Topic 8 - Acids & Bases Flashcards
Arrhenius’ theory
Acid: substance that ionises in water to produce H+
Alkali: soluble base that ionises in water to produce OH-
All alkalis are bases but not all bases are alkali.
Limitation of Arrhenius’ concept
the rxn between NH3 and HCl gas can’t be explained, as NH3 doesn’t contain OH-
NH3 (g) + HCl (g) -> NH4Cl (s)
Bronsted-Lowry theory
Acid: proton (H+) donor
Base: proton (H+) acceptor
In an aq soln, a proton can be represented as either hydrogen (H+) or hydronium (H3O+)
in what conditions will H3O+ form?
when a water molecule forms a coordinate bond with a proton
common acids are referred to as:
- monoprotic: donates 1 proton
eg. HCl - diprotic: donates 2 protons
eg. H2SO4 - triprotic: donates 3 protons
eg. H3PO4
What can be concluded in a reversible rxn involving an acid/base? Give an example.
- the acid/base is weak as they don’t fully dissociate
e.g. CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)
CH3COOH: BL acid
H3O+: conjugate acid
conjugate
if it’s a backwards rxn instead of forwards, the conjugate base will act as a base
conjugate acid-base pair
- conjugate acids and bases will differ from one another by a single proton
- they are called conjugate acid-base pairs
Amphiprotic species
species that can act as either BL acid or BL base depending on the rxn
e.g. HCO3 - (aq) + H2O (l) CO3 2- (aq) + H3O+ (l)
HCO3 - (aq) + H2O (l) H2CO3 (aq) + OH- (aq)
zwitter ion
acts as an acid in the presence of a strong base by donating a proton, and vice versa for strong acids
eg. H2O
Requirements for BL
Acid: must be able to dissociate and release H+
Base: must be able to accept H+ (have lone e- pair)
Amphiprotic: must possess both a lone e- pair and a H+ ion
difference between amphiprotic and amphoteric
- amphiprotic specifically related to BL theory (where emphasis is on proton transfer)
- amphoteric has a broader meaning, describing a substance that can act as both acid & base even in rxns that don’t involve proton transfer
types of bases
- metal oxides/hydroxides
- ammonia
- soluble carbonates
- hydrogen carbonates
why doesn’t HNO3 release H2 gas?
because of its oxidising properties
ACID + METAL -> ?
Acid + metal -> salt + H2
ACID + BASE -> ?
acid + base -> salt + water
ΔH(neut)
enthalpy change occurring when an acid and base react together to form 1 mol of water
for all strong acids & bases, enthalpy change is very similar: ΔH = -57 kJ/mol
ACID + CARBONATE -> ?
acid + carbonate -> salt + H2O + CO2
acid turns phenolphthalein…
colourless
acid turns methyl orange…
red
acid turns litmus paper…
red
acid tastes…
sour
bases taste…
bitter
bases turn litmus paper…
blue