The Relative Strength of Oxyacids and Its Application Manus Monroe a n d Karl Abrams Indian Valley Colleges, Novato, CA 94947 While teaching introductors and first-sear general chemistry, we have ha4 many students ask for technique to guide them through a portion of descriptive inorganic chemistry. In responst? lo tjleir requests, WP have (lrveloped a simple technique for predicting the relative acidic s t r e n a h of uxyaridnimd .--- - - thpir ~ ~ anions. We have aoolied .. this twhnioue tu earrectly predicting anion hydrolysis reactions, aqueous Brdnsted-Lowrv acid-base reactions. and which salts will di$solve with &ong aqueous acids. his article is a detailed approach to teaching and applying this qualitative method of predicting the strength of different oxyacids to a student population unfamiliar with the concepts of K, and pK.. Oxyacids and Their Strengths We begin this topic by reviewing with students that a binary acid may contain more than one hydrogen atom, will have only one nonmetal atom, and will not contain oxygen (e.g., HC1, H2S). Next, we discuss that while an oxyacid (ternary acid) may have more than one hydrogen atom, it must have two types of nonmetal atoms, one of which is oxygen (e.g., HN03, H2S04). TOconclude OUT introduction to acids, we discuss the terms very strong, strong, weak, and very weak acids. We inform st;denrs that R wry strong acid is approximately 99.W'o ionic in form, i.e., for onc mole of a very strong arid, about 99.999 of it will vxist as ions in water. A strone acid is approximately 99.9% ionic. In contrast, a weak acid is apnroximatelv 90% molecular in form. while a verv weak acid is about 99.99% molrcular. Our terms fnr acidic strengths are based u ~ m nLinus Pauline's classifications' whirh follow.
-
Very strong acid,
Ke,= 1OS
Strong acid, Weak acid, and Very weak acid,
K.,
K.,
-- lo3
" 10W2
K., 2
-
Predlctina Relative Acidic Strengths - of Oxyacids Based upon the work done by Pnuling during the 1930's ( 1 J, we use the followinr.. simolr vwcedure fur qualitatively pre. . dicting the relative strength of an oxyacid from its formula: 1. Determine the number of available hydrogen ions by counting the hydrogens written first in a formula (e.g., HzSOr has two
availahle hydrogen ions). 2. Determine the number of oxygen atoms. 3. Calculate the difference between the number of oxygen atoms
and available hydrogen ions. This difference is always positive for acids and represents their Relative Acidic Strength (RAS). 4. The followingRAS values are used to ascribe acidic characteristics: Acidic Characteristics RAS Values For Acids 3 2 1 0
very strong
strong weak very weak
We now give to students the examples listed in Table 1for reviewine this s i m ~ l procedure. e After discussing the examples, we intnrm st;dents that polyl,n,ric oxyacids dissociate in water and usually ~ r o d u r one r hydrogen or hvdn)nium ion and one oxyanion: w e often present t h e following examples:
'
Pauling, Linus, "General Chemistry," 3rd ed., W.H. Freeman and Company. pp. 499-502.
Volume 62 Number 1 January 1985
41
Table 1. Example Appllcatlons of RAS Procedure Number of Name at Oxyacid
Formula
Number
.Availahla -..
Of
Hydrogen ions
oxygen atoms
pwchloric permanganic Chioric sulfuric chloro~s acetic phosphoric hypochlwous
4 4
3 4 2 2 4 1 3
boric
Table 2. Examples to Revlew Technique for Approximating Acldlc Strength of AcidlBase Oayanions
Relative RAS
Acidic
Values
Strength
3
very strong very strong strong
3 2 2 1 1 1
strong
weak weak weak very weak very weak
0 0
RAS Values Acid
For Acids
H&O, HsW4
2 (strong)
H&Os
0(vW
Anions and Their RAS Value HS0,-: 1 (weak) H2P04-:0 (very weak) H2BOs-: -1 (extremely weak)
1 (weak)
weak)
Anions and Their RAS Value Besa Oxyanion
HPOd2-: -1
(extremelyweak1 HBOIP-: -2
(ultraweak)
ally less than 1%and that an anion hydrolysis reaction always produces a basic solution.
While we show two arrows in each equation, we leave until later in our discussion how to evaluate which arrow, if any, should be larger. Here, we just inform students about the simole meanine of the two different-sized arrows. We call such nd acidbase oxyanions because they ion; as ~ ~ 0 ~ - - aH2POamay either donate or accept hydrogen ions. For example:
Base Oxyanlons and Hydrolysis We complete our discussion on predicting hydrolysis reactions by investigating the relative strength of hase oxyanions. When the charge on a hase oxyanion is added to the RAS value of its parent acid and sums to a -1, -2, or -3 RAS value, then one-step hydrolysis will occur, and the solution will be basic. For example:
At this point, we disruss with students that when an oxyanion can no longer prduce a hydrogen ion, then that anion can only accept thrm, and thus (via Brdnsted-Lowry definition) it is called a hase oxyanion, e.g.,
(Na3P04 has a -2, NaaAsOa has a -3, and NaClO has a -1 RAS value.)
S042-(aq)+ H+(aq) 2 HSO,-(aq) CzHzOz-(aq) + Ht(aq) HC2H&(aq) Predicting Relatlve Acidic Strength of AcidlBase Oxyanlons and Hydrolysis Reactions In general, the relative acidic properties of any anion are reduced by afactor of 105each time the charge becomes more negative by a value of one ( I ) . For example, the acidic properties of HP0d2- are approximately lo5 times less than those of H*P042-. Based upon this concept of reduced acidic properties, we can estimate the relative acidic strength of an acidbase oxyanion by simply adding its negative charge to the RAS value of its parent acid. This calculated value may he either positive or negative and is used with an expanded version of our RAS acid scale to ascribe acidic characteristics: RAS Values for Acids and Oxyanions 3 2 1
o -1
-2. -3
=
(-1)
Acidic Characteristics very strong strong weak very weak extremely weak ulna weak. or virtually none
(Approximate K. Values) (1081 (lo3) (lo->) (lo-') (
