Chapter 4

Question 1

Bonsted-Lowry acids

Answer 1

proton donator

Question 2

Bronsted-Lowry bases

Answer 2

proton acceptor

Question 3

conjugate pair

Answer 3

any pair of molecules or ions interconverted through the transfer of a proton; differ by one proton

Question 4

conjugate base

Answer 4

form of the acid after it donates a proton

Question 5

conjugate acid

Answer 5

form of the base after it accepts a proton

Question 6

When a Bronsted-Lowry acid has more than one receptor site, where do atoms bond?

Answer 6

atoms of the other species bond to site which forms the most stable molecule

Question 7

how to determine where atoms bond on a Bronsted-Lowry acid with more than one receptor site

Answer 7

1.) determine the receptor sites for the element (eg hydrogen) 2.) draw all resonance structures for each of the resulting molecules 3.) determine the most stable molecule 4.) the most stable molecule represents the most likely formed molecule in reality

Question 8

how do pi electrons act as Bronsted-Lowry bases?

Answer 8

the double or triple bond breaks, and the pi-electrons form a bond with a proton of hydrogen

Question 9

pK_a

Answer 9

-log(Ka)

Question 10

relationship between acidity and pK_a

Answer 10

• higher pK_a, weaker acid
• lower pK_a, stronger acid

Question 11

relationship between strength of acid and strength of conjugate base

Answer 11

• stronger acid, weaker conjugate base - weaker acid, stronger conjugate base

Question 12

determine K_eq of reaction

Answer 12

1. ) acid-base equation
2. ) pK_a of acid vs pK_a of conjugate acid
3. ) equilibrium lies towards higher pKa
4. ) subtract smaller from larger to get ΔpK_a
5. ) K_eq = 10^(ΔpKa) - if equilibrium lies left: the exponent is negative - if equilibrium lies right: the exponent is positive

Question 13

What does a single unit increase/decrease represent in terms of acidity?

Answer 13

a tenfold increase/decrease in the acidity of the compounds compared

Question 14

pKa ethane (CH₃-CH₃)

Answer 14

51

Question 15

pK_a (CH₃=CH₃)

Answer 15

44

Question 16

pK_a ammonia (NH₃)

Answer 16

38

Question 17

pK_a hydrogen (H₂)

Answer 17

35

Question 18

pKa water (H₂O)

Answer 18

15.7

Question 19

pK_a acetic acid (CH₃CO₂H)

Answer 19

4.7

Question 20

pK_a hydrochloric acid, hydrogen chloride (HCl)

Answer 20

-7

Question 21

structural effects on acidity

Answer 21

• atoms with more stable conjugate bases better support negative charge
  1. ) within period: more electronegative atoms better able to support negative charge
  2. ) within column: larger atoms better accomodate negative charge
  3. ) delocalization of charge: more delocalized the charge, the more stable the conjugate base (stronger the acid)

Question 22

effect of electronegativity on acidity of molecule

Answer 22

• can be seen across period: acidity of molecule containing C < acidity of molecule containing N < acidity of molecule containing O
• electronegativity also depends upon hybridization: extra e^-smore stable in orbitals of greater s-character, because s-orbitals lower in energy; therefore, greater s-character increases the acidity of the molecule

Question 23

effect of size of atom on acidity of molecule

Answer 23

• negative charge “diffuses” across larger atoms, so larger atoms (further down a group) have lower pK_a values

Question 24

effect of delocalization of charge on acidity of molecule

Answer 24

• more resonance forms of conjugate base = more stable conjugate base = stronger acid
• inductive effect

Question 25

inductive effect

Answer 25

through sigma-bonds, so NO resonance - more electronegative atoms withdraw e^- density through sigma-bonds, which spreads negative charge 1. ) inductive effect increased (ie, greater drop in pK_a) for more electronegative atoms (F \> Cl) 2. ) inductive effect increased for greater number of electronegative atoms (3 Cls \> 2 Cls \> 1 Cl) 3. ) inducitve effect decreased as electronegative atoms further from negative charge (ie, pKa CF₃CH₂OH \< pKa CF₃CH₂CH₂OH)

Question 26

Lewis Acid (LA)

Answer 26

e^- pair acceptor (Bronsted-Lowry acids are H⁺ donors, and therefore a particular type of Lewis Acid)

Question 27

Lewis Base (LB)

Answer 27

e^- pair donor (Bronsted-Lowry bases are H⁺ acceptors, therefore a particular type of Lewis Base)