Acids and Bases Flashcards
Arrhenius acid
a substance that dissociates in water to produce hydrogen ions, H+
Arrhenius base
a substance that dissociates in water to produce hydroxide ions, OH-
shortfalls of Arrhenius theory
works with many compounds in water, but misses some (particularly bases), and clearly doesn’t describe non-aqueous solutions
bronsted Lowry acid
a substance that can give a hydrogen ion (proton donor)
bronsted Lowry base
a substance that can take a hydrogen ion (proton acceptor)
difference/similarity between BL bases and Arrhenius bases
D: B-L bases do not need to contain OH-
S: they can generate OH- when in water
general equation for a Bronsted Lowry acid and base interacting
HA + B <—> BH+ + A-
conjugate acid-base pairs
chemical species whose formulas differ by only one H+
acid dissociation
HA + H2O <–> H3O+ + A-
- an equilibrium
- H2O solvent is an (almost) pure liquid so is not in equilibrium expression
- Ka only describes the reaction of an acid with the solvent H2O as the base
- the stronger the acid, the larger the Ka
base dissociation
B + H2O <–> OH- + BH+
- stronger the base, the larger the Kb
amphiprotic
can both donate and accept protons
amphoteric
can act as both an acid and a base
dissociation of water
H2O + H2O <–> H3O+ + OH-
Kw = [H3O+][OH-] = 1 x 10^-14
pH =
-log[H3O+]
[H3O+] =
10^-pH
pOH =
-log[OH-]
[OH-] =
10^-pOH
basic solution
neutral solution
acidic solution
- [H3O+]<[OH-], pH > 7
- [H3O+] = [OH-], pH = 7
- [H3O+]>[OH-], pH < 7
strong acid
fully dissociates in water
Ka»_space; 1
weak acid
partially dissociates in water
Ka = 1
inert acid
does not dissociate in water
conjugates:
strong acid <-> ‘inert’ base
weak acid <-> weak base
‘inert’ acid <-> strong base
compare strong acids/bases and weak acids/bases as electrolytes
strong acids and strong bases are strong electrolytes. we assume they ionise completely in water.
weak acids and weak bases are weak electrolytes. they ionise to a limited but detectable extent in water
The levelling effect
for a strong acid in water, the ‘active proton donor’ isn’t HA, its H3O+
for a strong base in water, the active proton acceptor is OH-
- different strong acids usually have different Ka’s
- however, in water, they exhibit the similar acidic properties, i.e. the acid strength of H3O+
pKa + pKb =
14
distinguish between pH, pKa and Ka
pH measures the acidity of the solution and depends on the absolute [H3O+]
pKa and Ka reflect the strength of the acid molecule and depends on the relative concentration at equilibrium
percent dissociation
a common and useful description of a weak acid in solution
% dissociation = [HA]dissociated/[HA]initial x 100
factors that affect acid strength
- degree of polarity of H-A bond
- strength of H-A bond
- oxoacids
degree of polarity of H-A bond
depends upon the electronegativity of A
the more polar the H-A bond, the stronger the aid
strength of H-A bond
depends on the size of the A atom. the larger the A atom, the longer/weaker the bond, the stronger the acid
along the period, the most important factor is
electronegativity
down the group, the most important factor is
the H-A bond strength
oxoacids
YOm(HO)n
- eg H3PO4
- if same structure, different Y: acid strength increases as the electronegativity of Y increases
- a more electronegative Y pulls electron density away from O-H bonds, making it easier for H+ to dissociate
- the strength of oxoacids also increases with m, the number of lone oxygen atoms
- the electronegative oxygen atoms pull electron density from the chlorine, making it more positive, which in turn weakens the O-H bond
salt solutions from conjugates of strong acids and strong bases are
neutral
strong acids
HClO4, H2SO4, HNO3, HCl, HBr, HI
strong bases
LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2, Ba(OH)2
salts that are derived from a weak base and strong acid yield
acidic solutions
why does weak base + strong acid give acidic solutions?
forms weak conjugate acid of a weak base which then dissociates
eg NH4Cl (see slides)
- solutions of NH4Cl are acidic because the ammonium ion (NH4+, a weak acid) will dissociate while the chloride ion does not
salts, such as NaCN, that are derived from a strong base (eg NaOH) and a weak acid (eg HCN) yield basic solutions
see slides for example
Ka>Kb
Ka<Kb
solution contains excess H3O+ ions so pH < 7
solution will contain excess OH- ions so pH>7
Lewis acid
a species that accepts an electron pair
Lewis base
a species that donates an electron pair
hydrated metal cations
small, highly-charged metal ions (eg Al3+) form complexes in water
the resulting complexes are proton donors
acid rain
- pollutants such as sulfur oxides and nitrogen oxides dissolve in rain to form acids
- buildings and monuments made of marble and limestone eroded by acid rain
state the 2 equations for acid rain
ions that do not react appreciably with water to produce either H3O+ or OH-
conjugate cations from strong bases:
- alkali metal cations (group 1)
- alkaline earth metal cations (group 2)
conjugate anions from strong monoprotic acids
- Cl-, Br-, I-, NO3-, ClO4-
because they are inert
