Acids and bases Flashcards
What is a bronsted acid?
Proton donor
What is a bronsted base?
Proton acceptor
What is used to measure the strength of acids?
pKa scale
Why is the value of water ignored in the equilibrum constant equation?
It is large and constant (55.56 moldm-3)
What equation dictates the strength of acid and why?
HA + H2O A- + H3O+
Energy of products and reactants are looked at and compared to determine if the reaction is favourable
What happens to pKa when the HA becomes a stronger acid?
Becomes lower
What does the pKa of the acid depend on?
The stability of the conjugate base
The more stable A- is…
The easier the deprotonation and the lower the pKa
The stronger the base A-…
The weaker the conjugate acid HA
The stronger the acid HA…
The weather the conjugate base A-
What is the pKa of HI?
-10
What is the pKa of HCl?
-7
What is the pKa of H2SO4?
-3
What is the conjugate base of H2SO4?
HSO4-
What is the pKa of H3O+?
-2
What is the conjugate base of H3O+?
H2O
What is the pKa of HSO4-?
2
What is the conjugate base of HSO4-?
SO4 2-
What is the pKa of a carboxylic acid?
5
What is the pKa of H2S?
7
What is the pKa of NH4+?
9
What is the conjugate base of NH4+?
NH3
What is the pKa of a phenol?
10
What is the pKa of an alcohol?
15
What is the pKa of H2O?
15
What is the conjugate base of H2O?
OH-
What is the pKa of a ketone?
20
What is the pKa of an alkyne?
25
What is the pKa of NH3?
33
What is the conjugate base for NH3?
NH2-
What is the pKa of a benzene ring?
43
What is the pKa of an alkane?
48
What are the 5 factors that affect pKa and acidity?
Weak A-H bonds Stability of the conjugate base Delocalisation of the negative charge Inductive effects Hybridisation
How does bond strength affect pKa?
The stronger the A-H bond, the higher the pKa
The weaker the A-H bond, the lower the pKa
What causes a A-H bond to be strong?
If there is a good overlap of the s orbital from hydrogen and the p orbital of the molecule attached
How does stability of the conjugate base (A-) affect pKa?
The negative charge being on the electronegative atom in the conjugate base
The more electronegative the atom, the more acidic and the lower the pKa (eg. F- is more acidic (pKa = 3) than -CH3 (pKa = 48) )
How does delocalisationt of the negative charge affect pKa?
It makes A- more stable and HA more acidic, lowering the pKa
How can C-H become acidic?
If the negative charge can be stabilised by resonance
How do inductive effects affect pKa?
They stabilise A- and increase the acidity of HA
What effect do electron donating groups do the pKa?
Raise it
What do flurorines do to a molecule with a negative charge?
They stabilise the negative charge by induction electron withdrawal
How does hybridisation affect pKa?
Looking at Sp3, sp2 and sp orbitals, the more ‘s’ character, the lower the energy and the lower the pKa. More acidic
Why is the more stable A- the weaker it will be as a base?
It doesn’t need the proton back if it stable
What is pKaH?
The protonated form
What is the pKaH of NH2-?
The same as the pKa of NH3 which is 33
Why is NH3 a stronger base4 than H2O?
The lone pair on nitrogen is much easier to donate than the lone pair on oxygen and N is less electronegative
The higher the pKaH of a base…
The weaker the acid it comes from and the stronger the base A- will be
What happens when you increase the electron density on nitrogen?
The base strength increases
Why are tertiary amines less basic than primary or secondary amines in solution?
The positive charge cannot be stabilised by solvation as once protonated there is only one chance for hydrogen bonding compared to three on primary amines
How does hybridization affect pKaH?
The more s character in an orbital, the lower the pKaH and the weaker it is as a base
How does conjugation affect base strength?
The more conjugation, the more stable the electron pair so the base strength decreases
