Acid-base and pH I Flashcards
Why acids and bases are important?
- acid /base reactions in our body
- food contains different acids/bases
- most drugs are weak acid or bases
3 main definitions of acid/base
-Arrhenius
-bronsted lowry
-lewis theory
Arrhenius
- acid : produces H+ and an anion in water
- base : produce HO- and a cation in water
- Neutralisation : produces a salt (ionic compound, anion of an acid + cation of a base)
Bronsted-Lowry
Acid : a H+ donor
Base : a H+ acceptor
Acid-base reaction : H+ is transferred from an acid to a base
Lewis Theory
Lewis acid : accepts e- (electrophile), e- deficient
Lewis base : donates e- (nucleophile) to a nucleus with an empty orbital, e-rich
Form covalent bond
Arrhenius base
KOH
Lewis acid
BF3
Lewis base
AsH3
Bronsted-Lowry conjugate base/ acid
- HNO2 (acid) / H3O+ (conj. acid)
- H2O (base) / H3O+ (conj. base)
Bronsted-Lowry strong acid
powerful proton doner
Bronsted-Lowry strong base
High tendency to accept protons
Bronsted-Lowry weak acid
-conjucated acid of strong base.
-weak tendence to donate protons
Bronsted-Lowry weak base
-conjugated base of strong acids
weak tendency to accept protons
acid-base reaction
equilibrium favours the formation of the weaker acid or base
Water
-can be acid or base
-AMPHOTERIC SOLVENT
ACID? BASE?
“Any species that contains hydrogen can potentially act as an acid, and any
compound that contains a lone pair of electrons can act as a base”
Ka
acidity constant
K
equilibrium constant
Strong acids, ka=
-large Ka
-completely dissociated (ionised)
weak acids , Ka?
-small value Ka
-produce less dissociated
smaller the pKa..
stronger the acid
larger the pKa…
weaker the acid
Kw - ionisation constant H2O
10^-14
because
Kw = (H3O+)(HO-)
( 10^-7 )^2
= 10^-14
pKa (acid) + pKb (conjugated base of acid)
pKw = 14
Basicity
Basicity relates to the ability of a compound to use its nonbonding electrons
to combine with a proton
What influence acidity and basicity?
- Stability of the conjugate base
- Electronegativity
- Bond energy
- Electron donating and withdrawing
- Hybridisation
- Resonance/ delocalisation
Stability of the conjugate base…
- To assess acidity = stabilisation of conjugate base
- To asses basicity = ability to use its nonbonding e- to combine with a proton
Electronegativity
- ACIDITY increase = More electronegative elements Help to stabilise the -ive charge of conjugate base (bigger Ka, lower pKa)
-BASICITY decrease = E- more electronegative elements are less likely to be donated to a proton
Bond energy
Acidity INCREASE= descending group
Increasing size of the atom and the corresponding improved ability to disperse the -ive charge over the atom. Weakening in bond strengths (with H)
Electron DONATING (acidity)
destabilising the acids conjugate base
Electron WITHDRAWING (acidity)
stabilising the acids conjugate base
-“attract the e- from the negatively charged atom of the conjugate base, which are then shared with all the atoms of the molecule
Electron DONATING (basicity)
Increase basicity - stabilises the conjugate acid
Electron WITHDRAWING (basicity)
Decrease basicity - destabilise the conjugate acid
Hybridisation effect..
Acidity C-H bond = C sp e- held closer, conjugated base MORE stable, more acidic compound
Hybridisation state of the C - sp3, sp2 or sp
Why is lone pair of e- in an sp2 or sp orbital more difficult to protonate (weak base) than e- in sp3 orbital?
The e- is held closer to the nucleus so therefore the attraction is greater
Delocalisation of charge in the conjugate base anion through resonance is a stabilising factor which…
INCREASE ACIDITY
Delocalisation of the -ive charge into the aromatic ring system
more acidic
