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Neutralization reactions - An acid lowers the pH of a solution. A base raises the pH of a solution. What happens if you mix them together? The reaction of a strong acid with a strong base will result in the formation of water and a salt of neutral pH. If just enough acid is added to a strong base, a neutral solution will result. If just enough base is added to a strong acid, a neutral solution will result. Stoichiometry of neutralization reactions - What is just enough? Notice is takes two moles of the base here to neutralize one mole of the sulfuric acid because it can contribute two protons. based on the concept of an equivalent one equivalent of acid can generate 1 mol of H+ one equivalent of base can generate 1 mol of OH- Normality = equivalents / L (Volume*normality = VN = moles of equivalents) 1M HCl = 1N 1M H2SO4 = 2N 1M Ca(OH)2 = 2N To neutralize an acidic or basic solution, an equal number of equivalents of OH- and H+ must react. Problem: What volume of 1M KOH must be added to neutralize 100mL of 0.25M H2SO4. What will the pH of the resulting solution be? In neutralization reactions, it is easiest to deal with equivalents: For neutralization to occur, we need to have the number of equivalents of base = number of equivalents of acid NbaseVbase = NacidVacid The 1M KOH solution is 1N The 0.25 M H2SO4 solution is 0.5N (1N)(Vbase ) = (0.5N)(0.100L) Vbase = 0.05 L = 50 mL Neutralization reactions results in the formation of salts with little or no acid base character and water. Dilution calculations - when working with acids the concentration of acid is often manipulated by dilution: Ex: 50mL of 1M HCl is diluted with water to a final volume of 100ml. What is the molarity of the final solution. molarity = mol / L The number of moles remains unchanged by the dilution; consequently: M1V1 = moles = M2V2 (1M)(0.050L) = M2(0.100L) M2 = 0.5M Controlling the pH of solutions - Buffer solutions Buffers resist changes in pH when acids or bases are added to them. Buffers contain a weak acid and its salt or a weak base and its salt: Examples:
Buffers contain both a weak acid and a weak base. As both are weak, the buffer solutions are not strongly acidic or basic. Futhermore, the presence of both an acid and base allows for the pH of the solution to be “buffered” (maintains constant pH) against the addition of strong acids or bases. For instance, CH3COOH / NaCH3COO buffer CH3COOH / CH3COO- The acid CH3COOH reacts with added base to remove OH- CH3COOH + NaOH à NaCH3COO + H2O instead of generating OH- the NaOH is neutralized The base CH3COO- reacts with added acid to remove H+ NaCH3COO + HCl à CH3COOH + NaCl instead of generating H+ the HCl is neutralized. Acid rain 1. Acids relevant to acid rain Many compounds of nonmetals with oxygen dissolve in water to form acids: CO2(aq) + H2O(l) --> H2CO3(aq) (carbonic acid) SO2(aq) + H2O(l) --> H2SO3 (sulfurous acid) With oxygen present 2SO2(aq) + 2H2O(l) + O2(g) --> 2H2SO4 (sulfuric acid) Similarly, NO2 and NO form HNO3 and HNO2 Water in the environment is naturally acidic due to the dissolution of CO2, pH = 5.6 2. 1872 - acid rain - Robert Smith used the term to describe acidic precipitation at the start of the industrial revolution. Acid rain has a pH < 5.6. 3. Primary source of acid rain - burning of fossil fuels, primarily coal, oil (also called petroleum) and natural gas (statistics from the department of energy) More than 85 percent of the energy used by the United States comes from fossil fuels. Oil supplies about 40 percent of our energy; natural gas provides about 25 percent, while coal provides about 20 percent. These fuels were formed millions of years ago from plants and animals that died and decomposed beneath tons of soil and rock. Why fossil fuels? SO2 primarily from electrical utilities (65%) Coal is used primarily as a fuel in electric power plants. In fact, more than half of the electricity generated in the United States comes from plants that burn coal. Coal contains sulfur impurities which are converted to SO2 when burned. Coal from eastern states 3-10%(w/w) sulfur. Western states less than 1%(w/w) sulfur. S(s) + O2(g) --> SO2(g) NOx At high temperatures (such as when burning coal in an power generation plant or gas in your engine), N2 combines with O2 at an appreciable rate to yield NOxs N2 + O2 --> NO + NO2 (not balanced) 4. Some info: a. average annual pH in much of northeastern US and northeastern Europe = 4.0-4.5 b. fog in West Virginia, pH = 1.5, LA pH = 3 c. more than 1000 bodies of water in eastern US are acidified Ontario - 100 lakes devoid of fish 48,000 threatened in Ohio river valley and Great Lakes region 5. Effects of acid waters a. damage tree leaves b. damage tree bark - black forest in Germany - by 1990 nearly half of the trees in the Black Forest were damaged by acid rain c. Acid soils harm roots d. Can leach out necessary minerals e. Can add unwanted minerals and heavy metals Ex. Al(OH)3(s) + 3 H+(aq) --> Al3+(aq) + 3 H2O(l) , Al3+ harmful to roots and inhibits absorption of other nutrients f. Dissolves limestone statues and buildings CaCO3(s) + 2 H3O+ --> Ca2+ + 3H2O + CO2(g) 6. Tough choices and fixes a) acid water can be treated with limestone (base) - $$$ and short term Ironically, acid lakes are beautifully clear b) stop burning coal, stop using internal combustion engines (as now found in cars, lawn mowers, etc...) c) Some measures to stop/minimize damage i. plastic protecting material on statures - short lived ii. remove pollutants from coal burning and cars Clean Coal Technology Program - mid-1980s, the U.S. Government has invested more than $2 billion in cleaning up coal burning process - state-of-the-art processes can filter out 99% of particles and more than 95% of acid rain components (private companies and state governments have added another $4 billion) "flue gas desulfurization units," or "scrubbers |