Chemistry II Laboratory
Hydrolysis of salts will be used to study the acid-base properties of dissolved ions in aqueous solutions. The approximate pH of these solutions will be determined using acid-base indicators. A buffer solution will be prepared, and its ability to control pH will be studied, along with solutions that cannot functionas buffers. So, this experiment will serve to understand conjugate acid-base pairs, equilibria of weak acids and bases and to perform calculations involving ionic equilibria.
Salts are the products formed in neutralization reactions of acids and bases. Brønsted-Lowry acids are proton donors and bases are proton acceptors. In water, an acid can donate a proton to water to form H3O+ and theconjugate base; a base can accept a proton from water to form OH- and the conjugate acid. Also, when an acid and base undergo a neutralization reaction, the products usually include water and a “salt”. For example,
HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)
The dissolution of some salts into water can affect the pH. Nearly all salts are strong electrolytes and exist as ions inaqueous solutions and react with water to produce acidic or basic solutions; these reactions are called hydrolysis reactions. It is the dissolved ions that have the potential to undergo proton transfers with water to generate H3O+ or OH-. A weak acid is one which does not completely dissociate in or react with the water solution. The anions of this weak acid react with water to some extent to acceptprotons and generate OH-. Thus causing the solution pH to be greater than 7. Since there’s no complete dissociation of the weak acid, equilibrium is established. The equilibrium constant is called the acid dissociation constant and is given the symbol, Ka.
For instance the dissociation constant for, HA(aq) + H2O(l) A-(aq) + H3O+(aq), in which HA represents a weak acid; A- stands for itsconjugate base, will be calculate by :
Similarly, a weak base is one which does not completely dissociate in or react with the water solution. Instead, this equilibrium is established. The equilibrium constant is called the base dissociation constant and is given the symbol, Kb. In following HA represents a weak acid; A- stands for its conjugate base.A-(aq) + H2O(l) HA(aq) + OH-(aq)
Its dissociation constant would be found by completing the following equation:
Furthermore, a relationship is established between Ka and Kb. The product of the acid-dissociation constant (Ka) for an acid and the base-dissociation constant (Kb) for its conjugate base is the ion-product constant for water (Kw). Kw at 25oC is 1.010-14. Thisrelationship allows you to find Kb for the conjugate base of a weak acid or Ka for the conjugate acid of a weak base. The stronger an acid is, the larger its Ka and the weaker its conjugate base (with a smaller Kb). Likewise, the weaker an acid is, the smaller its Ka and the stronger its conjugate base (with a larger Kb). Anions derived from strong acids, such as Cl- and HCl- do not react with waterto affect the pH.
Now lets talk about cations, which are derived from weak bases. Cations react with water and increase the hydrogen ion concentration, forming an acidic solutions. For example, NH4 in water will react to form hydrogen ions.
NH4+(aq) + H2O(l) NH3(aq) + H3O+(aq)
Once again, a relationship is formed between Ka and Kb. In thiscase, Ka for NH4+, the conjugate acid of NH3, can be determined using the Kb of NH3 and Kw. Cations of the group 1A metals (Li+, Na+, K+, Rb+, Cs+) and the group 2A metals (Ca2+, Sr2+, Ba2+) do not react with water and are nonacids. They do not affect the pH of the solution.
In conclusion, the acidity, basicity, or neutrality of an aqueous salt solution can be predicted based on the strengths of...