Relative Strength of Acids & Bases (College Board AP Chemistry)

Why are some acids stronger than others?

• Acid strength is determined by the stability of the conjugate base of the acid
  • The more stable the conjugate base the stronger the acid 
  • Or, the stronger the acid, the weaker the conjugate base
  • An acid will donate a proton to form a conjugate base  

HA (aq)	+     H2O (l)		H3O+ (aq)	+    OH– (aq)
acid	base		conj. acid	conj. base

Table of acids and conjugate bases

Acid	Conjugate base
HNO3	NO3–
H2SO4	HSO4–
HClO4	ClO4–
HCl	Cl–
HBr	Br–
HI	I–

Factors that affect acid strength

Bond polarity

• In general, as the polarity of the H-A bond increases, the strength of the acid will increase
• For example, oxy acids (A-OH) react with water via the following reaction

AOH + H₂O AO^– + H₃O⁺

Oxy acids (A-OH), Polarity and pK_a value

Acid	Name	Polarity of O–H	*pKa
Cl–O–H	hypochlorous acid	Most polar	7.5
Br–O–H	hypobromous acid	-	8.7
I–O–H	hypoiodous acid	Least polar	10.7

*The lower the pK_a value, the stronger the acid

• The electronegativity of the halogen atom increases up the group, so the polarity of the O–H bond will increase up the group as well
• This is because of the positive inductive effect
  • The more electronegative the element (X) the more polar the O-H bond as more electron density is withdrawn from the O–H bond

The inductive effect in an oxy acid

The more electronegative X element withdraws electron density from the OH bond

• Therefore the acid is more likely to donate a proton

Bond strength

• The weaker the bond, the more easily the proton from HA occurs
• In general, the weaker the bond, the stronger the acid
• For example, the bond strength of the binary acids from Group 17 in the periodic table becomes stronger moving down the group
• This is due to the increase in bond length of the H-X bond and, therefore, the increased ability for the acid to donate a proton

Table to show the relative bond enthalpies and strengths of the Group 17 binary acids

Acid	Bond Enthalpy (kJ mol–1)	pKa Value *
H-F	567	3.2
H-Cl	431	-7.0
H-Br	366	-8.0
H-I	299	-10.0

*The lower the pK_a value, the stronger the acid

Stability of the conjugate base

• In general, the more stable the conjugate base, the stronger the acid
• This is because it is more likely that HA will donate a proton
• We can demonstrate this with the ethanoic acid and chlorine-substituted derivatives

pK_a values of ethanoic acid and chlorine-substituted derivatives table

Acid	pKa Value *
Ethanoic acid	4.8
Chloroethanoic acid	2.9
Dichloroethanoic aicd	1.3
Trichloroethanoic acid	0.6

*The lower the pK_a value, the stronger the acid

• The pK_a values of ethanoic acid and chloro-substituted derivatives show that the more electron-withdrawing groups there are on the carbon attached to the -COOH group, the stronger the acid

 Acid strength in chlorine substituted carboxylic acids

The more chlorine atoms there are in the carboxylic acids, the stronger the acid is

Trichloroethanoic acid is the strongest acid as:

• The O-H bond in CCl₃COOH is the weakest since there are three very strong electronegative Cl atoms withdrawing electron density from the -COOH group
• When the O-H is broken to form the carboxylate (-COO^-) ion, the charge density is further spread out by the three electron-withdrawing Cl atoms
• The carboxylate ion is so stabilised that it is less attracted to H⁺ ions

 Relative acidity of trichloroethanoic acid

The trichloroethanoate ion is very stable due to the distribution of charge density

• Ethanoic acid is the weakest acid as:
  • It contains an electron-donating methyl group which strengthens the O-H bond
  • The methyl group donates negative charge towards the -COO^- group which becomes more likely to accept an H⁺ ion

 Relative acidity of ethanoic acid

The ethanoate ion has increased electron density on the oxygen 

Strong Bases

• The same concept will apply to bases
• Examples of strong bases include Group 1 bases (NaOH, KOH)
• These strong bases will have weak conjugate acids
  • E.g. 

NaOH (aq)	+     HCOOH (aq)		H2O (aq)	+    HCOO–Na+ (aq)
base	acid		conj. acid	conj. base
pKb = 0.2			pKa = 14.00

• Weaker bases such as nitrogenous bases like ammonia and carboxylate ions, have stronger conjugate acids

NH3 (aq)	+     H2O (aq)		NH4+ (aq)	+    OH– (aq)
base	acid		conj. acid	conj. base
pKb = 4.25			pKa = 9.26