Describe an experiment that will allow you to place the following three bases in order of increasing base strength: NaCN, CH₃NH₂, Na₂CO₃.

Interpretation:

The bases has to be arranged in order of their increasing basicity.

Concept introduction:

The pH is a logarithmic scale used to specify the acidity or basicity of an aqueous solution.

The expression for pH is,

pH=−log[H+]                                                                                                         (1)

The pH varies from 0 to 14 for an aqueous solution.

pH=7(Neutral)pH>7(Basic)pH<7(Acidic)

The value of pOH is a measure of hydroxide ion concentration while pH is a measure of hydrogen ion concentration. The expression for pOH is,

pOH=−log[OH−]                                                                                                    (2)

The sum of pH and pOH is equal to 14 at 25 °C.

pH+pOH=14                                                                                                          (3)

An equilibrium constant (K) is the ratio of the concentration of products and reactants raised to appropriate stoichiometric coefficient at equilibrium.

For base CH3NH2,

  CH3NH2(aq)+H2O(l)⇌CH3NH3+(aq)+OH−(aq)

The relative strength of an acid and base in water can be also expressed quantitatively with an equilibrium constant as follows,

Kb=[CH3NH3+][OH−][CH3NH2]

An equilibrium constant (K) with subscript b indicates that it is an equilibrium constant of the base in water.

For base CN−,

    CN−(aq)+H2O(l)⇌HCN(l)+OH−(aq)

The equilibrium constant, Kb for CN− in water is expressed as,

Kb=[CN−][OH−][HCN]

For base CO3−2,

    CO3−2(aq)+H2O(l)⇌HCO3−(aq)+OH−(aq)

The equilibrium constant, Kb for CO3−2 in water is expressed as,

Kb=[HCO3−][OH−][H2CO3]

Polyprotic acid is capable of donating more than one proton. The ionization constant for each successive loss of a proton is about 104 to 106 which is smaller than the previous ionization step. Because of these reason, the first ionization constant (Kb1), is much larger than the second ionization constant (Kb2), so the hydroxide ion concentration in the solution results almost entirely from the first step. The hydroxide ion produced in the second step can be neglected. Therefore, hydroxide ion concentration in the solution of CO3−2 is calculated entirely from the first ionization step.

Given:

Refer Appendix H and I for Kb values of given bases.

The value Kb for CH3NH2 is 5×10−4.

The value Kb for CN− is 2.5×10−5.

The value Kb for CO3−2 is 2.4×10−8.

The initial concentration of each solution is 0.10 M.

Set up an ICE table for the reaction of any base B with water.

EquilibriumB(aq)+H2O(l)⇌BH+(aq)+OH−(aq)Initial(M)0