Ch 8: Aqueous Solutions: Chemistry of the Hydrosphere

Transcription

1 Ch 8: Aqueous Solutions: Chemistry of the Hydrosphere H 2 S + Cu 2+ CuS(s) + 2H + (Fe, Ni, Mn also) HS O 2 HSO energy (supports life) 1

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3 Figure taken from Principles of Biochemistry, 2nd Ed. By Lehninger, Nelson, and Cox Chapter Outline 8.1 Solutions and Their Concentrations 8.2 Dilutions 8.3 Electrolytes and Nonelectrolytes 8.4 Acids, Bases, and Neutralization Reactions 8.5 Precipitation Reactions 8.6 Oxidation-Reduction Reactions 8.7 Titrations 8.8 Ion Exchange 3

4 Concentration of Solutions - Molarity The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. Mass-to-mass ratios: Mass-to-volume ratios: mg/kg solvent mg/l solvent Parts per million: 1 g 10 6 g = mg 10 3 g = mg L d H2O = o C Molarity (M) = moles of solute liters of solution Sample Exercise 8.1: Converting Mass-per- Volume Concentrations into Molarity Vinyl chloride (C 2 H 3 Cl, MW = 62.49) is one of the most widely used industrial chemicals. It is also one of the most toxic, and it s a known carcinogen. The maximum concentration of vinyl chloride allowed in drinking water in the U.S. is mg/l. What is that concentration in moles per liter? Plan: mg/l g/l mol/l = Molarity mg L x 1 g 1000 mg x 1 mol g = 3.2 x 10-8 mol/l 4

5 Sample Exercise 8.2: Converting Mass-per-Mass Concentrations into Molarity A water sample from the Great Salt Lake in Utah contains 83.6 mg Na + per gram of lake water (soln). What is the molar concentration of Na + ions if the density of the lake water is g/ml? AW Na = Plan: mg Na + g mol 1 g soln ml L d soln = g/ml mol L = M 83.6 mg Na + x 1 g 1000 mg 1 mol Na+ x g = x 10-3 mol Na + 1 ml soln 1 g soln x g x 1 L 1000 ml = x 10-4 L M = mol L x 10-3 mol Na = + = 4.22 M x 10-4 L Sample Exercise 8.3: Calculating the Quantity of Solute Needed to Prepare a Solution An aqueous solution called phosphate-buffered saline (PBS) is used in biology research to wash and store living cells. It contains ionic solutes including 10.0 mm Na 2 HPO 4 2H 2 O (MW = 178.0). How many grams of this solute would you need to prepare 10.0 L of PBS? MM Plan: mm M mol g use the volume Useful equation: M x V = moles 10.0 mmol L x 1 mol 1000 mmol x 10.0 L x g mol = 17.8 g M x V = moles 5

6 Chapter Outline 8.1 Solutions and Their Concentrations 8.2 Dilutions 8.3 Electrolytes and Nonelectrolytes 8.4 Acids, Bases, and Neutralization Reactions 8.5 Precipitation Reactions 8.6 Oxidation-Reduction Reactions 8.7 Titrations 8.8 Ion Exchange Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution. Dilution Add Solvent Moles of solute before dilution (i) M i V i = = Moles of solute after dilution (f) M f V f 6

7 Dilution Equation: M i V i = M f V f Sample Exercise 8.4: Diluting Solutions The solution used in hospitals for intravenous infusion called physiological or normal saline - is M NaCl. It may be prepared by diluting a commercially available standard solution that is 1.76 M NaCl. What volume of standard solution (V i ) is required to prepare 10.0 L of physiological saline? Collect data: M i = 1.76 M V i =? M i V i = M f V f M f = M V f = 10.0 L add H 2 O Add 881 ml of 1.76 M NaCl, and then dilute to the mark (10.0 L) V i = M f V f M i V i = (0.155 M )(10.0 L) (1.76 M) = L = 881 ml 7

8 Chapter Outline 8.1 Solutions and Their Concentrations 8.2 Dilutions 8.3 Electrolytes and Nonelectrolytes 8.4 Acids, Bases, and Neutralization Reactions 8.5 Precipitation Reactions 8.6 Oxidation-Reduction Reactions 8.7 Titrations 8.8 Ion Exchange 8

9 Strong Electrolytes: Nearly 100% dissociated into ions Conduct current efficiently Examples: ionic compounds such as NaCl, strong acids like HCl NaCl(s) Na + (aq) + Cl (aq) HCl(aq) + H 2 O(l) H 3 O + + Cl - (aq) Figure from John J. Fortman, J. Chem. Ed. Vol. 71, No. 1, 1994, p

10 Weak Electrolytes: Only partially dissociate into ions Slightly conductive Example: weak acids such as acetic acid Hydronium Ion: H 3 O + The higher the concentration of H 3 O + in solution, the stronger the acid. + 10

11 Non-electrolytes Substances in which no ionization occurs. There is no flow of electrical current through the solution Examples: molecular or covalent compounds such as ethanol 11

12 Chapter Outline 8.1 Solutions and Their Concentrations 8.2 Dilutions 8.3 Electrolytes and Nonelectrolytes 8.4 Acids, Bases, and Neutralization Reactions 8.5 Precipitation Reactions 8.6 Oxidation-Reduction Reactions 8.7 Titrations 8.8 Ion Exchange 12

13 Acids Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. Cause color changes in plant dyes (litmus paper). React with certain metals to produce hydrogen gas. 2HCl (aq) + Mg (s) MgCl 2 (aq) + H 2 (g) React with carbonates and bicarbonates to produce carbon dioxide gas 2HCl (aq) + CaCO 3 (s) CaCl 2 (aq) + CO 2 (g) + H 2 O (l) Aqueous acid solutions conduct electricity. Limestone dissolving in acid = cave Most metals dissolve in acids to produce H 2 13

14 Strong Acids and Bases Strong Acids/Bases: Completely ionized in aqueous solution (i.e., strong electrolytes) Memorize the 6 strong acids All other acids are weak KNOW! 14

15 Bases Have a bitter taste. Feel slippery. Many soaps contain bases. Cause color changes in plant dyes (litmus paper). Aqueous base solutions conduct electricity. A Brønsted acid is a proton donor A Brønsted base is a proton acceptor base acid conjugate acid conjugate base For every acid there is a conjugate base, and for every base there is a conjugate acid (conj. acid-base pairs). 15

16 Water: Acid or Base? Water as Base: HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl (aq) Water as Acid: NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH (aq) Amphiprotic: Acts as an Acid or Base. Preface: Double-Displacement Reactions Many of the reactions in this chapter follow this pattern: (+) (-) (+) (-) (+) (-) (+) (-) AB + CD AD + CB 16

17 Acid-Base (Neutralization) Reactions acid + base salt + water HCl(aq) + NaOH(aq) HOH(aq) + NaCl(aq) H 2 SO 4 (aq) + 2 KOH(aq) 2 HOH(aq) + K 2 SO 4 (aq) Net Ionic Equations 1. Write the balanced molecular equation. 2. Write the ionic equation showing the strong electrolytes completely dissociated into cations and anions. 3. Cancel the spectator ions on both sides of the ionic equation Tips: Do not break apart solids, liquids (such as H 2 O), and gases (e.g. CO 2 ) Do not break apart weak electrolytes (such as weak acids, e.g. HC 2 H 3 O 2 ) Carbonic acid, H 2 CO 3, decomposes into CO 2 and H 2 O An acid such as H 2 SO4 breaks apart into 2H + (aq) + SO 4 2-, not H SO

18 1. Write the balanced molecular equation. H 2 SO 4 (aq) + 2 KOH(aq) 2 H 2 O(aq) + K 2 SO 4 (aq) strong strong Not a strong strong electrolyte!! 2. Write the ionic equation showing the strong electrolytes completely dissociated into cations and anions. 2 H + SO K + 2OH - 2 H 2 O 2 K + SO Cancel the spectator ions on both sides of the ionic equation 2 H OH - 2 H 2 O Net ionic equation: H + + OH - H 2 O Sample Exercise 8.5: Writing the Net Ionic Equation for a Neutralization Reaction Write the net ionic equation describing the reaction that takes place when acid rain containing sulfuric acid reacts with a marble statue (calcium carbonate) - 1. Write the balanced molecular equation. H 2 SO 4 (aq) + CaCO 3 (s) H 2 O(l) + CO 2 (g) + CaSO 4 (aq) strong solid Not strong strong* electrolytes!! 2. Write the ionic equation showing the strong electrolytes completely dissociated into cations and anions. 2H + (aq) + SO 4 2- (aq) + CaCO 3 (s) H 2 O(l) + CO 2 (g) + Ca 2+ (aq) + SO 4 2- (aq) *generally speaking, CaSO 4 is insoluble in water, but not in strong acid. More on this later when we study solubility rules. 18

19 Sample Exercise 8.5: Writing the Net Ionic Equation for a Neutralization Reaction 3. Cancel the spectator ions on both sides of the ionic equation 2H + (aq) + SO 4 2- (aq) + CaCO 3 (s) H 2 O(l) + CO 2 (g) + Ca 2+ (aq) + SO 4 2- (aq) Net ionic equation: 2H + (aq) + CaCO 3 (s) H 2 O(l) + CO 2 (g) + Ca 2+ (aq) 19