Chapter 6: Acid-Base Reactions

Question 1

bronsted lowry base reactions

Answer 1

• an acid is a proton donor
• a base is a proton acceptor
• after donating the proton the acid becomes a conjugate base
• after accepting the proton the base becomes a conjugate acid
• depending on relative acidities the reaction might be reversible or irriversible

Question 2

under Lewis acid-base reactions

Answer 2

• an acid is an electron pair acceptor
• a base is an electron donor
• lewis acids are often metals or metalloids
• lewis bases have a lone pair

Question 3

H ion

Answer 3

• single protons H cations, exist in the vacuum of space, they do not exist under standard conditions
• the proton associates with the most appropriate base in the mixture: usually water, a base or the solvent)
• some sources just show H to avoid confusion

Question 4

reaction coordianates of Acid-base reactions

Answer 4

• all acid base reactions/steps have a single transition state (for the transfer of the proton from the acid to the base)
• reactions with a strong acid are exergonic
• reactions with weak acids are endergonic
  (generally not always)
• whichever has more stable (lower energy) product is stronger) - assuming both reactions are possible

Question 5

qualitative estimates of acidity

Answer 5

• it is often useful to compare the relative acidity of two acids
• all acid-base reactions are in equilibrium
• the stronger the acid (or the base) the more th equilibrium is shifted right
• the best way to predict the relative strength of a neutral acid is by looking at the stability of its conjugate base
• anything that stabalizes the conjugate base, especially if it stabalizes the formal negative charge, increases acidity

Question 6

electronegativity

Answer 6

• the more electronegative an atom is the better it can stabalize extra electron desity (an anionic charge)
• when comparing two acids (with other factors being equal) the acid with the more electronegative atom becoming aniocic will be more acidic

Question 7

induction

Answer 7

• this also applies if there are electronegative atoms nearby = pulls electron density away, makes the anion more easily stabilized)
• when comparing two acids (with other factors being equal) the acid with more electronegative atoms near the future anion will be more acidic
• strength of induction depends on electronegativity, number of atoms and proximity (the closer they are the more stable they are)

Question 8

atomic size

Answer 8

• the larger the atom and its orbitals are the better it can stabalize extra electron density (an anionic charge)
• when comparing two acids (with other factors being equal ) the acid with the larger arom becoming anionic will be more acidic
• only applies if the row in the periodic table changes

Question 9

hybridization

Answer 9

• when comparing two acids (with other factors being equal)
• the acid with more s character at the carrying the acidic H will be more acidic

Question 10

resonance stabilization

Answer 10

• if the electron density on the conjugate base can be delocalized by resonance then the conjugate base is much more stable
• when comparing two acids with other factors being equal the acid with more delocalization of the anion will be more acidic
• this is the most important factor when comparing acids of the same element

Question 11

how to apply these factors to a cationic acid

Answer 11

• the best way to predict the relative strenght of a neutral acid is by looking at the stability of its conjugate base
• not all acids are neutral sometimes they are cationic (often conjugate acids
• the best way to predict the relative strength of a cationic acid is by looking at the stability of the acid itself
• anything that destabilizes the acid especially things that destabalize the positive formal charge increases acidity
• in other words, for both neutral and cationic acids analyze the relative stabilities of the compounds with formal charges
  if the cationic charge can be stabalized by resonance that stabilizes the acid, which makes it less acidic ( this only applies is hybridization is the same in both examples)

Question 12

when comparing two acids

Answer 12

• when comparing two acids, the acid woth fewer alkyl groups adjacent to the cationic atom will be more acidic

Question 13

Pka tables

Answer 13

• the smaller the PKA the more acidic the acid is
• its easy to compare acids using PKA values directly
• exact pka values for many compounds have already been measured
• ranges and values for common compounds and functional groups are normally tabulated and used as approximate values

Question 14

two common styles of pka tables

Answer 14

• organizing by structural similarties: useful to see what effect some change might have. difficult to use if you need to find a specific value
• organizing by pka values: useful to find a specific value, difficult to see patterns if the exact molecule is not listed

Question 15

using pka tables to compare acids

Answer 15

• directly compare the values
• whichever has the smaller pka is more acidic
• the stronger the acid the weaker the the conjugate base
• the weaker the acid the stronger the conjugate base

Question 16

using pka to determine equilibrium

Answer 16

• after an acid base reaction the products are a new conjugate acid and conjugate base
• pka values can be used to determine the equilibrium (side with the weaker acid is favoured)

Question 17

pka and ph

Answer 17

• if the water is the solvent the pH can be measured
• this is the value for how acidic the solution is itself
• it is possible to use the value of Ph and pKa of an acid in the solution to determine the protonation state of the compound
• this is exactly the same as comparing two pkas to determine which side an equilibrium favours