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Chapter 8: Water as a resource
Homework assignment: From the end of the chapter, questions 3,5,7,9,13,19,21,23,27,29,35,41,44,47,54,59,60.
From where do we get potable (fit for consumption) drinking water?
Surface water: from lakes, rivers, reservoirs Ground water: pumped from wells drilled into underground aquifers
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What is a solution?
A solution is a homogeneous mixture of uniform composition. Solutions are made of solvents and solutes.
Substances capable of Substances dissolved in a dissolving other substances- solvent- usually present in the usually present in the greater lesser amount amount
When water is the solvent, the solution is an aqueous solution.
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Working with solutions: concentration terms (1)
Parts per hundred (percent) 20 g of NaCl in 80 g of water is a 20% (w/w) NaCl solution
Parts per million (ppm) (w/w) solution: 1 g solute 1 mg solute 1 mg solute 1 ppm = = = 1,000, 000 g water 1,000 g water 1 L water
Parts per billion (ppb) (w/w) solution:
2 g Hg 2 ×10-6 g Hg 2 µg Hg 2 ppb Hg = = = 9 3 1 L H O 1×10 g H 2O 1×10 g H2O 2
Working with solutions: concentration terms (2)
Molarity (M) = moles solute/liter of solution
Consider a 1.0 M NaCl solution: [NaCl] = 1.0 M = 1.0 mol NaCl/L solution Because the molar mass of NaCl is 58.44 g, 1 liter of this solution would have 58.44 g NaCl enough water to give a total volume of 1.0 L.
This solution also is 1.0 M in Na+ and 1.0 M in Cl- [Na+] = 1.0 M and [Cl-] = 1.0 M This solution contains 22.99g sodium ions and 35.44g chloride ions
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What is the concentration (in M and mass %) of the resulting solution when you add 5 grams of NaOH to 95 mL of water?
Answer: 5% NaOH, 1.25M NaOH.
see solution properties worksheet for additional questions.
Properties of water: polarity
Lewis structures Space-filling Charge- density
Region of partial negative charge
Regions of partial positive charge Charge-density
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Electronegativity is a measure of an atom’s attraction for the electrons it shares in a covalent bond
On periodic EN Values assigned by Linus Pauling, table, EN winner of TWO Nobel Prizes increases
Polar covalent bonds A polar bond forms between atoms that have highly different electronegativity values. The electrons are NOT shared equally between the two atoms in this case, and the electrons spend more time around the more electronegative atom.
Partial charges reflect the fact that the O-H bond is polar.
Water is polar due to (1) the polarity of the covalent bonds, (2) the number of bonds, and, (3) the shape of the molecule.
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Determine the polarity of these bonds and molecules
O-H C-H
N-H CH4
NaCl NH3
H-H CO2
F-F
C-F
Water forms a network of hydrogen bonds
Polarized bonds allow hydrogen bonding to occur.
A hydrogen bond is an electrostatic attraction between an atom bearing a partial positive charge in one molecule and an atom bearing a partial negative charge in a neighboring molecule. The H atom must be bonded to an O, N, or F atom. Hydrogen bonds typically are only about one-tenth as strong as the covalent bonds that connect atoms together within molecules. H–bonds are intermolecular bonds. Covalent bonds are intramolecular bonds.
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Solid water forms a hexagonal lattice
There is a network of hydrogen bonds that forms within the structure. Six hydrogen bonds connect groups of six water molecules. Note the spaces within the structure.
Properties of aqueous solutions: solubility
Substances that will dissociate in solution are called electrolytes.
Ions are simply charged particles-atoms or groups of atoms.
They may be positively charged – cations,
Or negatively charged- anions.
Dissolution of NaCl in Water
The polar water molecules stabilize the ions as they break apart (dissociate).
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Polyatomic ions are a stable arrangement of two or more atoms
Na2SO4 (sodium sulfate) dissociates in water to form:
Na+ and The sulfate group stays together in sodium ions solution. Sulfate ions What is the ratio of sodium to sulfate ions in solution? Also see ionic substances worksheet.
Aqueous solution properties: conductivity
When ions (charged particles) are in aqueous solutions, the solutions are able to conduct electricity.
(a) Pure distilled water (non-conducting) (b) Sugar dissolved in water (non-conducting): a nonelectrolyte (c) NaCl dissolved in water (conducting): an electrolyte
see conductivity part of water properties worksheet
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Simple generalizations about ionic compounds allow us to predict their water solubility
Hard water contains high concentrations of dissolved calcium and magnesium ions. Soft water contains few of these dissolved ions. 2+ 2– Ca (aq) + CO3 (aq) → CaCO3(s) A hardness of 10 ppm indicates that 10 mg of CaCO3 could be formed from the Ca2+ ions present in 1 L of water.
One way to define an acid is as a substance that releases hydrogen ions, H+, in aqueous solution.
Because the hydrogen ion has no electron, and only one proton (hence the positive charge), the hydrogen ion sometimes is referred to as a proton. Consider hydrogen chloride gas, dissolved in water: H O HCl(g) 2 H+(aq) + Cl–(aq) (a proton)
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Formation of hydronium ions H+ ions are much too reactive to exist alone, so they attach to something else, such as water molecules.
When dissolved in water, each HCl donates a proton (H+) to an + H2O molecule, forming H3O , the hydronium ion. Hydronium ion. Often we - H The Cl (chloride) ion simply write H+, but + remains unchanged. O+ understand it to mean H3O H H when in aq. solutions.
The overall reaction is:
+ – HCl(g) + H2O(l) H3O (aq) + Cl (aq) hydronium ion
A base is any compound that produces hydroxide ions (OH-) in an aqueous solution Characteristic properties of bases • Bitter taste • Slippery feel when dissolved in water • Turn red litmus paper blue
Sodium hydroxide produces one equivalent of OH- H O NaOH(s) 2 Na+ (aq) + OH- (aq) Sodium hydroxide Sodium ion Hydroxide ion
Calcium hydroxide produces 2 equivalents of OH–
Ammonia reacts with water to produce one equivalent of hydroxide ions
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Neutralization of acids and bases Acids and bases can react with each other in neutralization reactions. For example, hydrochloric acid and sodium hydroxide can react and neutralize each other.
In neutralization reactions, hydrogen ions from an acid combine with the hydroxide ions from a base to form molecules of water. The other product is a salt (an ionic compound).
Neutralization reactions can be represented in ionic form or net ionic form, for example, the reaction of hydrobromic acid with barium hydroxide.
The ionic reaction contains spectator ions, whereas the net ionic reaction does not.
The pH of a solution is a measure of the concentration of the H+ ions present in that solution
pH = -log[H+]
What is the pH of a 1mM HCl solution? What is the pH of a 10mM HCl solution?
Since pH is a log scale based on 10, a pH change of 1 unit represents a power of 10 change in [H+]. That is, a solution with a pH of 2 has a [H+] ten times that of a solution with a pH of 3.
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How do we measure the pH of a basic solution?
The ion product of water, Kw, is a constant:
+ - -14 o Kw = [H ][OH ] = 1 x 10 (at 25 C)
If we know the hydroxide ion concentration, we can calculate the [H+], and use the pH expression to solve for pH. For example, what is the pH of a 15mM NaOH solution? If [H+] = [OH-], the solution is neutral (pH = 7) If [H+] > [OH-], the solution is acidic (pH < 7) If [H+] < [OH-], the solution is basic (pH > 7)
The pH of rain - where is rain most acidic in the US?
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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:
CO2(g) + H2O(l) H2CO3 (aq) carbonic acid
+ - H2CO3 (aq) H (aq) + HCO3 (aq)
The carbonic acid dissociates slightly, leading to rain with a pH around 5.3. Carbonic acid is an acid anhydride.
What are the main culprits of acid rain?
Sulfur dioxide (SO2) emissions are highest in regions with many coal- fired electric power plants, steel mills, and other heavy industries that rely on coal. Allegheny County, in western Pennsylvania, is just such an area, and in 1990 it led the United States in atmospheric SO2 concentration.
The highest NOx emissions are generally found in states with large urban areas, high population density, and heavy automobile traffic. Therefore, it is not surprising that the highest levels of atmospheric NO2 are measured over Los Angeles County, along with Oakland/Berkley/San-Francisco, Miami, Dallas, New York, Boston, and Chicago.
Oxides of sulfur and nitrogen are acid anhydrides, literally, acids without water. Their reactions with water produces protons and lowers the pH of the solution.
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Effects of acid rain (1): damage to material
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.
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Effects of acid rain (2): damage to human health
NO2 , NO3, and SO2 are known lung irritants. Exposure typically is accompanied by lung disorders such as asthma, bronchitis, and lung cancer.
Effects of acid rain (3): impaired visibility
NO2, NO3, and SO2 react with water and their products aggregate around particulates, forming haze that reduces visibility.
hazy day, 20 mile visual range clear day, 100 mile visual range
Effects of acid rain (4): damage to lakes and streams
This effect is not as severe in the midwest because the vast majority of the lakes are surrounded by limestone. This results in the “buffering” the pH of the lakes by reactions of the acid with calcium carbonate and hydrogen carbonate ion. In New England and Upstate NY, lakes are surrounded by granite and cannot be “buffered” this way.
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Controlling acid rain
NOx and SOx emission reduction were explicit goals of the clean air act amendments of 1990. So far:
NOx emissions due to utilities and industry have declined by using new technologies such as low NOx burners. In contrast, the transportation sector continues to present a challenge. Catalytic converters are improving but the number of vehicles continues to increase.
SOx emissions are tackled using 3 main strategies: 1. Switching to low sulfur coal 2. Coal cleaning. Using water, the coal is crushed, washed with water, and the sulfur-containing compounds are removed by filtration. 3. Scrubbing. Combustion emissions are passed through a wet slurry of powdered limestone. The main problem with each of these strategies is cost.
Two water purification techniques:
Distillation Reverse osmosis
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