Acids and Bases Part 1

Theories of acids and bases One of the earliest theories concerning acids came from Lavoisier in 1777. He believed that an acid could be defined as a compound of oxygen and a nonmetal. This theory had to be dismissed when the acid HCl was proved to be made of hydrogen and chlorine only- with no oxygen. To hold true, any definition of an acid has to be valid for all acids.

Theories of Acids and Bases The next big step forward came when Arrhenius in 1887 suggested that an acid could be defined as a substance that dissociates in water to form hydrogen ions (H+) and anions. He said a base dissociates into hydroxide ions (OH-) and cations. Arrhenius also recognized that hydrogen and hydroxide ions could form water and the cations and anions could form a salt. He was very close to the theory that is widely used to explain acids and bases today, but his focus was only on aqueous systems making it somewhat limited. A broader theory was needed.

Theories of Acids and Bases In 1923Lowry of England, and Bronsted of Copenhagen independently published similar conclusions regarding the definition of acids and bases. Their findings became known as the Bronsted-Lowry theory. This theory focuses on the transfer of H+ ions during an acid-base reaction: acids donate H+ while bases accept H+. For example, in the reaction between HCl and NH3: HCl + NH3  NH4+ + Cl- HCl transfers H+ to NH3 and so acts as an acid, NH3 accepts the H+ and so acts as a base. The theory can therefore be stated as: A Bronsted-Lowry acid is a proton (H+) donor. A Bronsted-Lowry base is a proton (H+) acceptor.

Theories of Acids and Bases The act of donating cannot happen in isolation-there must always be something present to play the role of acceptor. In Bronsted-Lowry theory, an acid can therefore only behave as a proton donor if there is also a base present to accept the proton. Consider the generic acid HA and baseB: HA + B  A- + BH+ HA acts as an acid, donating a proton to B, while B acts as a base, accepting the proton from HA If we look at the reverse reaction, BH+ acts as an acid, donating its proton to A- while A- acts as a base accepting the proton from BH+.

Theories of Acids and Bases Acid HA has reacted to form the base A-, while base B has reacted to form acid BH+ HA + B  A- + BH+ The acid-base pairs related to each other in this way are called conjugate acid-base pairs and they differ by just one proton. HA and A- are conjugate acid-base pairs, and B and BH+ are conjugate acid-base pairs.

Theories of Acids and Bases Worked example: Label the conjugate acid-base pairs in the following reaction: CH3COOH(aq) + H2O(l)   CH3COO-(aq) + H3O+(aq) Solution: CH3COOH / CH3COO- = conjugate acid-base pair acid base H2O / H3O+ = conjugate acid-base pair base acid The fact that in a conjugate pair the acid always has one proton more than its conjugate base makes it easy to predict the formula of the corresponding conjugate for any given acid or base.

Theories of Acids and Bases Some species can act as acids and bases. Water can act as an acid or base depending on how it reacts. Example: CH3COOH(aq) + H2O(l)  CH3COO-(aq) + H3O+(aq) acid base base acid NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq) base acid acid base NH3 and HCO3- also act as both acids and bases in the above reactions. Substances which can act as acids and bases in this way are called amphoteric or amphiprotic. In order to be amphoteric, a species must have a lone pair of electrons.

Theories of Acids and Bases Work the following problems and turn in: Write the conjugate base for each of the following: HF HNO2 H3O+ NH3 H2PO4- H2CO3 Write the conjugate acid of the following: 7. CN- 9. H2O 11. CO3-2 8. PO4-3 10. OH- 12. NH3

Theories of Acids and Bases Deduce the formula of the conjugate acid of the following: SO3-2 16. NO3- CH3NH2 17. F- C2H5COO- 18. HSO4- Deduce the formula of the conjugate base of the following: 19. H3PO4 22. H2SO4- 20. CH3COOH 23. OH- 21. H2SO3 24. HBr

Theories of Acids and Bases For each of the following reactions, identify the Bronsted- Lowry acids and bases, and the conjugate acid-base pairs. 25. CH3COOH(aq) + NH3(aq)  NH4+(aq) + CH3COO-(aq) 26. CO3-2(aq) + H3O+(aq)  H2O(l) + HCO3-(aq) 27. NH4+(aq) + NO2-(aq)  HNO2(aq) + NH3(aq)