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Magazine Chapter 2: Q43 P (page 139)
Consider the following proposed Bronsted-Lowry acid-base reactions. In each case, draw the products of a transfer of the most acidic proton on the acid to the most basic site on the base. Use Appendix 4 to find the pKa values for the acids and the pKb values for the bases. Then determine which side of the reaction is favored, either reactants of products.
(a)
(b)
(c)
(d)
(e)
(f)
Short Answer
(a)
(b)
(c)
(d)
(e)
(f)
Step by step solution
Bronsted – Lowry concept of acid and base
According to Bronsted-Lowry, an acid can be defined as any substance (molecule or ion) that has the tendency to donate a proton (H+) to any substance whereas a base can be defined as any substance (molecule or ion) that has the tendency to accept a proton (H+) from any other substance.
Conjugate acid-base pair
The remaining part of an acid after losing a proton (H+) will have a tendency to accept a proton (H+). Hence, it will behave as a base. These pairs of substances that differ from one another by a proton (H+) are known as conjugate acid-base pairs. Consider a general example of an acid:
Step 3: Ka and pKa , Kb and pKb
Ka is known as the acid dissociation constant and its value indicates the relative strength of the acid. pKa is defined as the negative logarithm (base 10 ) of Ka . Mathematically, pKa =-logKa . pKa value of strong acids is usually zero or even negative while the pKa value of weaker acids are greater than 4.
Kb is known as the base dissociation constant and its value indicates the relative strength of the base. pKb is defined as the negative logarithm (base 10 ) of pKb . Mathematically, pKb =-logKb .
Equilibrium positions of acid-base reactions
The formation of weaker acid and weaker base is favored by the acid-base equilibrium. Larger pKa means weaker is the acid while larger pKb means weaker is the base. Both the weaker acid and weaker base exist as either reactants or products and they are always present on the same side of the equation.
Prediction of equilibrium
(a)
The weaker acid and weaker base are both present in the reactant side, hence the equilibrium will favor the reactants.
(b)
The weaker acid and weaker base are both present in the product side, hence the equilibrium will favor the products.
(c)
The weaker acid and weaker base are both present in the reactant side, hence the equilibrium will favor the reactants.
(d)
The weaker acid and weaker base are both present in the reactant side, hence the equilibrium will favor the reactants.
(e)
The weaker acid and weaker base are both present in the product side, hence the equilibrium will favor the products.
(f)
The weaker acid and weaker base are both present in the product side, hence the equilibrium will favor the products.
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