2: The Strength of Acids and Bases

Acids that are completely dissociated in water are referred to as strong acids. The term strong refers to the ability of such acids to donate protons to water molecules. An example of a strong acid is the above mentioned hypochloric acid (HCl):

HCl(aq)  +  H2O  ↔  H3O+(aq)  +  Cl-(aq)

It is a strong acid because the reaction goes almost completely to the right while the reverse reaction occurs only to a very small extend. Thus, almost no HCl molecules are present in the aqueous solution. Instead the product species H+ (in the form of H3O+) and Cl- are present.

Weak acids are only partially dissociated in water, with a high proportion of the acid remaining in solution. An example for a weak solution is acetic acid CH3COOH:

CH3COOH(aq)  +  H2O  ↔  H3O+(aq)  +  CH3COO-(aq)

Most of the acetic acid remains as molecules of CH3COOH in solution while only a mere fraction is ionized to form H3O+and CH3COO-.

An example of a strong base is NaOH. It is completely dissociated in water. Other bases, such as NH3, react only a little in water making it a weak base.

The following chart lists a series of common acids in the order of decreasing strength, as well as their conjugate bases in the order of increasing strength (table 1).

Table 1: List of common acids in the order of decreasing strength, as well as their conjugate bases in the order of increasing strength

table acids and conjugate base sin order of strength

Sources:

  • D.A. McQuarrie, 2011. P.A. Rock, E.B. Gallogly, General Chemistry, 4th edition, pp. 732. University Science Books.
  • D.D. Ebbing & S.D. Gammon, 2012. General Chemistry, Cengage Learning, 10th edition, pp. 528. Cengage Learning.
  • M.S. Cracolice, E. Peters, 2015. Introductory Chemistry: An Active Learning Approach, 6th edition, pp. 495. Cengage Learning.
  • B. Averill & P. Eldredge, 2011. General Chemistry: Principles, Patterns, and Applications, p. 1445. Saylor Foundation. https://open.umn.edu/opentextbooks/BookDetail.aspx?bookId=69.
  • OpenStax  College,  2015. Chemistry, ch. 14.3 Relative Strengths of Acids and Bases. OpenStax College, 11 March 2015.  <http://cnx.org/content/col11760/latest/>
Last modified: Tuesday, 27 September 2016, 5:48 PM