Predicting the Direction of Acid/Base Reactions

The ability to predict the outcomes of acid-base reactions, which are very common in chemistry, is extremely beneficial. Many different things can change the outcome of an acid-base reaction including heat and pressure. Knowing the K(acid dissociation constant) and the K(base association constant) is the best way to predict the direction of an Acid-Base Reaction.

Introduction

Given the reaction:

 OH- CH3CH2SH Ka 2.5x10-29 6.8x10-6 Kb 1.5x10-4 2.7x10-22

Using the Ka and Kb values, one can predict which molecule will act as the acid in this reaction and which molecule will act as the base.  For OH- the Ka is extremely small in relation to Kb, so it will act as a base in this reaction.  For CH3CH2SH, the Kb is extremely small in relation to Ka so CH3CH2SH will act like an acid.

Problems

For problems 1 through 3:

In which direction will an acid-base reaction move, given the following factor?

1.  K is Big

2.  K is Small

3.  K=Q

(For extra review, check out the ChemWiki page on K and Q here!)

For problems 4-6:

Given the following information, finish the equation and determine the acid and the base

4.        NaOH          +          HCl   ⇔  ?   +   ?

 NaOH HCl Ka 2.5x10-20 6.8x10-7 Kb 1.5x10-2 2.7x10-27

5.        H2O           +           HCl      ⇔   ?   +   ?

 H2O HCl Ka 3.5x10-26 1.8x10-4 Kb 7.5x10-25 9.7x10-24

6.       H2O           +         HC2H3O2  ⇔  ?   +   ?

 H2O HC2H3O2 Ka 5.5x10-10 3.4x10-3 Kb 5.3x10-10 7.2x10-19

1. Reaction goes towards products (to the right)
2. Reaction goes towards reactants (to the left)
3. Reaction stays the same
4.  Products:  H2O + NaCl
1. NaOH is the Base
2. HCl is the Acid
5. Products: H3O+ + Cl-
1. H2O is the Base
2. HCl is the Acid
6. 6.  Products: H3O+ + C2H3O2-
1. H2O is the Base
2. HC2H3O2 is the Acid

References

1. Mortimer, Charles E. Chemistry a Conceptual Approach. 2nd ed. New York: Van Nostrand Reinhold, 1971. Print.
2. Keenan, Charles W., and Jesse H. Wood. General College Chemistry. 4th ed. New York: Haper and Row, 1971. Print.

Contributors

• Ryan Benoit (UCD)
• Dakota Miller (SWCC)