Équilibre Acid-Base

Question 1

What does p mean? (acid-base eqm)

Answer 1

-lg [sth]
eg
pH = -lg [H+]
pOH = -lg [OH-]

Question 2

Define Kw

Answer 2

• Kw = [H+][OH-]
• eqm constant
• for pure water at 25°C, Kw = 1.0x10^-14mol² dm^-6 (in data booklet) (since for pure water [H+]=[OH-]=√ (10^-14)=10^-7)

Question 3

pKw=? What can be derived at 25°C?

Answer 3

pKw
=-lgKw
= pH + pOH (since Kw = [H+][OH-])
so,
14 = pH + pOH at 25°C

Question 4

At 25°C, how do pH, [H+] and [OH-] compare in neutral, acidic and alkaline sol n?

Answer 4

neutral: [H+]=[OH-], pH=7
acidic: [H+]>[OH-], pH<7
alkaline: [H+]<[OH-], pH>7

Question 5

If temp of water increase, how does Kw and pH change? Why?

Answer 5

Kw decreases, pH decreases,
since self-ionisat n of water is endo so Le Chatelier’s Principle applies (NOTE: water is always neutral as [H+]=[OH-]; pH change bcos of change in [H+]

Question 6

Define Ka

Answer 6

For dissociat n of acid HA,
HA ⇌ H+ + A-

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

so,
pKa = -lg Ka

Question 7

What is the formula calculating [H+] of weak acid/bases?

Answer 7

acids: [H+] = √(c x Ka)

bases: [OH-] = √ (c x Kb)

where c is initial conc of acid/base

Question 8

Define Kb

Answer 8

For dissociat n of base B
B + H2O ⇌ BH+ + OH-

Kb = [BH+][OH-]/[B]

so,
pKb = -lg Kb

Question 9

What is the relationship between Ka & Kb of a conjugate acid base pair?

Answer 9

Ka x Kb = Kw (at all temp)
eg
at 25°C, Ka x Kb = 10^-14 mol²dm^-6

Question 10

What kind of salt is formed in each rxn:
- Strong base + strong acid?
- Weak base + strong acid?
- Strong base + weak acid?

Answer 10

• Strong base + strong acid
  neutral salt
• Weak base + strong acid
  acidic salt
• Strong base + weak acid
  basic salt

Question 11

Define buffer sol n

Answer 11

one which can maintain a fairly constant pH when small amt of acid or base is added to to it

Question 12

What are the 2 types of buffer sol n? What are their components?

Answer 12

• Acidic buffer:
  weak acid + salt of its conjugate base
  eg CH3COOH + CH3COONa
• Basic (alkaline) buffer
  weak base + salt of its conjugate acid
  eg NH3 + NH4Cl

Question 13

What are the 2 formulae for buffer sol n?

Answer 13

acidic buffer:
pH = pKa + lg ( [conjugate base]/[acid] )

basic buffer:
pOH = pKb + lg ( [conjugate acid]/[base] )

Question 14

Explain how acidic buffer works with two examples

Answer 14

*use single arrow (bcos full rxn)
1.
eg eqn: CH3COO- + H+ –> CH3COOH

When small amt of acid, H+, is added, the added H+ is removed as (eg CH3COOH). [H+] is slightly changed, hence pH remains fairly constant

2.
eg eqn: CH3COOH + OH- –> CH3COO- + H2O

When small amt of base, OH- is added, the added OH- is removed as (eg CH3COO- and H2O). [OH-] is slightly changed, thus pH remains fairly constant

Question 15

What to take note when calculations involve mixing?

Answer 15

new, effective conc = amt/new total volume

Question 16

Explain how basic buffer works with two examples

Answer 16

*use single arrow (full rxn)
1.
H+ + NH3 –> NH4+

When small amt of acid H+ is added, added H+ is removed as (eg NH4+). [H+] is slightly changed hence pH remains fairly constant

2.
OH- + NH4+ –> NH3 + H2O

When small amt of base OH- is added, added OH- is removed as (eg NH3 and H2O). [OH-] is slightly changed, thus pH remains fairly constant.

Question 17

Define max buffer capacity. What equations can be derived?

Answer 17

occurs when buffer sol n contain equal conc of weak acid/base and its salt

• acidic buffer: pH = pKa
• basic buffer: pOH = pKb

Question 18

How does pH of human blood remain fairly constant?

Answer 18

Carbonic acid/hydrogencarbonate ion (H2CO3/HCO3-) buffer pair plays vital role in maintain pH of human blood at almost constant pH value 7.4
H2CO3 (aq) ⇌ H+(aq) + HCO3-(aq)

Question 19

What is the criteria for choice of indicator?

Answer 19

an indicator is chosen for titration when:
- its pH transit n range lies within region of rapid pH change
- indicator gives distinct colour change at end pt

Question 20

What are common suitable indicators and region of rapid pH change for each of following titration type:
- SA-SB
- SA-WB
- WA-SB
- WA-WB

Answer 20

• SA-SB: pH 3-11, any indicator
• SA-WB: pH 3-7, methyl orange, screened methyl orange
• WA-SB: pH 7-11, bromothymol blue, phenolphthalein
• WA-WB: no sharp pH change, no suitable indicator

Question 21

What is a polyprotic acid?

Answer 21

An acid which can donate > 1H+ per acid molecule & it undergoes stepwise ionisat n