P- Acid-Base Equlibria Flashcards
State Brønsted–Lowry theory
- acid = proton donor
- base = proton acceptor
How does an acid produce H+ ions in solution?
H+ reacts w/ H2O molecules by giving a proton
Equation for reaction between a strong acid and water.
HA (g) + H2O (l) –> H3O+ (aq) + A- (aq)
- H3O = hydroxonium ion
- simpler ver: HA (aq) –> H+ (aq) + A- (aq)
def + examples of strong acids
- completely ionise
- eg. HCl, H2SO4, HNO3, H3PO4
def + examples of weak acids
- partially ionise
- eg. all organic acids (carboxylic) ethanoic CH3COOH, only inorganic weak acid- HF
In a reaction, how much of a strong acid is turned into products?
100%
Give an example of a monoprotic acid
HCl –aq–> H+ (aq) + Cl- (aq)
1 mole of HCl gives 1 mole of H+ –> monoprotic
- HNO3- strong monoprotic
Give an example of a diprotic acid
H2SO4 (l) –aq–> 2H+ (aq) + SO4 2- (aq)
1 mole of H2SO4 gives 2 moles of H+
has a ↓ pH for the same conc as monoprotic ∵ gives ↑ moles of H+
Give an example of a triprotic acid
H3PO4 –> 3H+ (aq) + PO4 3- (aq)
1 mole of H3PO4 gives 3 moles of H+
Define pH.
pH = -log10 [H+]
- [ ]- conc in moldm^-3
p: -log10
ALWAYS GIVE TO 2dp
Equation for reaction between a weak acid and water
HA (aq) ⇌ H+ (aq) + A- (aq)
[H+] = ?
rearrange pH formula
[H+] = 10 ^-pH
Give an equation for the dissociation of HF.
HF (g) ⇌ H+ (aq) + F- (aq)
Calculating pH from [H+]:
What is the pH if [H+] = 1 x 10^ 4
4
1 x 10^4 –> 4
check both values are roughly the same
Formula for [H+] of diluted acid.
[H+] = original [H+] x org. vol/ diluted vol (cm3)
HA (l) ⇌ H+ (aq) + A- (aq)
Ka = ?
[H+] x [A-]
Ka = ——————–
……………..[HA]
- only for weak acids!!
- [H+] = [A-]
- ↓ Ka - weaker acid (↓pKa)
pKa = ?
pKa = -log10 Ka
- Ka = 10^ -pKa
What limits the range of pH scale?
solubility - saturated solution- x dissolve any more
Is water an acid or a base?
Amphoteric- both
What is the equation for water at equilibrium?
H2O (l) ⇌ H+ (aq) + OH- (aq)
What is the conc of H+ at room temp?
1 x 10^ -7 moldm-3
pH 7.00
Kw = ?
Kw = [H+][OH-]
1 x 10^ -14 mol2dm-6 at stp
stp = 298K, 100kPa
pH value for neutral
varies w/ temp
def for neutral
[H+] = [OH-]
Is the dissociation endothermic or exo + why?
endo
- ↑ temp ↑ Kw – eq. shifted to RHS
What is a strong base + give examples?
fully ionises in water
- eg. NaOH, KOH, Ca(OH)2, Ba(OH)2
What is a weak base + give an example.
partially ionises in water
- eg. NH3 {NH3 (g) + H2O (l) ⇌ NH4 + (aq) + OH- (aq)}
What are [H+] and [OH-] in strong bases?
they are different!
Kw still 1 x 10^-14
How do you compare strengths of bases?
see how readily they produce OH- in solution-
1. contain OH- (eg. NaOH (s) –> Na+ (aq) + OH- (aq) )
2. take H+ from H2O (eg. CaO (s) + H2O (l) –> Ca2+ (aq) + 2OH- (aq) )
Formulas for the half neutralisation of a weak acid
when half of HA molecules reacted w/ OH-:
- [HA] = [A-]
- Ka = [H+]
- pKa = pH
What are acid buffers made of?
weak acid + salt of weak acid
eg. CH3COOH + CH3COO- (both at high conc.»_space; [H+]
How does an acidic buffer work?
when H+ (acid) added:
- CH3COO- + H+ –> CH3COOH
- H+ removed by CH3COO- –> x much change in [H+] —> pH stable
when OH- added:
- CH3COOH + OH- –> CH3COO- + H2O
- OH- removed by CH3COOH –> pH stable
What is a basic buffer made of?
weak base + salt of weak base
How does a basic buffer work?
eg. NH3 + NH4+
H+ added:
- NH3 + H+ –> NH4+
- H+ removed by NH3 –> [H+] barely changes –> pH stable
OH- added:
- OH- + NH4+ –> NH3 + H2O
- OH- removed by NH4+ –> pH stable
What is the equivalence point?
when solutions are mixed in exactly the right proportions as in the equation
What is the end point?
when indicator changes colour
