Acids and Bases Flashcards
Bronsted Lowry definition of acids and bases
acids are proton donors
bases are proton acceptors
Lewis acids and bases
Lewis acids are electron pair acceptors
Lewis bases are electron pair donors.
Lewis acid/base reactions frequently result in the formation of coordinate covalent bonds.
strong acid
one that dissociates completely in water
ex) HCl
weak acid:
dissociation does not go into completion.
weak acid is HF
strength of the acid is directly related to how much the products are favoured over the reactants.
Ka= acid dissociation constant.
if Ka >1 : then products are favoured and we say the acid is strong
if ka<1 : reactants are favoured and the acid is weak.
Ka and acid strength
the larger the ka , the stronger the acid.
the smaller the ka, the weaker the acid.
what are the common strong acids?
HI HBr HCl HClO4 H2SO4 HNO3 assume that any acid not on this list is weak acid.
why is HF weak?
F is smaller then Cl , Br, I
The more stable the conjugate base is the stronger the acid.
larger anions are better ale to spread out their negative charge making them more stable.
HF is the weakest of the H-X acids bc it has the least stable conjugate base, due to its size.
we can apply the same ideas to identify weak and strong bases.
Kb= base dissociation constant. the larger the kb the stronger the base and the smaller kb= weaker base.
strong bases
group 1 hydroxides ex NaOH
Group 1 oxides Li2O
some group 2 hydroxides BaOH2, SrOH2, CaOH2
Metal amides ex NaNH2
weak bases
ammonia NH3, conjugate bases of many weak acids
the conjugate base of a strong acid has no basic properties in water
ex) Cl- from HCL
the conjugate base of a weak acid is a weak base
ex) F- from HF
the weaker the acid the more the reverse reaction is favoured.
and the stronger the conjugate base!
the conjugate acid of a strong base has no acidic properties in water
ex ) Li + from LiOH , does not have any acidic properties in water.
the conjugate acid of a weak base…
acts as a weak acid! the weaker the base , the stronger the conjugate acid .
polyprotic
has more then 1 proton to donate, ex would be carbonic acid H2C03- in its dissociation . HCO3- first acts as a CB then it acts as an acid : therefore that means HCO3- is amphoteric : when a substance can act either as an acid or a base.
The ion product constant of water
water is amphoteric , it reacts with itself in a bronzed lowery acid base reaction and produces H3O+ and OH-
this is called self ionization of water.
the equilibrium expression is
kw= [H30+][OH-]
written as Kw : equilibrium expression for auto ionization of water , so kw is the ion-product constant of water.
at 25 degrees Celsius what is kw
1 x 10^-14
kw varies with?
temperature! like all other equilibrium constants!
kw will not be affected by concentration
ex) if we add an acid the [h3o+] will increase, equilibrium will be disturbed and the reverse reaction will be favoured. decreasing the [OH-] . Same thing happens if we increase [OH-], reverse reaction will be favoured. in either case, THE PRODUCT of [H30+] [OH-] will remain equal to kw
pH= - log [H+]
[H+] = 10^-pH
since [H+] = 10^-7
ph of water is 7.
pOH= -log [OH-]
[OH-] = 10^-pOH
Since [H+][OH-] =10^-14 at 25 degrees Celsius.
the values of pH and pOH satisfy a special relationship at 25 degrees Celsius.
pH +pOH = 14
rule of thumb about pH calculation
[H+] = y x 10^-n (n is a whole number)
if y is a number between 1 and 10, pH will be between (n-1) and n .
in general p of something = - log of that something
so pKa = -logKa
pKb= -logKb
larger the Ka , stronger the acid. but p means negative log of something. therefore, LOWER pKa value will mean higher acid strength.
acid with a lower pKa value is stronger acid and therefore
lower pKb value = stronger base
for any acid - base conjugate pair at 25 degrees celcius we will have
KaKb = Kw = 1 x 10^-14
as well as
pKa +pKb = 14
weak acids come to equilibrium with their dissociated ions. for a weak acid at equilibrium the concentration of undissociated acid will be much greater then the concentration of hydrogen ion.
to get the pH of a weak acid, you need to use the equilibrium expression.
so set it up as Ka= products/reactants
and if Ka is very small means its a very weak acid and not a lot of it dissociated . then we can assume the x part at the bottom is negligible.
when Ka is <10^-4 –> we can assume that the x being added or subtracted is negligible
when determining how much base we need to add to an acidic solution or how much acid we need to add to a base , in order to cause complete neutralization, we need the following formula
a x [A] x Va = b x [B] x Vb
a= number of acidic hydrogens per formula unit
b= constant tells us how many H+ ions the base can accept.
remember, regardless of the strengths of the acids or the bases…
all neutralization reactions go to completion.
neutralization reactions are exothermic
a salt is an ionic compound ! consists of cation and an anion
in water the salt will dissociate into cation and anion and depending on how these ions react with water , the resulting solution will be either acidic or basic or pH neutral.
cation of the salt
group 1 cations or larger group 2 cations : don’t react with water .
but the cation will result in acidic solution if its a stronger acid then water : NH4+, be2+, Cu 2+, Zn2+, Al3+, Cr 3+, Fe3+
anion
it won’t react with water if its the conjugate base of a strong acid.
but if its stronger base then water it will produce a basic pH ( THAT will be if its a CB of a weak acid).
Buffer
a buffer is a solution that resist change in pH when a small amount of acid or base is added. the buffering capacity comes from the presence of a weak acid and its conjugate base - or. a weak base and its conjugate acid in roughly equal concentrations
the Henderson hasselbach equation : for any buffer solution containing a weak acid and salt of its conjugate base.
pH= pKa + log ( conjugate base/weak acid)
to design a buffer solution
we choose a weak acid whose pKa is close to the desired pH. An ideal buffer solution would have [weak acid]= [conjugate base] , so then ph=pka
we can also design a buffer solution by choosing a weak base and salt of its conjugate acid such that the pea of the base is close to the pOH as possible.
the Henderson hasslebach equation would be:
pOH= pKb + log ( conjugate acid/weak base)
what is an indicator?
an indicator is a weak acid that undergoes a colour change when its converted to its conjugate base.
how do we select our indicator?
select one whose pKa value is convenient for our purposes
rule of thumb (but dosent always work) : an indicator will change colour in the range +/- 1pH unit from the pka
acid-base titration
is an experimental technique to determine the identity of an unknown weak acid or weak base by determining its pka or pkb.
- titrations can also be used to determine the concentrations of any acid or base solution.
procedure for acid base titration
adding a strong acid or strong base of known identity and concentration - the titrant- to a solution containing a the unknown base or acid. titrant is added in small discrete amounts , the ph of the solution will be recorded.
buffering region
where the pH changes very slowly . as the HF= F- concentration for ex.
then the solution will suddenly loose its buffering capability and ph will increase dramatically.
acid base equivalence point
when just enough base is added completely neutralize the HF
half equivalence point
the pka = pH
Polyprotic acid
the titration curve for the titration of a polyprotic acid like H2CO3 will have more than one equivalence point. the number of he equivalence points is equal to the number of ionizable hydrogens the acid can donate.
