8. acids and bases (sorry booboo i dont understand) Flashcards
ionic theory
in aq sol, hydrogen ions are hydrated to form hydroxonium ions (H3O +)
base: substance that can neutralize an acid
alkali: a base soluble in water
bronsted-lowry acids and bases + conjugate acids and bases
BL acid: can donate a proton
BL base: can accept a proton
conjugate base: species remaining after the acid as lost a proton
conjugate acid: species remaining after base has accepted a proton
substances (eg. water) that can donate or accept protons are amphiprotic
calculate pH of strong acids and bases
eg. pH of 0.1 mol/dm³ of NaOH
[OH-] = 0.1 mol/dm³
[H+] = 1e-14 / 0.1 = 1e-13
pH = -lg(1e-13) =13
acid deposition
oxides of sulfur:
sulfur dioxide occurs naturally from volcanoes
S(s) + O2(g) -> SO2(g)
sulfur dioxide in sunlight is oxidized to sulfur trioxide
SO2(g) + 1/2 O2(g) -> SO3(g)
oxides can react in the air to form sulfurous acid and sulfuric acid
SO2(g) + H2O(l) -> H2SO3(aq)
SO2(g) + H2O(l) -> H2SO4(aq)
oxides of nitrogen:
nitrogen oxides occur naturally from electrical storms
N2(g) + O2(g) -> 2NO(g)
nitrogen monoxide is produced in jet engines and combustion engines, oxidation of nitrogen monoxide occurs in the air
2NO(g) + O2(g) -> 2NO2(g)
nitrogen dioxide reacts with water to form nitric acid and nitrous acid
2NO2(g) + H2O(l) -> HNO3(aq) + HNO2(aq)
OR
oxidized to nitric acid directly by oxygen in the presence of water
4NO2(g) + O2(g) + 2H2O(l) -> 4HNO3(aq)
add calcium oxide or calcium hydroxide into lakes to neutralize acidity
Lewis acids and bases
LA: accept pair of electrons
LB: donate pair of electrons
in the process, a coordinate (both electrons provided by one species) covalent bond is formed between the acid and base
pOH, pH, pKw
Kw - ionic product of water
= [H+] x [OH-]
*dissociation of water is endothermic => Kw will increase as temperature increases
pH of water decreases at high temp = [H+] < [OH-]
- lg(Kw) = - lg[H+] - lg[OH-]
pKw = pOH + pH
at 25°C, Kw = 1e-14, pH + pOH = 14
weak acids and bases
dissociation of a weak acid:
HA(aq) ⇌ H+(aq) + A-(aq)
acid dissociation constant (Ka) = [H+] x [A-] / [HA]
* for weak acids, equilibrium conc of acid assumed to = initial conc
=> Ka = [H+]² / [HA]
reaction of weak base:
B(aq) + H2O(l) ⇌ BH+(aq) + OH-(aq)
base dissociation constant (Kb) = [BH+] x [OH-] / [B]
- the weaker the base the smaller the value of Kb and the larger the value of pKb
salt hydrolysis
salt: strong acid + strong base = neutral in aq sol (ions are fully dissociated)
salt: strong base + weak acid = alkaline in aq sol
salt: strong acid + weak base = acidic in aq sol
buffer solutions
buffer solutions are resistant to change in pH when a small amount of acid or base is added
acidic buffer solutions:
weak acid + salt of chosen acid + strong base
eg. ethanoic acid + sodium ethanoate
weak acid: partially dissociated in solution
salt: fully dissociated into its ions => conc of ethanoate ions is high
acid added: extra H+ ions are removed as they combine with ethanoate ions to form undissociated ethanoic acid => unaltered [H+]
alkali added: OH- removed by their reaction with undissociated acid to form water => unaltered [H+]
alkali buffer solutions:
weak base + salt of chosen base + strong acid
titration curves and indicators
strong acid + strong base:
S-shaped curve (large change in pH at equivalence point => max gradient (vert line))
point of inflexion when [H+] = [OH-]
weak acid + strong base:
S-shaped with buffered region before equivalence point (pH > 7) as more base is added
weak acid + weak base:
no sharp point of inflexion => should not be used for analytical chemistry
indicators:
titrations with strong acids - methyl orange
titrations with strong bases - phenolphthalein
