Acids and bases -- L 5 Bronsted Lowry Acid Base Theory

Recap of Previous Acid-Base Theories

Arrhenius Theory

  • Introduced in Grade 10.

    • Arrhenius Acid: Donates protons (H+) in aqueous solution.

      • Example: Hydrochloric acid (HCl) ionizing into H+ and Cl-.

    • Arrhenius Base: Donates hydroxide ions (OH-) in aqueous solution.

      • Example: Sodium hydroxide (NaOH) dissociating into Na+ and OH-.

Modified Arrhenius Theory

  • Introduced in Grade 11, where water is a reactant.

    • Modified Arrhenius Acid: Reacts with water to form hydronium (H3O+).

      • Example: HCl reacting with water to produce H3O+ and Cl-.

    • Modified Arrhenius Base: Reacts with water to produce hydroxide ions (OH-).

      • Example: Ammonia (NH3) reacting with water to form NH4+ and OH-.

  • Explains generation of acidic/basic environments without direct hydroxide or hydrogen presence.

Limitations of Previous Theories

  • Previous theories do not explain acid-base behavior in non-aqueous environments.

  • Cannot account for species acting as both acid and base, e.g., Sodium hydrogen carbonate (baking soda).

    • In aqueous solutions, baking soda behaves as a base, contrasting with its ability to act as an acid in stronger bases.

Behavior of Sodium Hydrogen Carbonate

In Water

  • Dissociates into Na+ and HCO3- (hydrogen carbonate ion).

  • HCO3- acts as a base by stripping a proton from water, generating OH-.

With Strong Bases

  • Dissociates into Na+ and HCO3- while the strong base dissociates into Na+ and OH-.

  • HCO3- can act as an acid and react with OH- to form carbonate ions (CO3^2-) and water, effectively reducing pH.

Amphoteric Nature of Baking Soda

  • Amphoteric: Substance that can act as both an acid and base in different contexts.

  • Amphiprotic Species: Species that can accept and donate protons.

    • Example: HCO3- acting as an acid (donating a proton) when mixed with OH- and as a base (accepting a proton) when mixed with H3O+.

Proton Transfer Model

Definitions

  • Bronsted-Lowry Acid: Proton donor.

  • Bronsted-Lowry Base: Proton acceptor.

Reaction Equation

  • Focus on strongest acid and base in the system.

    • Example: HCl is a strong acid donating a proton to water (acting as a Bronsted-Lowry base).

Conjugate Acid-Base Pairs

  • Conjugate Acid-Base Pair: Related species differ by one proton.

    • Example: Acetic acid (CH3COOH) as a conjugate acid and acetate ion (CH3COO-) as a conjugate base.

  • Acetic acid donates a proton to water forming hydronium ( H3O+); reverse reaction involves hydronium donating to acetate.

Hydrochloric Acid System

  • HCl as a strong acid typically dissociates completely into H3O+ and Cl-.

    • Conjugate acid-base pairs: HCl and Cl-, hydronium ion and water.

Strength of Acids and Bases

  • Strong acids have weak conjugate bases; weak acids have stronger conjugate bases.

    • Example: HCl is a strong acid with a weak conjugate base, Cl-.

Practice Questions

  • Identifying Bronsted-Lowry conjugate acid-base pairs based on differences in protons.

    • Example provided: Hydrogen carbonate ion (HCO3-) and carbonate ion (CO3^2-) is the correct pair.

Conclusion

  • The lecture highlights the importance of understanding Bronsted-Lowry theory within the context of acid-base reactions and equilibrium.

  • Next lecture will focus on predicting Bronsted-Lowry reactions in chemical systems.