In each pair, select the stronger acid. (a) Pyruvic acid

Predict the position of equilibrium, and calculate the equilibrium constant, Keq, for each acid-base reaction. (a) CH3NH2 1 CH3COOH EF CH3NH3 1 1 CH3COO2 Methylamine Acetic acid Methylammonium Acetate ion ion (b) CH3CH2O2 1 NH3 EF CH3CH2OH 1 NH2 2 Ethoxide ion Ammonia Ethanol Amide ion

Calculate Keq for a reaction with DG8 5 217.1 kJ/mol (24.09 kcal/mol) at 328 K. Compare this value to the 1 3 103 seen at 298 K.

Write an equation for the reaction between each Lewis acid-base pair, showing electron fl ow by means of curved arrows. (a) (CH3CH2)3B 1 OH2 9: (b) CH3Cl 1 AlCl3 9:

For each conjugate acid-base pair, identify the fi rst species as an acid or base and the second species as its conjugate acid or base. In addition, draw Lewis structures for each species, showing all valence electrons and any formal charge. (a) HCOOH HCOO2 (b) NH4 1 NH3 (c) CH3CH2O2 CH3CH2OH (d) HCO3 2 CO3 22 (e) H2PO4 2 HPO4 22 (f) CH3CH3 CH3CH2 2 (g) CH3S2 CH3SH

Complete a net ionic equation for each proton-transfer reaction using curved arrows to show the fl ow of electron pairs in each reaction. In addition, write Lewis structures for all starting materials and products. Label the original acid and its conjugate base; label the original base and its conjugate acid. If you are uncertain about which substance in each equation is the proton donor, refer to Table 4.1 for the relative strengths of proton acids. (a) NH3 1 HCl EF (b) CH3CH2O2 1 HCl EF (c) HCO3 2 1 OH2 EF (d) CH3COO2 1 NH4 1 EF

Complete a net ionic equation for each proton-transfer reaction using curved arrows to show the fl ow of electron pairs in each reaction. Label the original acid and its conjugate base; then label the original base and its conjugate acid. (a) NH4 1 1 OH2 EF (b) CH3COO2 1 CH3NH3 1 EF (c) CH3CH2O2 1 NH4 1 EF (d) CH3NH3 1 1 OH2 EF

Each molecule or ion can function as a base. Write a structural formula of the conjugate acid formed by reaction of each with HCl. (a) CH3CH2OH (b) O HCH (c) (CH3)2NH (d) HCO3

In acetic acid, CH3COOH, the OH hydrogen is more acidic than the CH3 hydrogens. Explain.

Which has the larger numerical value? (a) The pKa of a strong acid or the pKa of a weak acid (b) The Ka of a strong acid or the Ka of a weak acid

In each pair, select the stronger acid. (a) Pyruvic acid (pKa 2.49) and lactic acid (pKa 3.08) (b) Citric acid (pKa1 3.08) and phosphoric acid (pKa1 2.10)

Arrange the compounds in each set in order of increasing acid strength. Consult Table 4.1 for pKa values of each acid. O O (a) CH3CH2OH Ethanol C6H5COH Benzoic acid HOCO Bicarbonate ion O O (b) HOCOH Carbonic acid CH HCl 3COH Acetic acid Hydrogen chloride

Arrange the compounds in each set in order of increasing base strength. Consult Table 4.1 for pKa values of the conjugate acid of each base. O (a) NH (b) 3 CH3CH2O OH CH3CO OH HOCO O O CH3CO O CH3CO NH2 NH 3 O HOCO (c) H (d) 2O Ther

If the DG for a reaction is 24.5 kcal/mol at 298 K, what is the Keq for this reaction? What is the change in entropy of this reaction if DH 5 23.2 kcal/mol?

Calculate the Keq for the following reactions from the pKas given. State whether the reaction is exergonic or endergonic. pKa = 41.2 pKa = 15.9 CH3 CH2 O (a) HO CH3 CH2 pKa = 20.2 pKa = 15.9 O O O (b) HO H C pKa = 24 pKa = 71 C H H H C C (c) 4.

Answer True or False to the following statements about energy diagrams and reactions. (a) A reaction coordinate diagram is used to visualize the change in the internal energy of chemical structures that occurs during chemical reactions. (b) Thermodynamics is the study of the energies of structures that are represented by wells on reaction coordinate diagrams. (c) Kinetics is the study of the rates of chemical reactions. (d) One part of a reaction mechanism would be the understanding of which bonds break and form during a reaction. (e) Thermal reactions occur via collisions between molecules, and the more energy in those collisions the greater the rate of the reactions. (f) The enthalpy of a reaction is the sole determinant of whether it will or will not occur. (g) An exergonic reaction will always occur during the life span of the standard human being

Answer true or false to the following statements about the mechanism of acid-base reactions. (a) The acid and base must encounter each other by a collision in order for the proton to transfer. (b) All collisions between acids and bases result in proton transfer. (c) During an acid-base reaction the lone pair on the base fi lls the A-H antibonding sigma orbital.

In each of the following three reaction coordinate diagrams state whether (a) The reaction is exothermic or endothermic. (b) The reaction is the slowest, the fastest, or intermediate in rate. (c) If all three reactions have the same entropy change between the reactant and product, which reaction has the largest favorable DG . Reaction coordinate Reaction coordinate Reaction coordinate D H8 D H8 D H8 (a) (b) (c)

The acid-base chemistry reaction of barium hydroxide (Ba(OH)2) with ammonium thiocyanate (NH4SCN) in water creates barium thiocyanate, ammonia, and water. The reaction is highly favorable, but also so endothermic that the solutions cools to such an extent that a layer of frost forms on the reaction vessel. Explain how an endothermic reaction can be favorable.

Unless under pressure, carbonic acid (H2CO3) in aqueous solution breaks down into carbon dioxide and water, and carbon dioxide is evolved as bubbles of gas. Write an equation for the conversion of carbonic acid to carbon dioxide and water

Will carbon dioxide be evolved when sodium bicarbonate is added to an aqueous solution of each compound? Explain. (a) Sulfuric acid (b) Ethanol (c) Ammonium chloride

Acetic acid, CH3COOH, is a weak organic acid, pKa 4.76. Write an equation for the equilibrium reaction of acetic acid with each base. Which equilibria lie considerably toward the left? Which lie considerably toward the right? (a) NaHCO3 (b) NH3 (c) H2O (d) NaOH

Benzoic acid, C6H5COOH (pKa 4.19), is only slightly soluble in water, but its sodium salt, C6H5COO2 Na1, is quite soluble in water. In which solution(s) will benzoic acid dissolve? (a) Aqueous NaOH (b) Aqueous NaHCO3 (c) Aqueous Na2CO3

4-Methylphenol, CH3C6H4OH (pKa 10.26), is only slightly soluble in water, but its sodium salt, CH3C6H4O2 Na1, is quite soluble in water. In which solution(s) will 4-methylphenol dissolve? (a) Aqueous NaOH (b) Aqueous NaHCO3 (c) Aqueous Na2CO3

One way to determine the predominant species at equilibrium for an acid-base reaction is to say that the reaction arrow points to the acid with the higher value of pKa. For example, NH4 1 1 H2O ;9 NH3 1 H3O1 pKa 9.24 pKa 21.74 NH4 1 1 OH2 9: NH3 1 H2O pKa 9.24 pKa 15.7 Explain why this rule works

Will acetylene react with sodium hydride according to the following equation to form a salt and hydrogen, H2? Using pKa values given in Table 4.1, calculate Keq for this equilibrium. HC # CH 1 Na1H2 EF HC # C2Na1 1 H2 Acetylene Sodium Sodium Hydrogen hydride acetylide

Using pKa values given in Table 4.1, predict the position of equilibrium in this acidbase reaction, and calculate its Keq. H3PO4 1 CH3CH2OH EF H2PO4 2 1 CH3 CH2 OH2 1

For each equation, label the Lewis acid and the Lewis base. In addition, show all unshared pairs of electrons on the reacting atoms, and use curved arrows to show the fl ow of electrons in each reaction. C"O 1 H9Cl H H F2 1 BF3 BF4 2 (a) (b) C"O1 1 Cl2 H H H

Complete the equation for the reaction between each Lewis acid-base pair. In each equation, label which starting material is the Lewis acid and which is the Lewis base; use curved arrows to show the fl ow of electrons in each reaction. In doing this problem, it is essential that you show valence electrons for all atoms participating in each reaction. CH3 CH3 Cl CH3 Cl C Cl 1 1 ClAl CH3 CH3 CH3 (a) (b) C1 H H O CH3 CH3 1 CH 1 Br2 CH3 CH3 CH 1 CH3 1 (c) (d) H

Each of these reactions can be written as a Lewis acid/Lewis base reaction. Label the Lewis acid and the Lewis base; use curved arrows to show the fl ow of electrons in each reaction. In doing this problem, it is essential that you show valence electrons for all atoms participating in each reaction. C CH3 CH2 H CH ClH BrBr (a) CH3 CH3 CH3 CH2 CH Cl CH 3 CH2 CH C Br Br 3 CH2 (b) 54

The sec-butyl cation can react as both a Brnsted-Lowry acid (a proton donor) and a Lewis acid (an electron pair acceptor) in the presence of a watersulfuric acid mixture. In each case, however, the product is different. The two reactions are CH3 CH2 sec-Butyl cation CH3 CH3 O HH (1) CH 1 1 1 H2O C CH3 CH H2 CH3 CH2 sec-Butyl cation CH CH3 1 CH3 1 H3O1 (2) 1 H2O C CH3 CH H (a) In which reaction(s) does this cation react as a Lewis acid? In which does it react as a Brnsted-Lowry acid? (b) Write Lewis structures for reactants and products, and show by the use of curved arrows how each reaction occurs.

Write equations for the reaction of each compound with H2SO4, a strong protic acid. (a) CH3OCH3 (b) CH3CH2SCH2CH3 (c) CH3CH2NHCH2CH3 (d) CH3NCH3 CH3 O CH3CCH3 (e) O CH3COCH3

Write equations for the reaction of each compound in Problem 4.36 with BF3, a Lewis acid.

Label the most acidic hydrogen in each molecule, and justify your choice by using appropriate pKa values. (a) HOCH2CH2NH2 (b) HSCH2CH2NH2 (c) HOCH2CH2C#CH O (d) HOCCH2CH2SH HO O (e) CH3CHCOH O (f) H3NCH2CH2COH O (g) H3NCH2CH2CO (h) HSCH2CH2OH 4

Explain why the hydronium ion, H3O1, is the strongest acid that can exist in aqueous solution. What is the strongest base that can exist in aqueous solution?

What is the strongest base that can exist in liquid ammonia as a solvent?

or each pair of molecules or ions, select the stronger base, and write its Lewis structure. (a) CH3S2 or CH3O2 (b) CH3NH2 or CH3O2 (c) CH3COO2 or OH2 (d) CH3CH2O2 or H2 (e) NH3 or OH2 (f) NH3 or H2O (g) CH3COO2 or HCO3 2 (h) HSO4 2 or OH2 (i) OH2 or Br

Account for the fact that nitroacetic acid, O2NCH2COOH (pKa 1.68), is a considerably stronger acid than acetic acid, CH3COOH (pKa 4.76).

Sodium hydride, NaH, is available commercially as a gray-white powder. It melts at 800C with decomposition. It reacts explosively with water and ignites spontaneously on standing in moist air. (a) Write a Lewis structure for the hydride ion and for sodium hydride. Is your Lewis structure consistent with the fact that this compound is a high-melting solid? Explain. (b) When sodium hydride is added very slowly to water, it dissolves with the evolution of a gas. The resulting solution is basic to litmus. What is the gas evolved? Why has the solution become basic? (c) Write an equation for the reaction between sodium hydride and 1-butyne, CH3CH2C # CH. Use curved arrows to show the fl ow of electrons in this reaction.

Methyl isocyanate, CH3 !N" C " O, is used in the industrial synthesis of a type of pesticide and herbicide known as a carbamate. As a historical note, an industrial accident in Bhopal, India, in 1984 resulted in leakage of an unknown quantity of this chemical into the air. An estimated 200,000 persons were exposed to its vapors, and over 2000 of these people died. (a) Write a Lewis structure for methyl isocyanate, and predict its bond angles. What is the hybridization of its carbonyl carbon? Of its nitrogen atom? (b) Methyl isocyanate reacts with strong acids, such as sulfuric acid, to form a cation. Will this molecule undergo protonation more readily on its oxygen or nitrogen atom? In considering contributing structures to each hybrid, do not consider structures in which more than one atom has an incomplete octet

Offer an explanation for the following observations. (a) H3O1 is a stronger acid than NH4 1. (b) Nitric acid, HNO3, is a stronger acid than nitrous acid, HNO2. (c) Ethanol and water have approximately the same acidity. (d) Trifluoroacetic acid, CF3COOH, is a stronger acid than trichloroacetic acid, CCl3COOH.

Following is a structural formula for the tert-butyl cation. (We discuss the formation, stability, and reactions of cations such as this one in Chapter 6.) CH3 tert-Butyl cation (a carbocation) CH39C9CH3 (a) Predict all C9C9C bond angles in this cation. (b) What is the hybridization of the carbon bearing the positive charge? (c) Write a balanced equation to show its reaction as a Lewis acid with water. (d) Write a balanced equation to show its reaction as a Brnsted-Lowry acid with water.

Alcohols (Chapter 10) are weak organic acids, pKa 1518. The pKa of ethanol, CH3CH2OH, is 15.9. Write equations for the equilibrium reactions of ethanol with each base. Which equilibria lie considerably toward the right? Which lie considerably toward the left? (a) NaHCO3 (b) NaOH (c) NaNH2 (d) NH3

As we shall see in Chapter 19, hydrogens on a carbon adjacent to a carbonyl group are far more acidic than those not adjacent to a carbonyl group. The anion derived from acetone, for example, is more stable than is the anion derived from ethane. Account for the greater stability of the anion from acetone. Acetone pKa 22 O CH3CCH29H Ethane pKa 51 CH3CH2

2,4-Pentanedione is a considerably stronger acid than acetone (Chapter 19). Write a structural formula for the conjugate base of each acid, and account for the greater stability of the conjugate base from 2,4-pentanedione. H Acetone pKa 22 2,4-Pentanedione pKa 9 O CH3CCH29H O O CH3CCHCCH3

Write an equation for the acid-base reaction between 2,4-pentanedione and sodium ethoxide, and calculate its equilibrium constant, Keq. The pKa of 2,4-pentanedione is 9; that of ethanol is 15.9. H 2,4-Pentanedione O O CH3CCHCCH3 Sodium ethoxide CH3CH2ONa

An ester is a derivative of a carboxylic acid in which the hydrogen of the carboxyl group is replaced by an alkyl group (Section 1.3E). Draw a structural formula of methyl acetate, which is derived from acetic acid by replacement of the H of its 9OH group by a methyl group. Determine if proton transfer to this compound from HCl occurs preferentially on the oxygen of the C"O group or the oxygen of the OCH3 group

Alanine is one of the 20 amino acids (it contains both an amino and a carboxyl group) found in proteins (Chapter 27). Is alanine better represented by the structural formula A or B? Explain. NH2 O CH39CH9C9OH (A) NH3 O CH39CH9C9O (B)

Glutamic acid is another of the amino acids found in proteins (Chapter 27). Glutamic acid has two carboxyl groups, one with pKa 2.10 and the other with pKa 4.07. HO9C9CH29CH29CH9C9OH Glutamic acid NH3 O O (a) Which carboxyl group has which pKa? (b) Account for the fact that one carboxyl group is a considerably stronger acid than the other.

Following is a structural formula for imidazole, a building block of the essential amino acid histidine (Chapter 27). It is also a building block of histamine, a compound all too familiar to persons with allergies and takers of antihistamines. When imidazole is dissolved in water, proton transfer to it gives a cation. Is this cation better represented by structure A or B? Explain. N H Imidazole N N HH A N H2O OH or N H B N H 54