Acids And Bases Flashcards
Common strong acids formulas
HNO3, H2SO4
HCl, HBr, HI
HClO4, HClO3
Common string bases
H-, N3-, O2-, OH-, alkoxide ions like CH3O-
define Bronsted-Lowry acid
proton donor
cations, anions, or molecules
define Bronsted-Lowry bases
proton acceptor
molecules or anions
define acid/base
donates/accepts protons more readily than water
upon reaction with H2O, produces more H3O+ or OH- than the reaction of water with itself.
Bases must have what?
must have an atom with a non-bonding electron pair to accept H+
in an acid/base reaction there is no change in…
oxidation number
define Lewis acid
e- pair acceptors
Acid, Acceptor
define Lewis base
e- pair donors
a Lewis acid-base reaction results in…
bond formation
making a Lewis acid-base complex
what is an oxoacid?
group of acids with an oxo group
=O
strengths of oxoacids depends on
number of oxo- groups
1 = weak acid CH3CO2H ethanoic acid H2CO3 carbonic acid H2SO3 sulfurous acid H3PO4 phosphoric acid
more than 1 = strong acid HNO3 nitric H2SO4 sulfuric HClO4 perchloric acid HClO3 chloric acid
general rules for anions arising from weak/strong acids
anions arising from weak acids are weak bases
anions arising from strong acids do not change the pH of water
weakly basic anions (arising from weak oxoacids)
ethanoate ion
carbonate ion
sulfite ion
phosphate ion
not basic anions (arising from strong oxoacids)
nitrate ion
sulfate ion
perchlorate ion
acid strength define
tendency of an atom to donate a proton to a base
acidity of EHn trends
increasing –> and down
two factors influencing the acidity of EHn
POLARITY of E-H bond
more polar = more acidic (hence increase C —> F)
STRENGTH of E-H bond
longer bond = more acidic
(hence increase down, HI>HBr>HCl>HF)
what is the only HnX more basic than water?
NH3
weak base
how does the strength of a base relate to charge density?
Stronger base = HIGHER e- density, or higher e- pair availability, at the accepting atom
monatomic anions have higher charge density at X than HnX
hence the anion is more basic than HnX
Xn- acidity
all strong bases except halides
high charge density
reaction metal hydride and water
forms metal hydroxide and H2
because H- is a strong base, so ionic compounds w/ H- react completely with H2O.
formula for finding pH given [H3O+]
pH = -log[H3O+]
formula for finding [H2O+] given pH
[H3O+] = 10^(-pH)
Kw expression and standard value
Kw = [H3O+] [OH-] = 1x10^-14
the equilibrium constant for the auto ionisation of water
found from the equilibrium expression of auto ionisation equation
what is pKw formula?
pKw = pH + pOH = 14 at 25˚C
pKw = -logKw
define autoionisation of water
water exchanging protons with itself
H2O + H2O == OH- + H3O+
acid base conjugates
differ by one proton
conj base has one FEWER H+, hence on unit more negative
define amphiprotic
species can ACT as an acid or a base
weak/strong acids in relation to H2O and H3O+
H2O < weak acid < H3O+ < strong acid
strong acids are product favoured as the reactant acid > product acid (H3O+)
weak acids ==
labeled concentration of acids vs actual concentration
actual concentration is less due to reaction w/ H2O
STRONG = same as label, same pH
WEAK: depends on c(acid) and the strength of the weak acid, they react to different extents, hence different pH for the same c(acid)
metal (G1) oxides and water reaction
form metal hydroxide
because O2- is a strong base
O2- + H2O –> 2OH-
general rules of the strength of acid-base conjugate pairs
the weaker the acid, the stronger the conj base
if one member is STRONG, the other DOES NOT react with H2O to change its pH
what is the acidity constant?
Ka, a property of an acid-base conjugate pair.
acid + H2O == H3O+ conj base
Ka = [H3O+][conj base] / [acid]
or [H3O+][A-] / [HA]
Ka relationship with strength of acid
HIGHER Ka, STRONGER acid
higher tendency to donate H+
reactant favoured = weak, Ka<1
pKa formula
pKa = -logKa
Ka formula from pKa
Ka = 10^(-pKa)
pKa values for weak and strong acid/conj pairs
Weak acid-base conj pairs
2 < pKa < 13
stronger acid, LOWER pKa
strong acid, negative pKa
strong acid pKa and Ka values
high Ka, low (negative) pKa
strong bases pKa(conj acid) and Ka(conj acid) values
Ka(conj acid) lower, pKa(conj acid) higher
pKa for polyprotic acid, eg H3PO4
which to use for amphiprotic species?
one pKa for each acidic H
pK1(H3PO4) = pKa(H3PO4) pK2(H3PO4) = pKa(H2PO4-) pK3(H3PO4) = pKa(HPO4^2-)
for amphiprotic species, us pKa for the appropriate pair
eg. acid strength HPO4^2- use pK3 (HPO4^2-/PO4^3-)
base strength use pK2 (H2PO4-/HPO4^2-)
what is an ICE table?
Initial: c(reactant) and c(products)
Change: + or - change
Equilibrium = Initial + Change
used to calculate changes in concentration
Ka value for weak acid-base conj pair
Ka < 0.001
very small extent of reaction
assumptions for calculating [H3O+] from known Ka
assume [acid] = c0(acid)
because weak acid, so insignificant amounts react
strictly, c0(acid) = [acid] + [conj base]
assume [H3O+] = [conj base]
by reaction stoichiometry
VERIFY validity of assumption:
check that [H3O-] «< c0(acid)
and [H3O+] from weak acid»_space; [OH-] from water
all solutions containing weak acid/base or a mixture are at equilibrium with respect to these two equations
acid + H2O == H3O+ + conj base
H2O + H2O == OH- + H3O+
what is a buffer solution?
resists change in pH or dilution on addition of small amounts of strong acid/base
has active weak acid-base conj pair, both MAJOR SPECIES at that pH
for a buffer, when does pH=pKa?
where [acid] = [conj base]
so that Ka = [H3O+] x 1
for a buffer solution, when is pH
pH [base]
(as [H3O+] > Ka)
if [acid] < 10x [base], pH is within 1 unit of pKa
approximate formula of Ka
Ka = [H3O+] x c(conj base)/c(acid)
if both acid and base are major, small extent of reaction
so Ka = [H3O+] x n(conj base)/n(acid)
how is a buffer prepared?
by mixing amounts of acid and conjugate base until it has a specified pH
OR by reacting a solution containing one member of the conj pair with a strong acid/base to generate the second member
formula to use when the TOTAL concentration of the buffer is given
n(acid) + n(base) = c(buffer) x V(buffer)
unknown x unknown = known x known
use with Ka expression with n to calculate two unknowns
identify if a proton transfer reaction is reactant or product favoured
if reactant acid and base is STRONGER than product acid, PRODUCT favoured
if reactant acid weaker, reactant favoured
define dibasic
capable of accepting either 1 or 2 protons
eg. 1,2-diaminoethane
base only
accept 1 proton, the cationic form with one tetravalent N (has +ve charge on it) is both acid and base
accept another proton, and the di-cation is a stronger acid
define buffer region
pH = pKa +/- 1
the flat region on a titration graph
define major species
species present in relatively high concentration
reaction extent formula
= product concentration / initial reactant concentration x 100%
where one member of the conj pair is the reactant, the other is the product
numerical extent of reaction for strong / weak a/b
strong = 100%
weak < 10%
if weak, extent greatest w/ strongest a/b
if there’s a common ion, the extent of reaction is LOW because it already exists in solution
define equivalence point on a graph
the point halfway up the steep rise on the graph
key points on a titration curve
halfway along the flat buffer region: the point where [a]=[b] and pKa = pH
to either side, [a]>[b] or vice versa
equivalence point - halfway up steep rise
titration curve axis
pH vs volume of titrant added
define end point
the volume at which the indicator changes colour
factors influencing the shape of a titration curve
initial pH = substance in the flask
final pH = equal to that of diluted titrant
direction - which is added to which
strong = larger steep change section, change in pH of 8-10 units weak = smaller steep section, change in pH of 4-6 units
methyl orange indicator colours
red in acid form
yellow is base form
pKa = 3.7
colour changes over pH=pKa+/-1
bromocresol purple indicator colours
Yellow in acid form
Purple in base form
pKa = 6.3
Phenolphlalein indicator colours
Colourless in acid form
Purple in base form
pKa = 9.6
define polyprotic acid
has more than one acidic Hydrogen that can be removed sequentially by OH-
eg. H3PO4
find pH at equivalence points for amphiprotic anion
pH independent of concentration of the solute, given by
pH = 1/2 (pK1+pK2)
graphically, equivalence point halfway between the two flat buffer regions of pK1 and pK2
the extent of reaction is greater in a more ___ solution
dilute
because initial concentration low ??
smallest extent of reaction w/H2O when one product already exists as a major species in equilibrium
at equilibrium of a weak acid, major and minor species
major: weak acid, H2O
minor: conj base, H3O+, OH-
at equilibrium of a strong acid major and minor species
major: the conjugate base, and H3O+ (or OH- if base)
if both acid base members of a conj pair are in solution, major species are
both are major species
also the ion (eg. Na+, Cl-, K+)
acid hydrogens are always bonded to which element?
Oxygen.
more than one acidic H = polyprotic acid
