ch2 - Water STEP-BY—STEP GUIDE Major Concepts The special...

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Unformatted text preview: Water STEP-BY—STEP GUIDE Major Concepts The special properties of water; all of which are derived from its polarity and hydrogen—bonding capability, are central to the behavior ofbicmolec'ales dissolved in it. The shape anal polarity of the water molecule are responsible for its hydrogen-bonding capability. This capability is what gives water its internal coheu sion and its versatility as a solvent. An increase in entropy (randbnuiess) of the ions dissolving from a crystal lattice drives the dissolution of crystalline sub— stances, and an increase in entropy of surrounding water molecules drives the behavior of hydrophobic and amphipathic molecules in water. individual hy-' drogen bonds, ionic interactions, hydrophobic inter- actions, and van der Waals interactions are weak rela- tive to covalent bonds. However, cumulatively, these small binding forces result in strong aesociations that are critical to the function of a biomolecule. The ionization of water is an experimentally mea- surable quantity, which is expressed as Kw, the ion product of water This KW is the basis for the pH scale, which desig— nates the concentration of if“ in an aqueous solution. The pH of the environment and the pit“a (the negative log of the equilibrium constant) of each of the partic- ipating biomolecules are extremely influential in the reactions of biological systems. Bnfiers are mixtures of a weak acid (proton donor) and its conjugate base (proton acceptor}. Buffers will resist changes in pH most effectively when the concentrations of the proton donor and proton ac— ceptor are equal. Under these circumstances, the pH of the solution is equal to the pll’fa of the weak acid, as il~ iustrated by the Henderson-Hasselbalch equation: [proton acceptor] {proton donor} pH m mg + log ’. The phosphate and bicarbonate systems are critical buffer systems in cytoplasm and blood. Water acts as a reactant as well as a solvent in many biochemical reactions. A molecule of water is eliminated in condensation re- actions, added to bonds in hydrolytic reactions, and _ split in the process of photosynthesis. What to Review The use of significant figures in science is a topic covered in most introductory high school chemistry courses. The rules for determining significant fig— ' ores are simple, but sometimes forgotten. Rule 1: Nonzero digits are always significant. A number such as 34.78 has four significant figures; 2.85 has three. Rule 2: Any and all zeros between any two other sig— nificant digits are significant. A number such as 301 has three significant figures; but 24,360 has only four. Rule 3: When experimental or measured quantities are used in addition or subtraction, the answer should contain the same number of digits after the decimal point as does the least precise quantity. If you acid 14.08 m, 6.186 In, and £5 m, the actual sum is 35.266, but should he reported as 35 meters. Rule 4: When multiplying and dividing, the number of sig« nificant figures in the result is the same as the small- est number of significant figures in the original set of numbers. Rule 5: Conversion factors, integers, and pure numbers are exact and therefore can be consid— ered to have an infinite number of significant figures. They do not affect the number of significant figures in the result. 12 Chapter 2 Water Topics for Discussion Answering each of the fotlowing questions, especialiy in the context of a study group dis- cussion, should help you understand the important points of this chapter. 2.1 Weak interactions in Aqueous Systems ' Hydrogen Bonding Gives Water lts Unusual Properties 1.‘ What is the shape of the water molecule? How is its unequal charge distributed over that shape? 2. How do hydrogen bonds contribute to the high melting and boiiing points of water? 3. Why is ice less dense than liquid water? Water Forms Hydrogen Bonds with Polar Solutes 4. What other functional groups of biomolecules can form hydrogen bonds? Why don’t —CH groups participate in H bonds? Water Interacts Electrostatlcally with Charged Solutes 5. What are some of the functional groups of biomolecules that will interact with water electrostaticaily? ' 6. What is the force or strength of the ionic attraction between a Na+ and Cl“ (10 nm apart} in water? in benzene? Entropy Increases as Crystalline Substances Dissolve 7. Describe What occurs when a crystailine salt dissoives in water in terms of the enthalpy (H) and the entropy (S) of the system. Nonpolar Gases Are Poorly Soluble in Water 8. What physical property of oxygen could contribute to its concentration being a limiting factor for aquatic animals in deep water? 1L Nonpolar Compounds Force Energetioally Unla vorable Changes in the Structure of Water I' 9. What is the driving force behind the formation of micelles? 10. Amphipathicity is important to the structure (and therefore the function) or" which biomolecules? Step-By-Step Guide 13 Van o'er Waals interactions Are Weak lnteratomic Attractions 11. Van der Waals interactions are a weak, transient subcategory of which type of nonco— valent interaction? Weak Interactions Are Crucial to Macromolecular Structure and Function 12. How do the four types of weak interactions among biomolecules compare in strength to each other and to covalent bonds? 13. Why would a zipper or aVelcro® strip be an appropriate analogy to weak interactions in biochemical reactions? 14. How are tightly bound water molecules in DNA and proteins different from “free” water molecules? Salutes Affect the Colligative Properties of Aqueous Solutions 15. Which of the following would have the greatest effect on the freezing point of a liter of water: the addition of 2 moi of NaCl or 2 mol of glucose? 2.2 Ionizatioe of Water, Weak Acids, and Weak Bases Pure Water is Slightly ionized 16. What specific information does the equilibrium constant of a reaction provide? The ionization of Water is Expressed by an Equilibrium Constant 1?. Why is the ion product of water (Kw) at 25 °C always equal to 1 X 10””? The pit Scale Designates the H+ and on" Concentrations 18. How does Kw relate to the pH scale? Weak Acids and Bases Have Characteristic Dissociation Constants 19. Does a strong acid have a greater or lesser tendency to lose its proton than does a weak acid? Does the strong acid have a higher or lower K3? A higher or lower pKa? Titration Curves Reveal the pKa of Weak Acids 20. At a pH equal to the pit; of a weak acid, What can he said about the concentrations of the acid and its conjugate base? What point on a titration curve indicates the Mg of that weak acid? 14 Chapter 2 Water 2.3 Buffering against pH Changes in Bioiogicai Systems Buffers Are Mixtures of Weak Acids and Their Coniugate Bases 21. What are the two equilibrium reactions that are simuitaneously adjusting during an experimental titration of a weak acid? 22. What does the relatively flat zone of a titration curve teli you about the pH changes within that zone? The Henderson-Hasselbalch Equation Relates pH, pKa, and Buffer Concentration 23. How does the Henderson-Hasseibalch equation prove that the 'pKa of a weak acid is equal to the pH of the soiution at the nudpoint of its titration? 3 i Weak Acids or Bases Buffer Cells and Tissues against pH Changes 24. What is the importance of the fanctionai groups of proteins that act as weak acids or bases in biological systems? 25. What is the extracelluiar buffering system that is used by animals with lungs? What is the primary intraceiluiar buffering system? 26. Where and how does the phosphate buffer system function? 2‘7. What are the three reversibie equilibria involved in the bicarbonate buffer system in animals with lungs? Untreated Diabetes Produces Life- Threatening Acidosis 28. What is the most sensitive aspect of cell function (mentioned many times in this chapter) in reiationship to changes in pH? 2.4 Water as a Reactant 29. What general types of cellular reactions form water, and which coasume water? much of these reactions are endergonic and which exergonic? Step-By-Step Guide 15 2.5 The Fitness of the Aqueous Environment for Living Brganisms 30. What are some of the ways that organisms on earth exploit the special properties of wa- ter in terms of temperature regulation? Discussion Questions for Study Groups c Consider two sets of solutions: one set is composed of a solution of pH 1 and a solution of pH 2; the second set is composed of a solution of pH 11 and a solution of pH 12. Both sets thus comprise solutions differing from one another in pH by one pH unit. is the difference in El” concentration between the solutions in each set equal? Why or Why not? 16 Chapter 2 Water SELF-TEST Do You Know the Terms? ACROSS (3. Describes a solution with a {3+} of 1 x 10‘s. 5. V: 4. Hydro molecules can form I energetically favorable interactions with water molecules. pH and the pHa of a buffer. @ The equilibrium constant for the Cam /2 I 4. figPOZ m H”' + POE" describes a "V \ 5. Compound containing both polar and nonpolar regions. . Denotes the concentration of H+ (and . Hydro . The numbers 1, 10, 100, and 1000 are . A piot of pii vs. OH“ equivalents added is a . EA is a proton . . The point in a reversible chemical reaction at which the . Water is often referred to as the a “universal “because of its ability to hydrate moiecules and screen charges. 1 therefore of OH“) in an aqueous solution. . The ion product of water; it is 1 X n ~1a - . D I itmemwflnfltu amaaanauflamm a a R describes the relationship between E E reaction HA : if“ + A‘ is also called the constant, K... molecuies decrease the entropy of an aqueous system by causing water molecules to become more ordered. a E placed at equal intervals on a a scale. 5] E . Reaction in which two reactants combats to form a single product with the elimination water. curve . . Weak interactions that are crucial to the structure and function of macromoiecules. . Describes a solution in which [OI-1”] is greater than [1711+] . A mixture of a weak acid and its conjugate base. Enzymes show maximum activity at a characteristic pH rate of product formation equals the rate of product breakdown to the starting reactants. . A” is a proton radius is approximately twice the distance of a covalent radius for a single bond. (3 words) . Stable structures formed by lipids in water, which are held together by hydrophobic ‘mteractions. buffer system. a sea i. nemaaaammmmaaanemuna E was a seamen nameanauama fiflflflfifi E M «a. v3: ’2. /33. V14. .27- ,a enemas a a - saunanaaaaam ale E E 3 Ella..- S The electrostatic interactions between the hydrogen and oxygen atoms on adjacent H20 moiecules constitute a . {2 words) Dissolved molecuies. Covalent bond breakage by the addition of water. Water molecuies readily dissolve compounds such as NaCi because they screen interactions between Na+ and (31‘. pH at which [HAG] = lAc’]. H2003 “:2 H” + 3005‘ describes a system. buffer . Describes a sohztion in widen [FF] is greats:r than QOHW. 20 “12/3. Noncovalent bonds have weaker bond than covalent bonds. Self-Test 17 Do You Know the Facts? In questions 1—4, decide whether the statement is true or false, and explain your answer. 1. The oxygen atom'in water has a partial positive charge. 5 .9 Ffilfiifl. t O". 15/ 2. Each hydrogen atom of water hears a partiai positive charge. a! \uuf Tats 3. H bonds can form oniy between water molecules. "Fats t 4. H bonds are relatively weak compared to covalent bonds. Trv‘x 'C In questions 5—9, choose the one best response. 5. You are running your first marathon on a very warm day. You start to sweat heavily and realize that you may be in danger of dehydration. Why is severe dehydration potentialiy life—threatening? A. Water is a solvent for many biomolecules. B. Water is a chemical participant in many biological reactions. 0. Water is necessary for buffering action in the body. ' ' D. Water’s attraction to itseif drives hydrOphobic interactions. @> Ail of the above are true. 6. Which of the following is true of hydrogen bonds? ® The attraction between the oxygen atom of’a water molecule and the hydrogen atom of another molecule con- stitutes a hydrogen bond. = , ‘ B. Hydrogen bonds form as covalent bonds between positiveiy and negativeiy charged ions. 0. Hydrogen bonds form between nonpolar portions of biomolecules. D. A and B are true. ' E. A, B, and C are true. 7. Which of the following is true of pH? A. pH is the negative logarithm of §OH"}. B. Lemon juice, which has a pH of 2.0, is 60 times more acidic than armnonia, which has a pH of 12.0. G. Varying the pH of a solution will alter the pita of an ionizable group in that solution. ® Varying the pH of a solution will alter the degree of ionization of an ionizable group in that solution. B. All of the above are true. 8. Consider a weak acid in a solution witha pH of 5.0. Which of the following statements is true? A. The weak acid is a proton acceptor. B. The weak acid has a iower affinity for its proton than does a strong acid. C. At its pKa, the weak acid will be totally dissociated. ® The [at] is 10-5 M. B. All of the above are true. 9 Water derives all its Special properties from its: 'A. cohesiveness and adhesiveness. high boiling point and melting point. v C. smali degree of ionization. p f D. polarity and hydrogembonding capacity. ' E. high dielectric constant. 18 Chapter 2 Water 10. ri‘he pKa values for the three ionizable groups on tyrosine are pKa {WCOOHj m 2.2, 13Ka (WNHQ‘) t 9.11, and pKa (—R) = 10.07. in which pi-l ranges wili this amnio acid have the greatest buffering capacity? A. At all pH’s between 2.2 and 10.07 B. At pH’s near 7.1 QC) At pH’s between 9 and i0 D. At pH’s near 5.7 E. Amino acids cannot act as buffers. _ 11. in a typical eukaryotic cell the pH is usually around 7.4. What is the [fill in a typical eukaryotic coil? A. 000000074 M B. 6.6 nM @ 4 x is“8 D. 2.3 nM E. 7.4 x 10—5 M 12. Carbonic acid has a Ka of 1.70 X 1.0”“4 and acetic acid has a Ka of 1.74 X 10‘5. Which of the following is true? A. Carbonic acid has the higher Ka of the two and would therefore be the best buffer at pH 6. B. The acid with the larger K8 is a better proton acceptor. C. Carbonic acid is the stronger acid and has a lesser tendency to lose its proton compared to acetic acid. D. Neither carbonic acid nor acetic acid can be effective buffers at any pH. ® Acetic acid is a weaker acid and has a iesser tendency to lose its proton compared to carbonic acid. 13. As climbers approach the summit of a mountain they usually increase their rate of breathing to compensate for the “thinner air" due to the lower oxygen pressures at higher elevations. This increased ventilation rate resuits in a reduction in the ievels of 002 dissolved in the biood. Which of the following accurately describes the effect of lowering the [Cozlmssolved on blood pH? A. It will result in an increase in the dissociation of H3003 —~> iii" + H005“ and a drop in pH. B. it will result in an increase in the dissociation of H2003 ~—> H20 + (302 and a drop in pH. It will result in an increase in the association of H" + HCO§ —-> H2003 and an increase in pH. D. It will resuit in a decrease in the association of H‘“ + H005,“ -—> H2003 and a decrease in pH. Q E. Lowering [COZIdjssolved has no effect on blood pH. ( . You mix 100 ml of solution of pH 1 with 100 ml of a solution of pH 3. The pH of the new 200 ml solution will be: ‘ ® 1.0. B. 2.0. C. 3.0. D D. between pH 1.0 and pH 2.0. E. between pH 2.0 and pH 8.0. 15. The pH of a sample of blood is 7.4; the pH of a sample of gastric juice is 1.4. The blood sample has an ET“): A. 5.29 times lower than that of the gastric juice. 33. a miilion times higher than that of the gastric juice. C. 6,000 times lower than that of the gastric juice. a million times lower than that of the gastric guice. E. 0189 times that of the gastric juice. Applying What You Know 1. Certain insects can “skate” aiong the top surface of water in ponds and streams. What property of water allows this feat, and what bonds or interactions are involved? 2. When you are very warm, because of idgh envirorunentai temperature or physical exertion, you perspire. What propertyCies) of water is your body expioiting when it sweats? Self-Test 19 3. What is the absolute difference in [H+] between two aqueous solutions, one of p}; 2.0 and one of pH 3.0? What is the {OI-1"] of the solution of pH 2.0? 4. Formic acid has a pit; of 3.75; acetic acid has a pKa of 4.76. Wtach is the stronger acid? Does the stronger acici have a greater or lesser tendency to lose its proton than the weaker acid? 5. In a solution of pH 4.7 6, containing both acetic acid and acetate, What can you say about the concentrations of acetic acid (CEfiCOOI-I) and acetate {CH3000”) present? [proton acceptor] [proton donor} Show new it proves that the pKa of a weak acid is equal to the pH of the solution at the midpoint of its titration. 6. The HendersomHasselbaich equation is pH = pita + log Define pH and Mia and explain how they are different. 20 Chapter 2 Water ANSWERS Do You Know the Terms? A6ROSS 3. alkaline 4. philic 6. solvent 7. pH 8. Kw 9. Hendersonmliasselbaich 10. dissociation 14. phobic 15. iog 16. condensation £8. 19 21 22 24 25 26 27 titration . noncovalent . basic . buffer . optimum . donor . equilibrium . acceptor Do You Know the Facts? 1. 2. 5. E 11. C 6. A 12. E ’7. D 13. C 8. D 14. D 9. D 15. D 10. C False; the oxygen atom has a partiai negative charge. True; because the oxygen atom is more electronega~ tive than the two hydrogen atoms, the electrons are more often in the vicinity of the oxygen, giving each of the hydrogen atoms a partial positive charge. False; they can form between any electronegative atom (usually oxygen or nitrogen) and a hydrogen atom covalently bonded to another electronegative atom in the same or another moiecuie. . True; hydrogen bonds have a bond dissociation energy of 28 lei/moi, whereas covalent single bonds have a stabilization energy of approximately 200—460 kJ/mol. Applying What You Know I. 2. Extensive hydrogen binding among water mole- cuies accounts for the surface tension that allows some insects to walk on water. The high heat of vaporization of water, which is a measure of the energy required to overcome attrac» DOWN 1. van der Waals 2. micelles 4. phosphate 5. amphipathic 9. hydrogen bond 11. soiutes 12. hydroiysis 13. ionic 14. pKa 17. carbonate 20. acidic 23. energy tive forces between moiecuies, allows your body to dissipate excess heat through the evaporation of the water that is perspired. a. The soiution with a pH of 2.0 has a {at} of 10-2 M, and an [OH‘] of 10"12 M; the solution with a pH of 3.0 has a [n+1 of 10""3 M. The difference in {in} is 10‘2 M W 10"“3 M e 0.009 M. 4. Fortnic acid is the stronger acid. It has a greater tendency to lose its proton than does acetic acid. 5. At pH 4.76, the concentrations of acetic acid and acetate in the solution will be equai. 6. pH m pKa + log {proton acceptor] [proton donor} At the midpoint of the titration, [proton acceptor] m [proton donor}. The log of 1 m 0, so pH m pit”a + 0; pH = pKa- . pH is the negative logarithm of [H+] in an aqueous solution. It provides a standard way to measure the H4” concentration in an aqueous solution. pKa is the negative logarithm of an equilibrium constant. It is equal to the pH at which a weak acid is one-haif dis- sociated; i.e., the pH at which there are equal con— centrations of a weak acid and its conjugate base. The pit}. occurs at the midpoint of the titration curve of a weak acid, the center of the range providing the maximum buffering capacity of the conjugate acid» base pair. The pKa is an integral property of an ion— izabie group. It is the extent of ionization of an ion- izabie group that varies with the pH of the solution. ' - -.. Latina-g: ...
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ch2 - Water STEP-BY—STEP GUIDE Major Concepts The special...

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