Chemistry 2

Bronsted-Lowry Definition

  1. Water is a polar molecule with the oxygen atom being somewhat negative. Whenever a H+1 ion is dissolved in water, it is attracted to the negative oxygen. In fact, it forms a new ion with water molecules called the hydronium ion: H3O+1 . It is therefore correct to write aqueous hydrogen ions as H+1 or H3O+1.
  2. To illustrate how HCl dissolves in water we could do either of the following:
    1. HCl(g) ---> H+1(aq) + Cl-1(aq)
      or
    2. HCl(aq) + H2O(l) H3O+1(aq) + Cl-1(aq)
  3. According to Bronsted and Lowry you could think of a acid as a proton donor (H+1 donor) and a base as a proton acceptor. Looking at the dissolution of HCl in water from this perspective makes HCl the acid (H+1 donor) and water the base (H+1 acceptor). Because this is an equilibrium equation the hydronium ion can now be a proton donor and the chloride ion can be a proton acceptor. The terms used to describe the hydronium ion and chloride ion are conjugate acid and conjugate base respectively.
  4. The general definition can be symbolized as:
    HA(aq) + H2O(l) H3O+1(aq) + A-1(aq)
    acid
    		base
    		conjugate acid
    		conjugate base
  5. Using this expanded definition of acids and bases we can explain why NH3 is considered to be a base.
  6. NH3(aq) + H2O(l) OH-1(aq) + NH4+1(aq)
  7. Notice that in this case water acts as an acid, donating a proton to the ammonia molecule which is the base. This shows that water can be an acid or a base.