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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

    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)