CHAPTER 5 RESULTS SHOWING SCOPE AND QUALITY OF RESEARCH ON
8.2 DIFFICULTIES WITH TECHNICAL LANGUAGE IN ACID-BASE
Difficulty
number Difficulty Descriptions (In Bold) linked to Propositional Statements (coded)
Difficulty Classification R16 Diprotic acids can be treated as monoprotic acids.
pH calculations using pH = –log [H+] need systematic considerations of all the equilibria taking place. (9.7.1)
An Arrhenius diprotic acid dissociates in two stages (10.2.1.2) given by the equations:
H2SO4 HSO4– + H+ (10.2.1.2.1) and HSO4– SO42– + H+ (10.2.1.2.2) giving the overall equation as: H2SO4 SO42–
+ 2H+ (10.2.1.2.3)
A Brønsted diprotic acid ionizes in two stages (10.3.3) given by the equations:
H2SO4 + H2O HSO4– + H3O+ (10.3.3.1) and HSO4–
+ H2O SO42–
+ H3O+ (10.3.3.2)
Compounds in which a molecule or formula unit releases more than one H+ ion by dissociation or ionization will increase the [H+] (or [H3O+]) in solution accordingly. (8.2.2.1.2)
2
R17 Neutral and pH = 7are equivalent at all temperatures.
The ion-product constant for water, Kw, is equilibrium constant. (9.6.2.1.2) given by Kw = [H+].[OH–] or [H3O+].[OH–] (9.6.2.1.1)
Kw = 1.0×10–14 mol.dm-3, (9.6.3.1) only at 25oC. (9.6.3.1.1) Increasing temperature will increase Kw (9.6.3)
As [H+] increases the pH decreases.(9.4.2.1) pH will decrease with increasing temperature. (9.5.1)
We usually quote pH at the standard temperature of 25oC. (9.5.2) For a neutral solution, [H+] = [OH–] = Kw (9.6.2.2)
Neutral solutions have a pH of 7 (5.1.1)
3+
8.2 DIFFICULTIES WITH TECHNICAL LANGUAGE IN ACID-BASE
• Stronger Brønsted acids are better proton donors than weaker Brønsted acids. (8.3.1)
• A more concentrated solution contains more solute for the same amount of solution.
(1.2.0.1.1)
Few qualitative details substantiating the claims are published in any of the reports, and as a result the difficulty is classified at Level 2 or Emergent. For both the main Difficulty R1 and the sub-difficulties which follow, there appears to be no research on a similar conception of strong bases.
8.2.1.2 Sub-difficulty R1.1: Acid strength is shown by more hydrogen in a chemical formula
A possible corollary to the confusion between strength and concentration arises with the chemical formulae for acids and bases, where strong is inappropriately associated with more. In particular, Lin and Chiu (2007) report the conception that the number of H atoms or OH groups in chemical formula is a criterion for determining acid-base strength of solutions, as in the student quotation: “it [sulfuric acid] has two H…it ionizes H, that is hydrogen ion, I think sulphuric (sic) acid ionizes more." With the scant details reported, the classification is only Level 2, or Emergent. If students believe that concentration means the same as acid strength, then it is possible to reason that a greater ionic concentration arises from an acid with formula such as H2SO4 than from HCl and hence the former is a stronger acid. Further open ended research is needed. In the interim, the corresponding propositional knowledge should include the following:
• Compounds in which a molecule or formula unit releases more than one H+ ion by dissociation or ionization will increase the [H+] (or [H3O+]) in solution accordingly.
(8.2.2.1.2)
8.2.1.3 Difficulty R2: Strong acids have strong bonds
A difficulty of confusing strong acids with strong bonds has been shown among undergraduate and post-graduate students, as well as faculty staff, by Smith and Metz (1996). This research involved interviews concerning multiple-choice options depicting sub-microscopic representations of ions and/or molecules for hydrochloric acid, HCl, (a strong acid) and hydrofluoric acid, HF, (a weak acid). These authors report on undergraduate students who thought that strong acids such as HCl “won’t separate” and are “hard to dissociate”. Concerning HF, they also report: “Many students believe that a weak acid is easily pulled apart due to weak bonds or weak attractions between the charged species”. The authors describe the student conception as “A strong acid has a strong bond”. The conclusion arising from two sources of data (the students’ choice of diagram and the interview quotations) concerning two contrasting
chemical contexts, HCl and HF, indicates a consistent difficulty. Similar corroborating evidence for the conception of strong acids has been reported among senior secondary students (Ouertatani et al., 2007; Furió-Más et al., 2007; Ross & Munby, 1991) which allows the difficulty to be classified at Level 3+++, Partially Established in more than one context. In this difficulty, students appear to not differentiate between bond strength and acid strength. Here students have not accommodated a further meaning for strength and simply superimpose the bond strength conception onto acid strength.
Acid strength is shown quantitatively by the dissociation constant Ka. However, a definition is only one aspect of conceptual knowledge (White & Gunstone, 1992; Herron, 1996) and in this regard, many novices have a poor taxonomic understanding of the constants such as Kc, Ka, Kb
etc, sometimes not even recognizing them as all being equilibrium constants (Camacho & Good, 1989). Therefore, limiting propositional knowledge to definitions of strong and weak acids, as under Difficulty P21 (Section 7.4.3.5) or for the dissociation constant Ka will not sufficiently address the difficulty. In this regard, students also need to understand the significance of different values of the dissociation constant, Ka in terms of acid strength and the types of particles found in solutions of weak or strong acids. Furthermore, Furió-Más et al. (2005) emphasise the macroscopic evidence for acid-base strength in terms of the electrical conductivity of their solutions. These aspects are addressed with the following propositional statements:
• An Arrhenius acid HA, will dissociate as: HA H+ + A- (10.2.1.1)
• Ka is an equilibrium constant showing how well an acid dissociates (Arrhenius model) (8.2.4)
• Ka for an Arrhenius acid HA is given by
] HA [
] A ].[
H Ka [
−
= + (8.2.4.1)
• A low value for Ka indicates a minimal tendency for a molecular acid to dissociate/ ionize in water (Furió-Más et al., 2007). (8.2.4.1.1.2)
• Arrhenius acids or bases that are fully dissociated in solution exist mostly as ions (8.2.2.1.1)
• Arrhenius acids or bases that are partially dissociated in solution exist mostly as molecules with a few ions (8.2.2.2.1)
• Strong Arrhenius acids are strong electrolytes (8.2.3.1)
• Weak Arrhenius acids are weak electrolytes (8.2.3.2)
8.2.2 Difficulty with acid-base pairs
8.2.2.1 Difficulty R3: acid-base conjugate pairs are reactant pairs
Research has shown that students do not recognize conjugate acid-base pairs as reactant- product pairs; instead they believe them to be both reactants. In his research into this conception Schmidt (1995) built on earlier work (Sumfleth, 1987) and through a triangulated study he showed the consistency (over several chemical contexts and among many different student cohorts) of a conception, which he described as given above. This extensive research establishes the conception and allows classification of the difficulty as Level 4. Schmidt (1995) suggests that textbooks need to include discussion which distinguishes conjugate pairs from reactant pairs, because the term “acid-base pair” can apply to both, accordingly such propositional knowledge (Schmidt, 1995; 1997) needs to be made clear as follows:
• In the general Brønsted general reaction scheme: acid1 + base2 base1 + acid2 (10.3)
• Conjugate pairs are reactant/product pairs: acid1/base1 and base2/acid2 (10.3.1)
• Formulae for acid-base conjugate pairs differ by a proton, H+ (10.3.1.1)
8.2.3 Difficulties with ionization, dissociation and decomposition 8.2.3.1 Difficulty: R4: Ionization and dissociation are not distinguished
Grade 12 student conceptions investigated by Kousathana et al. (2005) through two multiple- choice items revealed two difficulties concerning ionization and dissociation. The first difficulty indicates that students knew the different processes occurring when molecular and ionic substances dissolved, but muddled the respective concept labels of ionization and dissociation. This is a linguistic rather than conceptual difficulty (Clerk & Rutherford, 2000;
Taber, 2001c) which is perpetuated in chemistry writing (see Sections 3.3.2.3 and 3.3.3.4) and will not be considered further here. The conceptual difficulty reported by Kousathana et al.
(2005) is that students confused the processes of dissociation and ionization that occur respectively when ionic and molecular compounds dissolve in water.
In this reported research, concerning ionic compounds, selection of the multiple-choice distractor: “Ions are created during the dissolution of ionic compounds” by over 10% of the Grade 12 students indicated that they did not understand that a solid ionic compound already contains ions, which water can release from a lattice structure. These students appear to have understood ionization in the context of molecular compounds but inappropriately transferred it to ionic compounds, and so not seen the need to extend their conceptual understanding to include a new concept, ionization. Besides knowledge of ionic bonding (which falls outside the
scope of the current analysis) the appropriate propositional knowledge is based on the correct option in the multiple-choice item as follows (Kousathana et al., 2005).
• Ionic solids are composed of ions (cations and anions) (8.2.5)
• Ionic solids dissociate into cations and anions when they dissolve in water. (8.2.5.1)
As Cokelez et al. (2008) have found, students are easily misled by equations which they think depict NaOH as a molecule like HCl, such as: NaOH(s) Na+(aq) + OH– (aq). Cokelez et al.
indicate that the process could be more clearly represented as: Na+OH–(s) Na+(aq) + OH– (aq). Consequently, the difficulty with dissociation and ionization may be found to originate from difficulties with the chemical symbolism, but further research is needed to verify this conjecture.
Kousathana et al. (2005) also show students confusion about molecular compounds. Almost 20% of the students chose the distractor: “ions are released during the dissolution of molecular compounds”. In a similar vein, Chiu (2005) reports that secondary students in Taiwan
“considered that the molecule always dissolved in a solution in ionic state”, but gives no further details. Both studies show that students are using a conception of dissociation for the molecular compounds instead of ionization, showing they have not seen a need to absorb a new concept with new terminology for ionization. Again, appropriate propositional knowledge as given below is based on that from Kousathana et al. (2005):
• Ions are formed when molecular Brønsted acids or bases dissolve in polar molecular solvents, such as water. (7.3.3.2.3)
As explained in Sections 3.3.2.3 and 3.3.3.4, a notion of ‘dissociation’ for both ionic and molecular compounds was chosen for the Arrhenius model whereas ionization was chosen for the Brønsted model, so it is also necessary to signpost the appropriateness of the terms within each model, as indicated by propositional knowledge given below:
• Arrhenius acids and bases dissociate into ions in aqueous solution. (8.2.1.1)
• Brønsted acid-base reactions include ionization. (7.3.3.2)
• The formation of one or more ions from neutral molecules is ionization. (7.3.3.2.2)
Until more is known about the nature of the difficulty, that is about whether it is due to confusion between the two models or poor understanding of the difference between ionic and molecular compounds (Furió-Más et al., 2007) or perhaps the chemical symbolism mentioned above, there is only a vague description of the difficulty: Ionization and dissociation are not distinguished and so it must be given a low classification – Level 2 or Emergent. Further research making use of free-response probes is needed. However, Southway (pers.com)
suggests that emphasis at high school should lie in what is in solution, rather than the model for the process by which it got there.
8.2.3.2 Difficulty R5: Dissociation is decomposition
From a national study in Taiwan, Chiu (2007) reports the conception: “[A] weak electrolyte exists as a molecule or ions in water because [a] weak electrolyte can just partially decompose”
which was identified among 13% of junior secondary and 34% of senior secondary students. I was unable to interpret this statement in the context of the study because supporting data was not published in English. Consequently, I am not clear whether this is a language difficulty of simply mis-labelling dissociation as decomposition – or a conceptual difficulty of actually thinking dissociation was decomposition into different compounds. Furthermore, the problem might have arisen in translating the research into English. Therefore, at this stage the difficulty is classified as Level 1 or Suspected. The propositional knowledge from IUPAC (McNaught &
Wilkinson, 1997) clarifies the two processes:
• Dissociation is the separation of the constituents of an ion pair. (8.2.1)
• Decomposition is the breakdown of a single molecular entity (8.2.1.2.1)
8.2.3.3 Summary of difficulties with acid-base terminology
In this section two categories of difficulties have been identified with respect to chemists’ acid- base terminology. In the first case students apparently presume that an old concept label (along with its meaning) is the same as that for a new concept, and hence they do not accommodate the new scientific concept. This is evident in the difficulties concerning acid strength (R1 and R2) and conjugate pairs (R3). The second category includes difficulties where two labels are used interchangeably for one muddled undifferentiated conception such as Difficulties R4 and R5 concerning dissociation, ionization and decomposition. For effective communication, students need to be inducted into chemists’ special terminology.