Topic 18 HL Acids and Bases

what is a lewis acid and an example

what is a lewis acid and an example

electron pair ACCEPTOR / electrophile

• ex. ions with + charges, H+, Fe 2+, period 2 outlaw ions (Be 2+, B 3+)

what is a lewis base with an example

electron pair DONOR / neucleophile / ligand

• ex. ions with negative charges, Br -, NH3 -, OH-

what kind of bond is formed when a lewis acid and base react together

coordinate covalent bond (because electron pair is donated from just one atom, the base)

whats the difference between a nucleophile and electrophile

nucleophile: base, electron pair donor, at least one lone pair of electrons

electrophile: acid, electron pair ACCEPTOR

what does the size of the Ka and pKa indicate?

larger Ka = more dissociation = stronger acid

smaller pKa = larger Ka = stronger acid

what does the size of the Kb and pKb indicate?

larger Kb = more dissociation = stronger base

smaller pKb = larger Kb = stronger base

what are important things to remember when calculating the pH of weak acids/bases

they always dissociate in a 1-to-1 ratio

use Ka or Kb = x²/original concentration

remember that x is very small, can be ignored (ex. 0.2-x = 0.2)

what is the % dissociation and what should it be for weak acids

% dissociation = [H+] / [weak acid] x 100 or the same for OH- and base

• should be < 5% for weak acid

what is a buffer solution

equilibrium system that (within limits) resists changes in acidity, maintains a fairly constant pH, even when a strong acid/base is added

why/when are buffers important

physiological → blood has a pH 7.3-7.4, can be fatal if below 7.0

industrial → treatment of sewage, electroplating, developing photo films

how can an effective buffer be made

1. weak acid + conjugate base (salt of weak acid) equimolar

ex. HF + NaF

2. weak base + conjugate acid (salt of weak base) equimolar

ex. NH3 + NH4Cl

3. strong base + excess weak acid OR

4. strong acid + excess weak base

ex. OH- + H2PO4 → H2O + HPO4 2-

what equations can ONLY be used for conjugate acid/base pairs

[H+] = Ka [HA] / [A-]

pH = pKa + log ([A-] / [HA])

pKa + pKb = 14

what is the buffering capacity and how does it change/not change

amount of extra acid/base that can be absorbed before the buffer is destroyed

• determined by concentrations of HA and A- NOT their ratio

• dilution will decrease the buffering capcity, but not the pH (as the ratio doesn’t change)

• changing temp changes the pH (Ka changes), but no change in capacity

what is the natural buffer found in human blood and how does it work

weak acid = H2CO3, conjugate base = HCO3 1-

OH - + H2CO3 → HCO3 - + H2O

H3O+ HCO3 - → H2CO3 + H2O

what is the relationshio between an acid and its conjugate base

weak acid → relatively STRONG base (at least dissociates into OH-, affects pH)

strong acid → WEAK base (conjugate base of the 6 strong acids don’t dissociate at all, no affect on pH)

what is the relationship between a base and its conjugate acid

weak base → STRONG acid

strong base → WEAK acid

what are the rules for if metallic ions affect pH

• those from group 1-2 do NOT dissociate into H+, no effect, EXCEPT: Be 2+

• other ions may dissociate, creating H+, acidic solution

• metal ions combine with OH to make a STRONG base

• if both parts of an ionic compound affect pH, need to look at pKa or pKb to see which is stronger (which has smaller pKa/pKb)

what is important to remember for reactions between STRONG acids and STRONG bases

• assume all 100% dissociates, thus [acid] = [H+] etc.

• if equal, pH = 7

• if not equal, find limiting reagent/ which has more

• find concentration of excess H+ or OH-, then find pH

what are the parts of the titration curve

1 buffering areas (before and after) where pH barely changes

equivalence point, should be when indicator changes colour, pH = 7 for strong acid and strong base

steps to finding halfway point of weak acid & strong base OR weak base & strong acid titration

1. write out equation

2. use pH = pKa

3. pKa is for the thing you create (if created a base, use pKa = 14 - pKb)

steps to finding equivalence point of weak acid & strong base OR weak base & strong acid titration

1. using ice chart of mols, find mols of conjugate acid/base produced

2. find concentration of conjugate acid/base produced

3. 2nd ice chart with mol/L for the dissociation of conjugate acid/base produced

4. sub in x for amount dissociated

5. find appropriate pKa or pKb (should be for the reverse of dissociation), sub and solve for x

6. use x to find pH (check if its OH- or H+)

what is the buffer in blood? write the equations for how its formed, and how it neutralizes an acid/base

CO2 + H2O → H2CO3

H2CO3 + H2O → H3O+ + HCO3-

OH- + H2CO3 → H2O + HCO3-

HCO3- + H+ → H2CO3