automobiles cannot be a contributor to SO2. ▪ Automobiles contribute the emissions of nitrogen oxides. But, gasoline does not contain nitrogen. Therefore, nitrogen oxides cannot be formed from burning gasoline. Literally, this is correct. However, to burn gasoline air must be moved in car engines, and air contains much nitrogen. Although usually not reactive, nitrogen reacts with oxygen at high temperature of car engines: N2(g) + O2(g) → 2NO(g) ▪ This reaction is not limited to car engines. The same reaction occurs when air is heated to a high temperature in the furnace of a coal burning electrical power plant. ▪ NO is very reactive, so it can react with oxygen to form NO2: 2NO + O2 → 2NO2 ▪ However, this reaction does not occur in a short time (e.g., while driving your car to work) because it requires high concentration of NO to proceed quickly. ▪ VOCs (volatile organic compounds and the hydroxyl radical (.OH) are the key players in rapidly producing NO2 from NO. The actual reaction pathways are quite complicated: VOC + .OH → A + O2 → A' + NO → A" + NO2 ▪ A, A', and A" represent reactive molecules synthesized from the VOCs.
Nitrogen dioxide
▪ Nitrogen dioxide is a highly reactive, poisonous, red-brown gas with a nasty odor. The hydroxyl radical reacts with nitrogen dioxide to rapidly form nitric acid: NO2(g) + .OH(g) → HNO3(l) ▪ Since the entire series of reactions involves byproducts from gasoline combustions such as VOCs, NO, and .OH, acid rain caused by nitrogen oxides is commonly observed in big cities such as Los Angeles, Houston, etc. ▪ Those three states (Ohio, Pennsylvania, Indiana) also lead in NOx emissions not because of heavy traffic but because of heat coal burning power plants. Again, high NOx emissions also are found in large urban areas with heavy automobile traffic.
Powerplants & Automobiles
▪ Power plants and automobiles emit SO2 and NOx. These contribute to haze. The tiny droplets of sulfuric acid, which is formed through the chain reactions (SO2 → SO3 → H2SO4), coagulate to produce large droplets. These droplets form an aerosol with particles. These particles of sulfuric acid do not absorb sunlight. Rather, they scatter sunlight, resulting in haze.
How does the sulfur get into the atmosphere?
▪ The burning of coal. ▪ Coal contains 1-3% sulfur and coal burning power plants usually burn about 1 million metric tons of coal a year! ▪ Burning of sulfur with oxygen produces sulfur dioxide gas, which is poisonous. S(s) + O2(g) → SO2(g) ▪ Once in the air, the SO2 can react with oxygen molecules to form sulfur trioxide, which acts in the formation of aerosols. 2 SO2(g) + O2(g) → 2 SO3(g)
X. Damage to lakes and streams
▪ The capacity of a lake or other body of water to resist a decrease in pH is called its acid-neutralizing capacity (ANC). ▪ The surface geology of the Midwest is limestone which has a great ANC because limestone reacts with H+ (acid) and thus reduces H+ (i.e., blocking the decrease in pH). Therefore, despite the deposition of acid rain the pH of the lakes in the Midwest would remain more or less constant. ▪ In contrast to the Midwest, many lakes in New England are surrounded by granite which is much less reactive and thus has low ANC. Consequently, many lakes show a gradual acidification ▪ As pH become low (more acidic), granite erodes and release Al3+ ions. Fish exposed to high concentrations of aluminum ions may develop a thick mucus on their gills that suffocates them. ▪ Additionally, aluminum ions (Al3+) react with water to generate H+, increasing the acidity, which in turn dissolves more aluminum ions to further aggravate the problem. ▪ Nitrogen saturation occurs when an area is overloaded with "nitrogen," that is, where the reactive forms of nitrogen entering an ecosystem exceed the system's capacity to absorb the nitrogen. ▪ Once nitrogen saturation develops, the nitrate ion accumulates with an accompanying rise in acidity. As a result, the soils have little ability to neutralize acidic precipitation before it runs off into the lakes and streams.
III. Contributors to acid rain
▪ The main contributors to acid rain are sulfur dioxide (SO2), sulfur trioxide (SO3), nitrogen monoxide (NO), and nitrogen dioxide (NO2). ▪ These compounds are collectively designated SOx and NOx, better known as "sox and nox."
But acid rain can have pH levels lower than 4.3-where is the extra acidity coming from?
▪ The most acidic rain falls in the eastern third of the United States, with the region of lowest pH being roughly the states along the Ohio River valley. ▪ The extra acidity must be originating somewhere in this heavily industrialized part of the country
VI. Forms of acid precipitation, haze, and human health
▪ The term acid deposition includes wet deposition and dry deposition. ▪ Wet deposition includes rain, snow, fog, and cloud-like suspensions of microscopic water droplets. ▪ Dry deposition includes solid particles (aerosols) of the acidic compounds such as ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4). ▪ Dry deposition can be just as significant as the wet deposition of the acids in rain, snow, and fog. These aerosols also contribute to haze. ▪ Acid deposition in other forms than rain is often more acidic and damaging than the acid rain itself.
Effects of acid rain: damage to marble
▪ These statues are made of marble, a form of limestone composed mainly of calcium carbonate, CaCO3. ▪ Limestone and marble slowly dissolve in the presence of H+ ions: CaCO3(s) + 2 H+(aq) → Ca2+(aq) + CO2(g) + H2O(l) ************************* ▪ acid rain not only affects the ecosystems, it also affects non biogenic systems like sculptures and buildings - in this case you see the damages to the marbles ▪ marble has the chemical composition of CaCO3... ▪ if you look at the bottom, that chemical reaction ... that marble and solid CaCO3 reacts with H+ ion which is from the acid rain... then that marble solid dissolves and becomes ion Ca2+ and CO2 gas and water and liquid, meaning that solid marble disappears - that's what you see in those two contrasting pictures... you see many spots of dissolution... this is because acid in general is very corrosive
Acids & Bases
▪ What is an acid? ▪ acid is a compound that can release H+ ion in solution ▪ for example, we know HCl is a famous acid (hydrochloric acid) - when we dissolve that HCl, it releases H+ , so that's an acid ▪ Another example is H2CO3, which is what they call carbonic acid - once it dissolves in water, it releases H+ so H2CO3 is another example of an acid ▪ Acid in general tastes sour ▪ What is a base? ▪ a base is a compound that can release OH- in solution ▪ for example, KOH is a base, and it releases OH- in solution ▪ another base is NaOH because it releases OH- ion in solution ▪ bases in general taste bitter and they are slippery to touch
What about the NOx?
▪ how do those nitrogen oxides get into the atmosphere? ▪ they are from automobiles because automobiles omit hydrogen oxides, the NOx - however, gasoline does not contain nitrogen ... so where does that nitrogen originally come from? ▪ to burn gasoline, air must be moved into car engines & air contains lots of nitrogen ▪ the ultimate source is air itself ▪ once that air gets into the engine, then N2 and O2 inside of the engine react with each other. - this reaction cannot occur at normal temperature, only at the high temperature, and inside the engine it is hot - in that hot environment, N2 and O2 can produce NO which is the first NOx ▪ that NO is very reactive, so it can react with oxygen to form NO2, the second NOx ▪ then NO2 emits from automobiles into the atmosphere, and then in the atmosphere, NO2 reacts with water vapor and oxygen gas , producing HNO3, the nitric acid ▪ since its an acid, it can release H+ , making acid rain
how does the sulfur (or the SOx) get into the atmosphere?
▪ it is from the burning of coal ▪ this is because coal contains 1-3% of sulfur ▪ mostly the power plants burn huge amounts of coal each year ▪ As you can see in the reaction right beneath, the sulfur in solid, which is coal, gets burned , meaning reacting with O2 gas, and producing SO2, which is the first SOx ▪ and then that SOx, if you get down to the bottom, second most row, that SO2 reacts with moisture in the atmosphere H2O, and it becomes H2SO3 which is an acid ▪ or that SO2 continues to react with the O2 in the atmosphere, and it becomes SO3... the other SOx ▪ that SO3 again reacts with the water vapor in the air, and it becomes H2SO4... another acid - so those to acids in red, they are all acids and they all originate from the sulfur in the coal
Contributors to acid rain
▪ major contributors to acid rain are oxides of sulfur and nitrogen ▪ sulfur oxide, we have examples of SO2, SO3, ▪ for nitrogen oxide, we have NO and NO2 - these compounds are collectively called SOx and NOx ... "sox and nox" - these are the contributors to acid rain
Why is rain naturally acidic?
▪ rain water is naturally acidic .. that means , even if there are no obvious signs of pollution, it is naturally acidic - Why? ▪ first of all, the atmosphere contains CO2 gas, and the water vapor in the atmosphere H2O reacts with that gas and produces H2CO3... ▪ that H2CO3 is carbonic acid ▪ that carbonic acid releases H+ ion ▪ since rain water naturally contains H+ ion, rain water naturally becomes slightly acidic, even if there are no significant signs of pollution - as a result , rain water carries a pH level of about 5.3 ▪ So, natural rain water, does not have a pH of 7, but is slightly acidic, at 5.3 - this is not the sign for acid rain at all ▪ acid rain has a pH level range from 3-5, it is lower than natural rain water
Rotation between pH and [H+]
▪ relation between pH and [H+] ion concentration ▪ suppose you have a solution whose pH is equal to 1 ▪ then the amount of hydrogen ion concentration in that solution is equal to 10^-1 molarity - you don't have to know about that concentration molarity, but just look at that number ▪ when you have a solution that has a pH value of 3... - then you have a hydrogen ion concentration of 10^-3 molarity ▪ pH 7 ... then you have an H+ concentration of 10^-7 molarity ▪ pH 10 ... H+ ion concentration of 10^-10 molarity ⭐️As you can see... in that relation, to find out the H+ ion concentration, you simply take that given pH and bring it to the superscript of the base 10 , with the (-) sign attached in the front ▪ Now let's look at the example question ▪ How many times is solution A of pH = 2 more acidic than solution B of pH = 4 ▪ first of all, solution A has a lower pH than solution B, so solution A must be much more acidic... the question is how many times - you may think 2 vs. 4 is twice as much...but that's not how it works ▪ to find the answer, we have to first find out the concentration of hydrogen ions existing in each solution ▪ SO, solution A has pH 2, which means it has a H+ concentration of 10^-2, which is equal to 0.01 ▪ Solution B has pH 4, which means it has H+ concentration of 10^-4 which is equal to 0.0001 ▪ So now , if you divide the bigger number, by the smaller number, then it becomes 10^2, which is 100... ▪ this means solution A has a strength that is greater than solution B by 100 times... - solution A is 100x more acidic than solution B
The double whammy of NOx emissions:
▪ the first problem is that they contribute to acid deposition that in turn forms haze and diminish air quality. ▪ The second problem is that NOx emissions are a form of reactive nitrogen for food production, which along with fertilizer will disturb the balance within the nitrogen cycle on our planet. ▪ With too much reactive nitrogen, ecosystems become overloaded. The origin of the reactive nitrogen doesn't matter—it could be from acid deposition or it could be from excess fertilization. Regardless of the source, the buildup of reactive nitrogen can have devastating consequences.
Analysis of rain for specific compounds confirms that the chief culprits are:
▪ the oxides of sulfur and nitrogen: - sulfur dioxide (SO2), sulfur trioxide (SO3), nitrogen monoxide (NO), and nitrogen dioxide(NO2). ▪ These compounds are collectively designated SOx and NOx and often referred to as "sox and nox"
Another mechanism responsible for producing sulfuric acid is:
▪ the reaction of the hydroxyl radical (•OH) with SO2: 2OH• + SO2 → H2SO4 ▪ This reaction goes faster in intense sunlight, because the hydroxyl radical is formed from ozone ad water in the presence of sunlight. ▪ Sulfur dioxide emissions are highest in states with many coal-fired electric power plants, steel mills, and other heavy industries that rely on coal. Ohio, followed by Pennsylvania and Indiana, leads the nation in SO2 emissions.
how are oxides of sulfur and nitrogen behaving as acid?
▪ they are acid anhydrides, meaning "acids without water" - when these anhydrides are added to water, then acid is generated ▪ For example, when SO2 reacts with water, it produces an acid H2SO3... ▪ OR.. when SO3 reacts with water, it produces H2SO4, which is again an acid - so they can now relesase H+ ▪ what about NOx? ▪ when NO2 reacts with water, it produces 4HNO3, so it can now release H+
Common substances & pH values
▪ you may know or may have heard of this scale called pH ▪ pH scale is used to indicate the strengths of acids and bases ▪ so as the pH value of a solution becomes less and less than 7, then that solution becomes more and more acidic - so as the pH value of a solution becomes greater and greater than 7, then it becomes more and more basic ▪ So let's look at some examples... ▪ the most acidic value listed above is lemon juice, it has a pH level a little over 2 ... ▪ then the soda pop coca cola is quite acidic ▪ then we have a pH range from around 3 to 5, that is acid rain pH, which is a result of pollution ▪ then normal rain water, not being affected by pollution is still slightly acidic, and we'll see the reason why later on ▪ Then if you look at the basic side, our blood is slightly basic, and household ammonia is very basic, close to 12 on the pH scale - so all of the soaps are basic - and as we know, when we touch soap it is slippery - Lye is the most basic
