This lesson coaches the student to recognize acid-base reactions that essentially go to completion (big K) or hardly go at all (small K). It starts off qualitatively, and establishes that the weaker acid and base predominate at equilibrium. There is an exercise in which the student is shown reactions, and challenged to find, with the help of a Ka table, the stronger/weaker acid/base, or the acid/base which predominates at equilibrium.
Next, this formula is developed: K = Ka/K'a
(where K'a is the acid constant of the back reaction. Various equivalent forms
are presented. There are guided exercises to evaluate several typical equilibrium
constants, and it is shown that the equation is consistent with the qualitative ideas
developed previously.
There is some coaching concerning strong electrolytes, and on the leveling effect of water on strong acids and bases. The strong, weak and very weak acids and bases are presented in a table, with H2O, H3O+ and OH- marking the divisions. Five classes of reactions are presented that are guaranteed ot "go": strong acid or base + water, H3O+ + OH-, weak acid + OH- and weak base + H3O+. The five inverse reactions are guaranteed not to "go".
The lesson ends with a game. Five acid-base reactions are presented, and the player must decide whether or not each goes. For full marks (20) one must answer correctly within 0.7 seconds. Wrong answers get zero and right answers get scores decreasing with time taken to answer.Recorded scores on this game are remarkable. Experienced middle-aged professional chemists score in the high eighties with some practice. On the other hand, students who have experience with computer games, and whose fingers move at light speed, frequently get scores as high as 100.
Home page | Acid-Base Lessons for Windows | Acids and Bases in Water | Neutralization Equilibrium
Updated July 6, 2001