The Bronsted-Lowry Theory of Acids and Bases

The Bronsted-Lowry Theory of Acids and Bases

more general than the Arrhenius theory
incorporates Arrhenius
used to allow for equilibrium reactions
extends to a wider range of species and reactions

Acid

donates a proton to another substance
gives away H⁺

Base

accepts a proton from another substance
receives an H⁺

Example :

NH₃ + H₂O « NH₄⁺ + OH^-

NH₃ gains a proton so it acts as a base

H₂O has lost (donated) a proton so it acts as an acid

Example :

CH₃COOH + H₂O « CH₃COO^- + H₃O⁺

CH₃COOH loses (donates) an H⁺, so it acts as an acid (of course - its acetic acid!)

H₂O gains a proton, so it acts as a base

** substances that can act as an acid or a base are amphiprotic

example : H₂O, H₂PO₄^-, HS^-, HCO₃^-

they can either lose a proton, or regain one.

They either possess a negative charge, and still have an easily removable hydrogen.

H₃PO₄ « H₂PO₄- « HPO₄^-2

H₂S « HS^- « S^-2

Definitions :

Monoprotic acid - can supply one proton

Diprotic acid - can supply two protons

triportic acid - can supply three protons

polyprotic acid - can supply more than one proton

In every Bronsted-Lowry rxn there is an acid and a base on both side of the equation

CH₃COOH + H₂O « CH₃COO^- + H₃O⁺

acid base base acid

a conjugate acid-pair - is a pair of chemical species that differs only by one proton

conjugate acid - has the extra proton

conjugate base - lacks the extra proton

Conjugate Pair	Conjugate Acid	Conjugate Base
NH₄⁺, NH₃	NH₄⁺	NH₃
H₂O, H₃O⁺	H₃O⁺	H₂O

A bronsten-Lowry acid-base reaction just involves an equilibrium proton transfer :

Conjugate Acid form of A + Conjugate Base form of B «

Conjugate Base form of A + Conjugate form Acid of B

Note :

Simple organic acids end with a COOH group, the H at the end of the group is the acid

CH₃CH₂COOH à CH₃CH₂COO^- + H⁺

Organic bases contain an NH₂ group or an NH group. The nitrogen atom accepts the H⁺

CH₃CH₂NH₂ + H⁺ à CH₃CH₂NH₃⁺

Strong and Weak Acids and Bases

A strong acid or base is 100% ionized in solution

NaOH à Na⁺ + OH^-

A weak acid or base is less than 100% ionized in solution

NH₃ + H₂O « NH₄⁺ + OH^-

equilibrium reactions involve weak acids and bases, NOT strong acids and bases

the terms weak and strong refer to the % ionization
the terms dilute and concentrated refer to the molarity of a solution

Strong Acids :

top six acids on the left side
* H₂SO₄ is only strong for the first dissociation
note the one way arrows, the reverse reactions DO NOT occur at all

H₃O⁺ « H⁺ + H₂O just shows that H⁺ is equivalent to H₃O⁺ : the net result of putting any strong acid in water.

Strong Bases :

the bottom two bases strongly dissociate in water, O^-2 and NH^2-

H₂O « H⁺ + OH^- is the result of putting any strong base in water

metal hydroxides are 100% dissociated in water (ie NaOH, KOH, Mg(OH)₂, Zn(OH)₂ etc

Weak Acids :

on the left, separated by equilibrium arrows from their conjugate bases on the right.
OH^- and NH₃ never act as acids in aqueous solutions.

Weak Bases :

on the right, separated by equilibrium arrows from their conjugate acids on the left
the six species on the right of the top section NEVER act as bases in aqueous solutions.

When a substance acts as an acid with water, H₃O⁺ is produced. The stronger the acid the greater the [H₃O⁺] produced

H₂S acting as an acid in water :

ie. H₂S_(aq) « H⁺_(aq) + HS^-

H₂S_(aq) + H₂O_(l) « H₃O⁺_(aq) + HS^-_(aq)

When a substance acts as a base with water, OH^- is always produced. The stronger the base, the greater the [OH^-] produced

HS^- acting as a base in water :

ie. H₂S_(aq) « H⁺_(aq) + HS^-

HS^- + H₂O « H₂S + OH^-

Table Info

the stronger an acid, the weaker its conjugate base, and vice versa

ex. Since IO3- is a very weak base, its conjugate acid, HIO3 is a relatively strong acid.

If a compound is on both the left and right side of the table, when comparing relative acid strengths look at the left side (higher - stronger), when comparing relative base strengths, look at the right side (lower -stronger)

Levelling Effect

all strong acids are 100% dissociated in aqueous solution and are equivalent to solutions of H3O⁺, while all strong bases are 100% dissociated in aqueous solution are equivalent to solutions of OH^-

Strong Acids in water :

1 M HClO₄ produces :

1 M H₃O⁺ + 1 M ClO₄^- (No dissociated HClO₄)

1 M HCl produces :

1 M H₃O⁺ + 1 M Cl^- ( No dissociated HCl)

in water,, all strong acids are 100% dissociated to form H3O+, hence the water has ‘levelled’ all strong acids to the same strength, therefore H3O+ is the strongest acid that can exist in aqueous solution.

**the top six acids have identical strengths in aqueous solution.

Weak Acids in water :

1 M HF produces :

0.97 M HF + 0.03 M H₃O⁺ + 0.03 M F^-

1 M CH₃COOH produces :

0.996 M CH₃COOH + 0.004 M H₃O⁺ + 0.004 M CH₃COO^-

the strongest base that can exist in aqueous solution is OH^-; all strong bases are 100% ionized to form OH^-,
O^-2 and NH^2- have the same strength in solution.