Chapter 9: Unit 4. Strengths of Acids and Bases

Strengths of Acids and Bases

Acids and bases can dissociate in water to produce their corresponding conjugate acids – bases pairs. The degree of such dissociation will determine the strength of the acids and bases. Stronger acids and bases dissociate completely (about 100%) while weaker acids and bases dissociate partially or slightly. Weaker acids and bases stay undissociated for the most part.

Strong Acids:

There are seven strong acids that can dissociate completely (about 100%):

HCl, HBr, HI, HNO 3 , H 2 SO 4 , HClO 3and HClO 4

Strong Acids Dissociation:

HClO 4 (aq) + H 2 O(l) à H 3 O + (aq) + ClO 4–(aq)

Dissociation goes to completion (designated with one arrow)

Strong Bases:

Bases are made of metals of group 1A and 2 A form strong bases that can dissociate completely (about 100%).

Group 1 A: LiOH, NaOH, KOH, RbOH, CsOH and FrOH

Group 2 A: Mg(OH) 2 , Ca(OH) 2 , Sr(OH) 2 , Ba(OH) 2and Ra(OH) 2

Strong Bases Dissociation:

Ba(OH)2 (s) + H2O(l)→ Ba 2+ (aq) + 2 OH–(aq) + H2O(l)

NaOH(s) + H2O(l)→ Na + (aq) + OH–(aq) + H2O(l)

H2O is a solvent.

Dissociation goes to completion (designated with one arrow)

Weak Acids:

All other inorganic as well as organic acids are weak except those seven strong acids. Examples of inorganic acids are: H2CO3 , HF, H3PO 4 and H2S. Examples of weaker organic acids are: CH 3 COOH (acetic acid), H2 C2O4 (oxalic acid), HCOOH (formic acid), C6H8O7 (citric acid) and C6 H8O6 (ascorbic acid).

Weak Acids Dissociation:

HF(aq) + H2O(l) ⇔ H3O + (aq) + F–(aq)

Dissociation does not go to completion (designated with double arrows)

CH3COOH (aq) + H2O(l)  ⇔H3O + (aq) + CH3COO–(aq)

Dissociation does not go to completion (designated with double arrows)

Weak Bases:

All other inorganic as well as organic bases are weak bases except those of group 1 A and 2 A metal hydroxides. Examples of inorganic weak bases are: H 2 O, NH 3 , Fe(OH) 3 , Al(OH) 3 and Cu(OH)2 . Examples of organic bases are: CH3NH2 (methylamine), C5H5N (pyridine), C3H4N2 (imidazole), C2H5NO2 (glycine) and other amines.

Weak Bases Dissociation:

NH3 (g) + H2O(l)  ⇔NH4 + (aq) + OH–(aq)

Al(OH)3 (s) + H2O(l)  ⇔ Al3+ (aq) + 3OH–(aq) + H2O(l) [H2O is a solvent]

CH3NH2 (l) + H2O(l) ⇔ CH3NH3 + (aq) + OH–(aq)

Dissociation does not go to completion (designated with double arrows)

The table below illustrates the strength of acids in decreasing order with perchloric acid being the strongest acid and the strength of bases in increasing order with methide ion being the strongest base:

https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_Chemistry_(OpenSTAX)/14%3A _Acid–Base_Equilibria/14.3%3A_Relative_Strengths_of_Acids_and_Bases

The videos below illustrate the ranking and the strength of acids and bases:

Monoprotic, Diprotic and Triprotic AcidsDissociation:

Monoprotic acids have one proton, diprotic acids have two protons and triprotic acids have three protons. Examples are given below:

Strong Monoprotic Acids: HCl, HBr, HNO3 , HClO4

Strong Monoprotic AcidsDissociation:

HNO3 (aq) + H2O(l) → H3O + (aq) + NO3–(aq)

Weak Monoprotic Acids: HF, HCN, H2O, HNO2

Weak Monoprotic AcidsDissociation:

HNO2 (aq) + H2O(l)⇔ H3O + (aq) + NO2–(aq)

Strong Diprotic Acids:H2SO4

Strong Diprotic AcidsDissociation:

The second step of sulfuric acid dissociation is a weak acid dissociation where is K a2 is small, and it is designated with double arrows indicating that the dissociation is partial and incomplete.

Weak DiproticAcids:H2S, H2CO3

Weak DiproticAcidsDissociation:

H2S(aq) + H2O(l) ⇔ H3O + (aq) + HS–(aq)

HS–(aq) + H2O(l) ⇔ H2O (aq) + S2–(aq)

Strong Triprotic Acids:There are no strong triprotic acids.

Weak Triprotic Acids:H 3 PO 4 (phosphoric acid) and C 6 H 8 O 7 (citric acid)

Weak Triprotic AcidsDissociation:

Phosphoric Acid Dissociation:

H3PO 4 (aq) + H2O(l)  ⇔ H3O + (aq) + H2 PO4–(aq) K a1 small

H2PO 4–(aq) + H2O(l) ⇔ H3O + (aq) + HPO4 2–(aq) K a2 smaller

HPO4 2–(aq) + H2O(l)  ⇔H3O + (aq) + PO4 3–(aq) K a3 smallest

Citric Acid Dissociation:

C6 H8O7 (aq) + H2O (l)⇔H3O + (aq) + C6 H7O7– (aq) K a1 small

C6 H7O7– (aq ) + H2O (l) ⇔ H3O + (aq) + C6 H6O7 2– (aq)

K a2 smaller C6H6O7 2– (aq) + H2O (l)  ⇔ H3O + (aq) + C6H5O7 3– (aq)

Ka3 smallest

Direction of Reaction:

In acid – base reaction, the stronger acid and the stronger base favor the products formation where weaker acids and bases are produced:

http://www2.ucdsb.on.ca/tiss/stretton/CHEM2/acid06.htm

HClO 4 (aq) + H2O(l) → H3O + (aq) + ClO4–(aq)

(Stronger Acid) (Stronger Base) (Weaker Acid) (Weaker Base) Reaction favorsproducts

On the other hand, in weaker acids – bases reactions favor the reactants side.

HCN(g) + H 2 O(l)⇔ H3O + (aq) + CN–(aq) Reaction favors Reactants

(Weaker Acid) (Weaker Base) (Stronger Acid) (Stronger Base)