Topic 18 - Acids / Bases Flashcards
Arrhenius acid definition
Substance with covalently bonded H atoms and dissociates in H2O to yield H3O+
Arrhenius base definition
Substances with OH in its formula that dissociates in H2O to yield OH
Arrehenius Acid-Base Reaction
Neutralization
a) Exothermic (-55.9 kJ mol-)
b) Formation of H2O (l) and salt
Strength of an acid / base
Extent of dissociation to produce [H+] and [OH−] in solution
Strong acid / base
Fully dissociate in solution (~100%)
Weak acid / base
Partially dissociate in solution (<5%)
Substances that behave as strong acids
HNO3
HCl
HBr
HI
H2SO4
HClO3
HClO4
Substances that behave as weak acids
a) HF
b) Oxoacids (# O atoms = > ionizable H+)
c) H is not bonded to O or to a halogen
d) Carboxylic acids (ROOCH)
Substances that behave as strong bases
Water soluble compounds containing OH- or O2-
a) G1
b) G2 (except Mg)
Substances that behave as weak bases
a) Those with an e- rich N atoms (Ammonia / Amines)
b) Anions of weak acids
Acid dissociation constant (Ka)
Measures the extent of acid dissociation in water
Water Autoionization Formula
2H2O(l)⇌H3O+(𝑎𝑞 )+OH−(𝑎𝑞 )
Reasons for Water Autoionization
Amphoteric nature of H2O
Ion-product constant of H2O (at 298K)
Kw = [H+][OH−] = 1.0×10−14
Relationship between H3O+ and OH-
Inversely proportional
a) As the concentration of one of these ions increases, the concentration of the other must decrease
When is a solution…?
a) Neutral
b) Basic
c) Acidic
a) H+ = OH-
b) H+ < OH-
c) H+ > OH-
pH
-log([H+])
Number of significant figures inside the logarithm should equal…
the number of decimal places in the number
pH + pOH = … (at 298K)
14
How is pH measured?
a) Acid-base indicators. Organic molecules where color depends on the solution’s acidity
b) pH meter (Probe with 2 electrodes)
Bronsted-Lowry acid definition
Proton donor
Bronsted-Lowry base definition
Proton acceptor containing a lone pair of e-
Brønsted-Lowry Acid-Base Reaction
Proton transfer process that can occur between gases, nonaqueous solutions, and heterogeneous mixture
Conjugate acid-base pairs
Two species which differ by H+
a) Acid becomes into the conjugate base
b) Base becomes into the conjugate acid
Relationship between the strength of a conjugate acid and a conjugate base
The weaker the acid/base, the stronger the conjugate base/acid
Amphiprotic definition
Substance capable of accepting and donating H+
A reaction proceeds to the greater extent in the direction in which…
a stronger acid and stronger base form a weaker acid and weaker base
Assumptions of weak-acid equilibria problems
a) [H3O+] from the autoionization of H2O is negligible
b) A weak acid has a small Ka, so we can neglect the change in its concentration to find its equilibrium concentration
% Acid dissociation formula
([HA]dis / [HA]i)*100
Why does Ka of successive ions differ by several orders of magnitude?
It is more difficult for the positively charged H+ ion to leave a singly charged anion than to leave a neutral molecule
Factors affecting the acid strength of nonmetal hydrides
a) Electronegativity of central nonmetal
b) Strength of the EH bond
Polyprotic Acids
Acids with more than one ionizable H+
Trends of the acid strength of nonmetal hydrides
a) Across a period: Increasess
b) Down a group: Increases
Factors affecting the acid strength of oxoacids
a) Electronegativity of central nonmetal
b) # O atoms around central nonmetals
Effect of concentration on the extent of acid dissociation
As the initial acid concentration decreases, the percent dissociation of the acid increases
Trends of the acid strength of nonmetal hydrides
a) Acid strength increases with the electronegativity of nonmetal
b) Acid strength increases with # of O atoms
Reasons for the acidity of hydrated metal ions
Small and highly charged, which withdraws sufficient e- density from OH bonds of the bound water molecules for an H+ to be released
Relationship between Ka and Kb of a Conjugate Acid-Base Pair
(Ka)*(Kb) = Kw
Acidity of a salt from strong acid and strong base
Neutral
(NaCl)
Acidity of a salt from strong acid and weak base
Acidic
NH4+ + H2O ↔ NH3 + H3O+
Acidity of a charged metal cation and anion of strong acid
Acidic
Al(H2O)63+ + H2O ↔ Al(H2O)5OH2+ + H3O+
Acidity of a salt from strong base and weak acid
Basic
NO2− + H2O ↔ HNO2 + OH−
Acidity of salts of amphiprotic anions or anions from weak acids / cations from basic cations
Depends on Ka and Kb
Leveling Effect of Water
An acid yields the cation and a base yields the anion that would be produced by autoionization of the solvent.
Why are all strong acids and
strong bases equally strong in water?
In water, the strongest acid
possible is H3O+ and the strongest base possible is OH−:
Method used to rank the relative strength of strong acids
Dissolve them in a solvent that is a weaker base than water, so it accepts their protons less readily
Lewis acid definition
Species that can accept a pair of electrons
Lewis base definition
Species that can donate a pair of electrons
Examples of Lewis Acid compounds
e- deficient compounds such as B/Al
Molecules with a polar double bond (e.g. SO2 / CO2)
Metal ion in complex ions
Lewis-acid base reaction
Donance and acceptance of an e- pair to form an adduct, a single species that contains a new covalent bond
