Predicting the Direction of Acid/Base Reactions
- Page ID
- 1286
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 Ka (acid dissociation constant) and the Kb (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 |
Answers
- Reaction goes towards products (to the right)
- Reaction goes towards reactants (to the left)
- Reaction stays the same
- Products: H2O + NaCl
- NaOH is the Base
- HCl is the Acid
- Products: H3O+ + Cl-
- H2O is the Base
- HCl is the Acid
- 6. Products: H3O+ + C2H3O2-
- H2O is the Base
- HC2H3O2 is the Acid
References
- Mortimer, Charles E. Chemistry a Conceptual Approach. 2nd ed. New York: Van Nostrand Reinhold, 1971. Print.
- 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)