Équilibre Acid-Base Flashcards
What does p mean? (acid-base eqm)
-lg [sth]
eg
pH = -lg [H+]
pOH = -lg [OH-]
Define Kw
- 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)
pKw=? What can be derived at 25°C?
pKw
=-lgKw
= pH + pOH (since Kw = [H+][OH-])
so,
14 = pH + pOH at 25°C
At 25°C, how do pH, [H+] and [OH-] compare in neutral, acidic and alkaline sol n?
neutral: [H+]=[OH-], pH=7
acidic: [H+]>[OH-], pH<7
alkaline: [H+]<[OH-], pH>7
If temp of water increase, how does Kw and pH change? Why?
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+]
Define Ka
For dissociat n of acid HA,
HA ⇌ H+ + A-
Ka = [H+][A-]/[HA]
so,
pKa = -lg Ka
What is the formula calculating [H+] of weak acid/bases?
acids: [H+] = √(c x Ka)
bases: [OH-] = √ (c x Kb)
where c is initial conc of acid/base
Define Kb
For dissociat n of base B
B + H2O ⇌ BH+ + OH-
Kb = [BH+][OH-]/[B]
so,
pKb = -lg Kb
What is the relationship between Ka & Kb of a conjugate acid base pair?
Ka x Kb = Kw (at all temp)
eg
at 25°C, Ka x Kb = 10^-14 mol²dm^-6
What kind of salt is formed in each rxn:
- Strong base + strong acid?
- Weak base + strong acid?
- Strong base + weak acid?
- Strong base + strong acid
neutral salt - Weak base + strong acid
acidic salt - Strong base + weak acid
basic salt
Define buffer sol n
one which can maintain a fairly constant pH when small amt of acid or base is added to to it
What are the 2 types of buffer sol n? What are their components?
- 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
What are the 2 formulae for buffer sol n?
acidic buffer:
pH = pKa + lg ( [conjugate base]/[acid] )
basic buffer:
pOH = pKb + lg ( [conjugate acid]/[base] )
Explain how acidic buffer works with two examples
*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
What to take note when calculations involve mixing?
new, effective conc = amt/new total volume
Explain how basic buffer works with two examples
*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.
Define max buffer capacity. What equations can be derived?
occurs when buffer sol n contain equal conc of weak acid/base and its salt
- acidic buffer: pH = pKa
- basic buffer: pOH = pKb
How does pH of human blood remain fairly constant?
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)
What is the criteria for choice of indicator?
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
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
- 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
What is a polyprotic acid?
An acid which can donate > 1H+ per acid molecule & it undergoes stepwise ionisat n
