Acid & Bases Flashcards
What is the common ion effect?
The shift in eqm caused by the addition of a compound having an ion in common with dissolved substance
What does the presence of the CIE have?
Suppresses ionisation of weak acid + base
What is the common ion in these equations?
CH3CONa (s) —> Na+ + CH3CO2-
CH3CO2H ⇌ H+ + CH3CO2-
CH3CO2-
What is the effect of having a weak acid + salt in a solution?
Ionisation of HA is suppressed by presence of A-
Hydrolysis of A- suppressed by presence of HA
Acid + conjugate base may react with one another
What happens when a strong base is added to a buffer?
Weak acid will give up H+
To transform base (OH-) into H2O + conjugate base
= added OH- consumed by reaction = pH changes only slightly
What happens when strong acid added to a buffer?
WB react with H+ from SA to form WA
H+ absorbed by A- = pH changes only slightly
What is a buffer?
Solution has ability to resist changes in pH upon addition of small amount of acid or base
How do you form a buffer?
WB + salt
WA + salt
What are some examples of buffers?
CH3COOH/CH3COO-Na+
NaH2PO4/NaHPO4
NH3/NH4Cl
Explain the titration CH3COOH with NaOH
Start at low pH + add OH-
Product of [H+][OH-] must main constant
= add OH- = [H+] conc decrease = pH increases
At pKa, [Ac-] + [HAc] are equal
So adding more OH- doesn’t change ratio very much
pH doesn’t change
What is the Henderson-Hasselbach equation?
pH = pKa + log conjugate base
———————–
acid
Solution containing 0.1M CH3COOH with 0.15M CH3COONa (pKa 4.76)
pH = 4.76 + log(0.15/0.1) = 4.94
Need a buffer with pH of 4.5
4.5 = 4.76 + log(base/acid)
log(base/acid) = 4.5-4.76= -0.26
(base/acid) = 10* -0.26 = 0.55
Use 1M CH3COOH with 0.55M CH3COONa
What is the equation used to predict the ability of a buffer to resist change in pH?
Change in H+
What does a higher conc mean?
Higher buffer capacity
What is involved in buffer capacity?
Ratio of [A-]/[HA] - influences pH of solution
Conc of [A-][HA] - control effectiveness of buffer
What does more A- + HA available molecules mean?
Less of an effect addition has on pH of system
Why must living systems be buffered?
Resist large variations in pH
What are the important buffers?
Bicarbonate
Phosphate
What happens if blood pH drops?
[H2CO3] increases
By protonation of H2CO3
H2CO3 rapidly loses H2O to form CO2 (aq)
Expelled as CO2 (g)
What happens if blood pH rises?
[H2CO3] increases
By deprotonation of H2CO3
Breathing rate = CO2 (g) converted to CO2 (aq)
Then converted to H2CO3 in capillaries of the lungs
How do you select buffer components?
Choose acid with pKa close to desired pH
Calculate ratio of base to acid using equ
Use at least 0.1M of lower component
Target = both to be 0.1-1.0M
What is ionisation?
Protonation or deprotonation = charged molecules
What are the 3 acidic species if HA dissolved in H2O?
HA - may be strong, weak or very weak
H2O - pKa = 15.7
H3O+ - pKa = -1.7
What happens if the acid has pKa value >15.7?
H2O stronger acid than HA = NO ionisation
What happens if acid has pKa value 1.7 - 15.7?
HA stronger acid than H2O BUT weaker than H3O+ = partial ionisation
What happens if acid has pKa <1.7?
HA stronger acid than H3O+ = complete ionisation
What is the strongest acid in aq solution?
H3O+
What is the strongest base?
OH-
What is the levelling effect?
Effect of solvent on property of acids + bases
What is the strength of SA levelled by?
Basicity of solvent
What is the strength of SB levelled by?
Acidity of solvent
What does the fraction in the acid form depend on?
Whether the functional group is acidic or basic
What happens if it was acidic?
Formula gives fraction unionised
What happens if it was basic?
Formula gives fraction ionised
What is the eqm between unionised + ionised?
Acidity constant Ka
What is % ionised equ for acid?
1 + 10* (pKa-pH)
What is % ionised equ for base?
1 + 10* (pH-pKa)
What is effect of ionisation on antibacterial potency of sulphonamides?
pH 11-7
Potency increases
= active species = anion
What is effect of ionisation on antibacterial potency of sulphonamides?
pH 7-3
Potency decreases
= only neutral form of compound can transport into cell
What is the pH of stomach?
1.5
What is aspirin’s pKa?
3.5
How much is aspirin ionised in the stomach?
99%
What is the pH of the blood?
7.4
How much is aspirin ionised in the blood?
99%
What is the Arrhenius definition of an acid?
Any substance which produces H+ in aq solution
What is the Arrhenius definition of a base?
Any substance which delivers OH- in an aq solution
What are cons of Arrhenius Theory?
No consideration of solvent effects
NH3 base with no OH?
What is the Bronsted-Lowry definition of an acid?
Any substance that donates H+
What is the Bronsted-Lowry definition of a base?
Any substance that accepts H+
What does it mean H2O can act as base + acid?
Amphoteric
What is acidity?
Measure of tendency to give up H+
What s basicity?
Measure of affinity for H+
What does a SA have?
Strong tendency to give up H+
What are cons of Bronsted-Lowry Theory?
Works nicely for H2O BUT fails to explain acid-base behaviour in other solvents
What is Lewis’ definition of an acid?
e- pair acceptor
What is Lewis’ definition of a base?
e- pair donor
What does a Lewis acid + base combine to form?
Adduct
What are cons of Lewis’ Theory?
Lowry acids are Lewis acids, BUT not all Lewis acids comply with Bronsted-Lowry definition?
What does Sorenson’s scale measure?
[H3O+]
How do you calculate pH?
-log[H3O+]
How do you calculate [H3O+]?
10*-pH
How do you calculate pOH?
-log[OH-]
How do you calculate [OH-]?
10*-pOH
What is Kw equation?
Kw = [H3O+][OH-]
What does Kw = at RTP?
1X10-14
What are examples of SA?
HCl = hydrogen halides
HClO3, HClO4 = oxyacids of halides
H2SO4, HNO3, H3PO4 = other oxyacids
What are examples of SB?
NaOH, KOH, CsOH = alkali metal hydroxides
Ca(OH)2, Sr(OH)2 = alkali earth metal hydroxides
Do SA + SB completely ionise in aq solutions?
YES
What are examples of WA?
HCOOH
CH3COOH
HF
HCN
What are examples of WB?
NH3
H2O
N(CH3)3
NH4OH
Do WA partially ionise in aq solutions?
YES
How do you calculate pKa?
-log(Ka)
How do you calculate Ka?
10*-pKa
What pKa value will a SA have?
Lower pKa
What pKa value will a WA have?
Higher pKa
Do WB partially ionise in aq solutions?
YES
What does eqm favour in aq solutions with WB?
Reactants
How do you calculate pKb?
-log(Kb)
How do you calculate Kb?
10*-pKb
What Kb value does a stronger base have?
Higher Kb
What Kb value does a weaker base have?
Lower Kb
What will a stronger base have?
Weaker conjugate acid
What will a stronger base have?
Higher pKa value
What does the pKa of an acid + pKb of conjugate base related to?
Related to Kw
Does a base have a pKa?
NO
Refers to its conjugate acid
eg. NH3 = NH4+
What is pH?
Level of H+ in solution
What is pKa?
Degree at which acid dissociates in solution
What is the equation for Ka?
[C]
What is the equation for the [H3O+] for weak acids?
√Ka X C
What is the equation for Kb?
[C]
What is the equation for the [OH-] for weak bases?
√Kb X C
How does electronegativity effect pKa?
As H bond becomes more polarised, H becomes more positive
= bond easier to break
More electronegative = stronger acid
How does anion size effect pKa?
Size + H held more loosely = bond easier to break
Larger size stabilises anion
What happens to acidity of halogens as atom size increases?
Acidity increases
How does resonance effect pKa?
Delocalisation of negative charge on conjugate base will stabilise anion = stronger acid
What is the resonance form?
Stabilises negative charge
What is resonance hybrid?
Actual structure
How does hybridization effect pKa?
Affects H bonded to atom
Higher % of s-character of hybrid orbital = L.P held closer to nucleus = more stable
How does inductive effect effect pKa?
Halogens ore electronegative
Pulls bonding e- towards itself
Pulls e- through sigma bond
Stabilises conjugate base through decrease of e- density around O2
