Chapter In each case the conjugate base is obtained by removing a proton from the acid: (a) OH (b) I (c)

Transcription

1 Practice Exercises 16.1 Conjugate acid base pairs (a), (c), and (f) (b) The conjugate base of I is I (d) The conjugate base of N 2 is N 2 and the conjugate base of N 4 is N 3 (e) The conjugate acid of C 2 3 is C 3 and the conjugate acid of CN is CN 16.2 In each case the conjugate base is obtained by removing a proton from the acid: (a) (b) I (c) N 2 (d) 2 P 4 (e) 2 P 4 (f) 3 P 4 (g) S (h) N In each case the conjugate acid is obtained by adding a proton to the base: (a) 2 2 (b) S 4 (c) C 3 (d) CN (e) N 3 (f) N 4 (g) 3 P 4 (h) 2 P CN and CN and 16.5 The Brønsted acids are 2 P 4 (aq) and 2 C 3 (aq) The Brønsted bases are C 3 (aq) and P 4 2 (aq) 16.6 P 4 3 (aq) C (aq) P 4 2 (aq) C (aq) base acid acid base 16.7 (a) 2 P 4 amphoteric since it can both accept and donate a proton (b) P 2 4 amphoteric since it can both accept and donate a proton (c) 2 S amphoteric since it can both accept and donate a proton (d) 3 P 4 not amphoteric: it can only donate protons (e) N 4 not amphoteric: it can only donate protons (f) 2 amphoteric since it can both accept and donate a proton (g) I not amphoteric: it can only donate protons (h) N 2 not amphoteric: it can only donate protons 16.8 P 4 2 (aq) (aq) P 4 3 (aq) 2 ; P 4 2 acting as an acid P 4 2 (aq) 3 (aq) 2 P 4 2 ; P 4 2 acting as a base 16.9 S 4 (aq) P 4 2 (aq) S 4 2 (aq) 2 P 4 (aq) The substances on the right because they are the weaker acid and base (a) < Br < I (b) P 3 < 2 S < (c) 2 < 2 Se < 2 Te (d) As 3 < 2 Se < Br (e) P 3 < 2 Se < I (a) Br is the stronger acid since binary acid strength increases from left to right within a period. (b) 2 Te is the stronger acid since binary acid strength increases from top to bottom within a group. (c) 2 S since acid strength increases from top to bottom within a group (a) 3 (b) 2 S 4 147

2 16.14 (a) 3 As 4 (b) 2 Te (a) I 4 (b) 2 Te 4 (c) 3 As (a) 2 S 4 (b) 3 As The acid strength decreases as follows: C 2 C > C 2 C > BrC 2 C (a) N 3 is the Lewis base since it has an unshared pair of electrons. is the Lewis acid since it can accept a pair of electrons (b) Na 2 is the Lewis base ( 2- would also be a correct answer since sodium salts are soluble, assuming the reactions are in water) Se 3 is the Lewis acid (c) Ag is the Lewis acid N 3 is the Lewis base (a) luoride ions have a filled octet of electrons and are likely to behave as Lewis bases, i.e., electron pair donors. (b) Be 2 is a likely Lewis acid since it has an incomplete shell. The Be atom has only two valence electrons and it can easily accept a pair of electrons. (c) It could reasonably be considered a potential Lewis base since it contains three oxygens, each with lone pairs and partial negative charges. owever, it is more effective as a Lewis acid, since the central sulfur bears a significant positive charge. Review Questions 16.1 A Brønsted acid is a proton donor, whereas a Brønsted base is a proton acceptor In a conjugate acidbase pair, the acid has one more hydrogen ion than does the base S 4 is not the conjugate acid of S 4 2 because 2 S 4 has two more hydrogen ions than does S 4 2. S 4 is the conjugate acid of S An amphoteric substance can act either as an acid or as a base. (aq) 2 3 (aq) (aq), in which water serves as a base N 3 (g) 2 (aq) N 4 (aq), in which water serves as an acid 16.5 A compound that can act either as a proton donor or a proton acceptor Acid strength increases from left to right in the same period and from top to bottom in the same family (a) The relative strength of the binary acids increases from top to bottom in a group of the periodic table, so we expect At to be a stronger acid than I. (b) The relative strength of oxoacids increases from bottom to top in a group of the periodic table, so we expect At 4 to be a weaker acid that Br In nitric acid, there are more oxygen atoms bound to the nitrogen atom than in nitrous acid. As the number of oxygen atoms increases, the pull on the electrons in the bond increases withdrawing electrons away from the hydrogen atom. This makes it easier to lose a hydrogen ion The relative strengths of the binary acids increases from top to bottom in a group of the periodic table, so we expect 2 S to be a stronger acid than 2. This is because the S bond strength is lower than the bond strength. 148

3 16.10 C 3 C 2 is a stronger Brønsted base than C 3 C 2 S. The S bond is weaker than the bond, therefore, C 3 C 2 S is a stronger acid than C 3 C 2 this in turn makes the conjugate base of the stronger acid a weaker base There are more oxygen atoms not attached to protons in 4 than in 2 Se 4. In addition, the chlorine atom is more electronegative than selenium which makes the bond more polarized in (aq) 2 3 (aq) (aq) is a stronger base than water and 3 is a stronger acid than The equilibrium would lie to the left, more so than to the right, so as to lie in the direction which favors formation of the weaker acid and base C is a stronger base than N This equilibrium should lie to the left, because if N 3 is a strong acid, then, by definition, N 3 must be a weak base Both 4 and N 3 are 100% ionized in water and, therefore, appear to be the same strength. To distinguish between these two, we would need a solvent that is a weaker proton acceptor than Water, formic acid, and acetic acid have similar but different dissociation constants. ence, it is possible to distinguish between them. In N 3 solution, formic acid and acetic acid are fully dissociated and appear the same strength The molecule on the right will be the stronger acid because the very electronegative atoms will stabilize the resulting ion The hydrogen on the tri-chloro compound is more acidic because the high electronegativity of will stabilize the ion. So 2 C is the stronger conjugate base Lewis acid: Electron pair acceptor Lewis base: Electron pair donor The ion accepts a pair of electrons from the oxygen atom of the water molecule. This makes the ion the Lewis acid and the 2 molecule is the Lewis base. 149

4 16.22 C N B C N B The Lewis base is the water molecule and the Lewis acid is the C The 2 ion has a complete valence shell. It can donate electron pairs functioning as a Lewis base. It cannot accept additional electrons and, therefore, cannot serve as a Lewis acid Sb 5 has an empty orbital, it can accept a pair of electrons to form a Lewis acid-base adduct In the reaction of calcium with oxygen, there is a complete transfer of the electron pair from the calcium to the oxygen, then the electrostatic attractions bind the two ions together. In a Lewis acidbase reaction, a covalent bond is formed between the two atoms and the electrons are shared The element would be classified as a metal if it had a characteristic metallic luster, was a good conductor of heat and electricity, was ductile and malleable, formed ionic compounds with nonmetals and formed an oxide that was a basic anhydride. The element would be a nonmetal if it formed covalent bonds with other nonmetals Metal oxides are typically basic so this must be a non-metal oxide B B B B This is due to the equilibrium: Cr( 2 ) Cr( 2 )

5 16.31 The charge density on the e 3 is higher, so it is more acidic Because they have such a small charge density, the metal ions have little impact and essentially zero interaction with 2 in an acidbase sense (a) 2 S 4 (b) 2 C 3 (c) 3 P (a) Cr 3 (b) Cr (c) Cr (a) Al Al (b) Al Al 2 2 Review Problems (a) (b) N 2 5 (d) 2 (e) 2 Cr 4 (c) C 5 5 N (a) N 2 (b) 2 S 3 (d) 4 I 6 (e) N 2 (c) CN (a) N 3 N 2 4 N 2 5 N 3 (b) (c) N 3 N 2 5 N 4 N 2 4 base acid acid base 2 P 4 2 C 3 C 3 2 P 4 (d) I 3 C C 2 4 I 3 151

6 16.39 (a) S 4 2 S 3 S 3 2 S 4 (b) (c) (d) 2 S 2 S 3 CN CN 2 2 Se 2 3 Se (a) Br, Br bond is weaker (b), more electronegative polarizes and weakens the bond (c) Br, larger Br forms a weaker bond with (a) 2 Se, larger central atom forms a weaker bond with (b) I, more electronegative atom (c) P 3, larger central atom (a) 2, because it has more oxygen atoms (b) 2 Se 4, because it has more lone oxygen atoms (a) I 4, because it has more oxygen atoms (b) 3 As 4, because it has more oxygen atoms (a) 3, because is more electronegative (b) 3, because the charge is more evenly distributed (c) Br 4, because the negative charge is more evenly distributed (a) 3 P 4, since P is more electronegative (b) N 3, because N is more electronegative (N 3 is a strong acid) (c) 4, because is more electronegative Cr( 2 ) 6 3 (aq) 2 (l) Cr( 2 ) 5 2 (aq) 3 (aq) 152

7 16.47 Mg() 2 is an ionic species consisting of Mg 2 ions and - ions. When the compound dissociates in water it produces - ions so the compound is a basic species. Si() 4 is a molecular compound with the groups attached to the silicon atom. This bonding weakens the bond so that a hydrogen ion will dissociate whe the compound is dissolved in water. Thus, Si() 4 is an acidic species N N Lewis acid: Lewis base: N Lewis acid: B 3 B B

8 16.51 Al Al Al Al Lewis Lewis base acid C C Lewis Lewis base acid 2- C C N N Lewis bases: N 2 and Lewis acid: S C C S Lewis bases: 2 on C 3 2 Lewis acids: S 2 and C 2 154

9 Additional Exercises Acid: (C 3 ) 2 N 2 Base: (C 3 ) 2 N Molecular equation NaN 2 N 4 Na 2N 3 Ionic equation Na N 2 - N 4 - Na - 2N 3 Net ionic equation N 2 - N 4 2N 3 N 4 is an acid and N 2 - is a base S 2 Na 2 S 3 2S 2 2Na S 2 is an acid and Na 2 S 3 is a base. rom a Lewis definition S 2 is an electron acceptor and S 3 2- is an electron donor. S 2 accepts an electron pair from one of the oxygen atoms on S 3 2- and the S 3 2- unit loses the oxygen atom Ethanamide, also known as acetamide, is a stronger acid and therefore a weaker base than N 3. The 3 CC unit withdraws electron density from the N 2 group thus making it easier for the hydrogen atoms on the N 2 unit to dissociate. K a for ethanamide is 2.34 x 10-1 while K a for N 3 is 5.56 x N 3 is a stronger base than N 3. This is due to the difference in the electron withdrawing strength of compared with. is more electronegative than and will withdraw more electron density from the nitrogen than hydrogen. Thus, the nitrogen atom in N 3 has a greater positive charge than it does in N 3. The greater positive charge means that is more difficult to add to N 3 than it is to add to N Ethanedioic acid (pk a1 = 1.23, pk a2 = 4.19) is stronger than butanedioic acid (pk a1 = 2.82, pk a2 = 5.70). or ethanedioic acid the two C groups are connected to each other. In the case of butanedioic acid the C groups are separated by C 2 groups. The closer the two C groups are to one another the more influence they have on weakening the - bond due to their electron withdrawing effects. In essence, you dilute the electronic affect of neighboring electron withdrawing groups as you increase their separation from the group they are influencing C C 3 C C (a) In 2 2, the oxygen atom helps to stabilize the 2 ion making it easier for 2 2 to lose a proton than can 2. (b) acidic Acid-base properties are based on the structure of the compound and on bond strengths. In the case of Na you have an ionic compound consisting of Na ions and - ions. Therefore, when added to water Na will dissociate into these ions. Thus, Na is a source of - ions and is a base. In the case of Al( 2 ) 3 () 3 the groups are covalently bonded to the Al atom. Depending on the conditions, the - group can react with an or an - react with the Al( 2 ) 3 () 3. In acid conditions an - on Al( 2 ) 3 () 3 will react with the. Complete neutralization will result in Al( 2 ) 6 3. In base - is added to the Al( 2 ) 3 () 3 molecule resulting in [Al( 2 ) 2 () 4 ] - being formed. 155

10 In the case of 4, the and atoms are very electronegative, withdrawing electrons and thus weakening the - bond. This increases the positive charge on the making it a very acidic hydrogen The equilibrium lies to the right since reactions favor the weaker acid and base acids: C 3 N 3, N 3 bases: N 2, C 3 N N 3 is the stronger acid will ionize more in the more weakly acidic solvent, 2. 2 is a very strong acid and the second equilibrium will be to the left. Multi-Concept Problems The structure with the bulkier 2 groups at apart is more stable The predominate species in solution can be determined from the appropriate equilibrium constants. Arsenic acid and arsenous acid are both weak acids therefore, the predominate species will be the molecular acid forms. The two salts are soluble and the equilibrium constants for their respective anion reactions with water are very small so the predominate species will be the anions. The predominate species in solution are: 3 As 4, 3 As 3, As 4 3-, and As

11 As As Arsenic acid Arsenous acid 3-3- As As Arsenate ion Arsenite ion 3 As 3 As As 4 As 3 157