Acids and Bases – An Alternate View- Lowry-Bronsted Theory

This theory represents a broader view of acids and bases than that presented by Arrhenius.
Any reaction that involves a transfer of hydrogen ions (protons) is regarded as an acid/base reaction.
The species that donates the hydrogen ion (proton) is defined as the acid in a particular reaction. It is the proton donor.
The species that accepts the hydrogen ion (proton) is defined as the base in a particular reaction. It is the proton acceptor.
Some species may behave as an acid in some reactions and as a base in other reactions.
Hence, in the Lowry-Bronsted theory, species are defined as acids or bases only in the context of a particular reaction.
Pairs like HCl/Cl^-, NH₄⁺/NH₃, H₂CO₃/HCO₃^-, H₂SO₄/HSO₄^-, are called conjugate acid-base pairs.
Each acid has a conjugate base that is formed by removing one hydrogen ion (proton) from the acid. This base then becomes capable of accepting a proton to reform its conjugate acid.

Some Conjugate Acid/Base Pairs

Upwards Increasing strength as a proton donor (acid)	Acid	H⁺	+	Base	Downwards Increasing strength as a proton acceptor (base)
	HCl	H⁺	+	Cl^-
	H₂SO₄	H⁺	+	HSO₄^-
	HNO₃	H⁺	+	NO₃^-
	H₃O⁺	H⁺	+	H₂O
	H₂SO₃	H⁺	+	HSO₃^-
	HSO₄^-	H⁺	+	SO₄^2-
	HF	H⁺	+	F-
	CH₃COOH	H⁺	+	CH₃COO^-
	H₂S	H⁺	+	HS^-
	H₂CO₃	H⁺	+	HCO₃^-
	HSO₃^-	H⁺	+	SO₃^2-
	NH₄⁺	H⁺	+	NH₃
	HCO₃^-	H⁺	+	CO₃^2-
	HS^-	H⁺	+	S^2-
	H₂O	H⁺	+	OH^-
	OH^-	H⁺	+	O^2-
	NH₃	H⁺	+	NH₂^-

Questions

1.Identify the conjugate base of the following acids.

(a) H₃PO₄ (b) HClO (c) H₂Se (d) HPO₄^2- (e) CH₃OH (f) H₂O₂

2.Identify the conjugate acid of the following bases. (a) HPO₄^2- (b) ClO₃^- (c) H₂O (d) NH₃ (e) NH₂^-

3.Reactions involving weak acids and weak bases are frequently written as equilibrium reactions. In the following reactions identify the substances acting as the Lowry-Bronsted acids and bases.

(a)HCO₃^- (aq) + F^-(aq) CO₃^2-(aq) + HF(aq)

(b) NH₄⁺(aq)+ HS^-(aq) NH₃(aq) + H₂S(aq)

4.Complete the following acid-base reactions.

(a) H₂S(aq) + NH₃(aq)

(b) H₂O(l) + HF(aq) + OH^-(aq)

5. (a) Complete reaction equations for the following acids and bases.

(i) H₂SO3 (aq) + HCO₃^-(aq)

(ii) H₂CO₃(aq) + SO₃^2-(aq)

(iii) H₂SO3(aq) + SO₃^2-(aq)

(b) In each of the above reactions identify the acid in both the forward and back reactions. Complete the table below and then use the K_a values to determine which acid in each reaction is the stronger.

	Acid (f’ward reaction)	K_a	Acid (back reaction)	K_a	Stronger acid
(i)
(ii)
(iii)

(c) For each of the above reactions predict whether the reaction will proceed more strongly to the right or the left. (i.e. predict whether the reactants or products will be favoured).

(i) (ii) (iii)

6. In an acid base reaction, if the products are favoured what can we determine about the relative strength of the two acids (i.e. acid of the forward reaction and acid of the back reaction)?

7. The following equations represent reactions that occur to an extent greater than 90% in the direction indicated.

1. H₂O(l) + CH₃O^-(aq)® CH₃OH(aq) + OH^-(aq)

2. HNO₃(aq) + H₂O(l) ® H₃O⁺(aq) + NO₃^-(aq)

3. H₃O⁺(aq) + CN^-(aq) ® HCN(aq) + H₂O(l)

4. HCN(aq) + OH^-(aq) ® H₂O(l) + CN^-(aq)

(a) Circle the stronger Lowry-Bronsted acid in each reaction.

(b) Determine the order from strongest to weakest for all the acids.

(c) The stronger an acid, the weaker is its conjugate base. Determine the order from strongest to weakest for all the bases.

8. Two examples of conjugate acid-base pairs are

(a) H₂CO₃/HCO₃^- and

(b) HCO₃^-/CO₃^2-

(i) For (a) write the reaction equation showing each species separately behaving as an acid and a base in water.

(ii) Write the appropriate expression for K_a and K_b.

(iii) Determine a mathematical relationship between K_a and K_b.

(iv) Repeat (i) and (ii) for (b).

(v) Compare the numerical value of K_a and K_b for HCO₃^-. Which is larger? Thus predict whether HCO₃^- is more likely to behave as an acid or a base in water.

9. Use the relationship derived in Question 8 and the table of K_avalues to determine the K_bvalues for the following bases.

(a) CN^- (b) CO₃^2- (c) SO₃^2- (d) CH₃COO^- (e) NH₃

10. Compare the K_a value for NH₃ with the calculated value of K_b for NH₃. Predict whether NH₃ is more likely to behave as an acid or a base in water.