Chapter 5: Acid and Base

Acid and Base

Acid are those substance that dissolves in water, producing the oxonium ion, H3O+.

Base are those substance that neutralizes acids, producing a salt and water as the only product.

Alkali is a soluble base which produces hydroxide ions. OH in water.

Bronsted–Lowry Acid and Base

  • Bronsted-Lowry Acid is a species that can donate protons.
    • HA (aq) + H2O (l) → H3O+(aq) + A(aq)
  • Bronsted-Lowry Base is a species that can accept protons.
    • B (aq) + H2O (l) → BH+(aq) + OH(aq)
  • Conjugate acid-base pairs are a pair of species that interconvert by the gain or loss of a proton.

 

  • Monoprotic Acids – each molecule can release one proton, e.g. HCl

HCl (aq) →  H+ (aq) + Cl- (aq)

  • Diprotic Acids – each molecule can release two protons in two steps, e.g. H2SO4

        H2SO4 (aq) →  H+ (aq) + HSO4- (aq)
        HSO4- (aq) →  H+ (aq) + SO4^2- (aq)

  • Diprotic Acids – each molecule can release two protons in two steps, e.g. H2SO4
  • Neutralization reactions:
    • acid + base → salt + water
  • Acid-carbonate reactions:
    • acid + carbonate → salt + carbon dioxide + water
  • Acid-metal reactions:
    • acid + base ⇌salt + hydrogen gas

Ionic Product of Water

Species that can act as both an acid and a base are called amphoteric, e.g. water. Water slightly dissociates into hydroxide and hydroxonium ions.

2H2O (I) ⇌ OH (aq) + H3O+ (aq)
H2O (I) ⇌ OH (aq) + H+(aq)

The equilibrium constant for the dissociation of water is:

The concentration of H2O is constant for a given temperature

Kc x [H2O] = [OH] [H+]​​

  • Ionic Product of water, Kw = [OH] [H+]

Kw, like other equilibrium constants, only changes with temperature. Kw can be used to calculate the pH of a strong base from its concentration

  • Strong bases dissociate almost fully, meaning the concentration of OH- ions is equal to the concentration of the base before dissociation: [base]=[OH]

PH

The concentration of H+ ions, often written as [H+], present in a solution determines how acidic it is. The pH scale is logarithmic to cover the wide range of hydrogen ion concentrations in aqueous solution

  • ​pH = -log10[H+]

The greater the concentration of H+ ions, the lower the pH value, and the more acidic the solution is

[H+] 10-pH

The pH of a strong acid can be calculated from its concentration:

Strong acids fully dissociate: HA (aq) ⇌ A (aq) + H+ (aq)

Therefore, the concentration of H+ ions is equal to the concentration of the acid before dissociation: [HA] = [H+]This is only the case for monoprotic acids, such as HBr. Diprotic acids, such as H2SO4, will require the concentration of the acid before dissociation to be multiplied by two to get the concentration of H+ ions.

Buffer Solution

A solution that resists the change in the pH when a small amount of either an acid or an alkali is added to it. Acid buffer are those that maintain the pH of less than 7. Alkaline buffers are those that maintain the pH of greater than 7.

Titrations

Acid base titration can be used to find the concentration of a sample of either an acid or a base. Titration curve is a graph that shows how pH varies with addition of acid or base to a base or acid.

Indicators

Indicators are those substance that are used to determine the end point of the titration. An indicator must have:

  • A change in color
  • An end point close to equivalence point
  • A clear change in color

 

Buffer Solution

Buffer solution are those solution that resists the change in its pH when a small amount of acid or base is added to it.

Acidic Buffer

These are the mixture of weak acid and the salt of the same weak acid.

CH3COOH + CH3COONa

Basic Buffer

These are the mixture of weak base and the salt of the same weak base.

NH4OH + NH4Cl