L3.1: Acids & Bases

• ● Acids must have Hydrogen (H+) in their chemical formula. (i.e. HCl, HA)

  ● Bases must have a Hydroxide (OH-) in their chemical formula. (i.e. NaOH)
• Arrhenius

  ● The earliest definition of acids and bases, which was suggested by Swedish Chemist Svante Arrhenius in the 1800s.

  ● This definition classifies these substances in terms of their behavior in water:

  ○ Acids

  ■ are H+ or H3O+ producers.

  ■ has H in its formula that dissociates in H2O to yield H3O+

  ○ Bases

  ■ are OH producers.

  ■ has OH in its formula that dissociates in H2O to yield OH-

  ● When acids and bases react, they undergo neutralization.
• Strong Acid

  ● Strong acids dissociate completely into ions in water. Meaning, every molecule of the acid (HA) breaks apart into ions.

  ● There are no more HA molecules in a diluted solution of strong acid.

  ★ HA = Acid = Simple Definition

  ■ H = Hydrogen

  ■ A = Element/Molecule
• Weak Acid

  ● Weak acids dissociate slightly into ions in water. Meaning, only a few HA molecules split into ions; most stay as HA.

  ● Most HA molecules are still associated in a diluted solution of weak acid.

  ● The equilibrium constant in terms of concentration (Kc), in weak acids, have very small values.
• Acid Dissociation Constant (Ka)

  ● Measures the strength of an acid, and is indicated by Ka

  Acid Strength | [H30+] | % of HA dissociated | Ka value

  • Strong Acid | High | High | Large
  • Weak Acid | Low | Low | Small
• Brønsted-Lowry

  ● A more general definition of acids and bases, compared to the earlier and older Arrhenius definition.

  ● The acid-base reaction of this definition, is a proton transfer process.

  ○ Reaction can occur between gasses, in aqueous/non-aqueous solutions, and in heterogeneous mixtures.

  ■ No longer being restricted to just H2O for reaction to occur.
• Brønsted-Lowry

  ● This definition classifies substances in terms of

  their transfer of a hydrogen (H+) atom.

  ○ Acids

  ■ are proton donors.

  ■ any species that donates an H+ ion

  ■ must contain H+ in its formula.

  ● All Arrhenius acids are Brønsted-Lowry acids.

  ○ Bases

  ■ are proton acceptors.

  ■ any species that accepts an H+ ion.

  ■ must contain a lone electron pair to bond to H+
• Arrhenius

  • Base must produce OH directly by itself in water. (Arrhenius base)
  • But NH3 doesn't have OH-, and it doesn't dissociate into OH− ; it reacts with water to form OH

  Brønsted-Lowry

  • NH3 accept proton from water (Brønsted base)
  • H2O donates proton to NH3 (Brønsted acid)
• Conjugate Acid-Base Pairs

  ● Pairs that consist of two substances that differ by exactly one proton, which is the Hydrogen (H+) atom.

  ● Every acid has a conjugate base, and every base has a conjugate acid.

  ★ Acid → loss of H+ → Conjugate Base

  ★ Base → gain of H+ → Conjugate Acid
• Conjugate Acid-Base Pairs

  ● The net direction of an acid-base reaction depends on the relative strengths of the acids and bases involved.

  ○ A reaction will favor the formation of the weaker acid and base.

  ○ This is noted because all Brønsted-Lowry acid-base reactions are equilibrium reactions, meaning they can go forward or reverse.

  ● The stronger the acid, the weaker the conjugate base and vice versa.
• Conjugate Acid-Base Pairs

  ● The stronger the acid, the weaker the conjugate base and vice versa.

  ○ When an acid reacts with a base that is farther down the list, the reaction

  proceeds to the right (Kc > 1).

  ● An amphoteric compound contains both a hydrogen atom and a lone pair of e- meaning it can be either an acid or a base.

  ○ Prime example of such is water or H2O.