CHAPTER 5 RESULTS SHOWING SCOPE AND QUALITY OF RESEARCH ON
7.5 DIFFICULTIES WITH OTHER ACID-BASE REACTIONS
7.5 DIFFICULTIES WITH OTHER ACID-BASE REACTIONS
Brønsted models rather than the operational model. Consequently, students would have been correct if they had applied the Arrhenius or Brønsted models, neither of which focuses on the formation of a salt. However neither report states overtly which model was expected in this case, and neither shows evidence of soliciting the students’ frame of reference. Consequently, this is inappropriately reported as a misconception.
7.5.2 Difficulties with hydrolysis
When salt dissolves in water, the resulting solution may be non-neutral if either anion or cation undergoes hydrolysis. Difficulties recorded with this observation and with explanations of it in sub-microscopic terms are described next.
7.5.2.1 Difficulty P25 concerning macroscopic aspects of hydrolysis
Difficulties with predicting observations of neutrality (or otherwise) of salt solutions are shown by the student quotation: “Salt is neutral …because it is only a salt. If it was acidic or basic, then we should call [it] acid or base, not salt” (Pinarbasi, 2007) and quantitative data on student estimates of the pH of a solutions of sodium chloride and sodium ethanoate (Bradley &
Mosimege, 1998). Both reports, however, give few details of the research. Consequently the descriptions are both classified at Level 2. Because of the scant research, I have considered these to be sub-difficulties concerning macroscopic observations of the neutrality (or otherwise) of aqueous solutions of salts, described as follows:
Difficulty P25.1: All salts have neutral aqueous solutions.
Difficulty P25.2: Sodium chloride does not have a neutral aqueous solution.
Further research may indicate that they are separate difficulties, if it shows two distinct patterns of students’ thinking. The following research questions could be addressed: “Do students believe all salts have neutral solutions or only some of them?”and “On what basis do students make these predictions?” In the interim, I propose that practical exercises are used to introduce and develop the following propositional knowledge in students:
• Salts may have neutral or non-neutral solutions (5.1.3)
• NaCl forms a neutral aqueous solution. (5.1.2)
• Sodium ethanoate will have basic solution (5.1.3.1)
Research by Lin and Chiu (2007, p 793) showed that some students relied on statements concerning the strength of acid and base from which a salt was derived as an end in themselves to predict acid or base character of solutions of salts. The authors termed this student model, the
“pithy formula model”. The problem highlights the importance of such knowledge being
taught in a meaningful way, otherwise students may memorize little more than a mnemonic. A meaningful explanation and prediction of hydrolysis effects demands understanding the system of ions in water and so necessitates propositional knowledge such as follows:
• Salts where ions are weaker Brønsted acids or bases than water will have neutral solutions (7.3.3.3.2.1)
• Salts where ions are stronger Brønsted acids than water will have acidic solutions.
(7.3.3.3.2.2)
• Salts where ions are stronger bases than water will have basic solutions (7.3.3.3.2.3)
7.5.2.2 Difficulty P26: There is no acid-base reaction between water and the ions from a salt.
Two research reports give evidence of senior secondary student poor conceptual understanding of hydrolysis of ions at sub-microscopic level (see Table 7.6 below).
Table 7.6 Summary of research on sub-microscopic understanding of hydrolysis
Reported conceptions Incidence Acceptable conception Authors
The whole salt undergoes hydrolysis. 45% The ions undergo hydrolysis. Furió-Más et al.
(2007) Students do not appear to know the cause of
hydrolysis.
Not given Hydrolysis is due to proton transfer between H2O molecules and cations or anions
Furió-Más et al.
(2007) Aqueous solution of NH4Cl would contain
equal concentrations of H3O+ and OH- ions and would consequently be neutral
27% to 28%
Greater concentration of H3O+ , so solution is acidic
Schmidt (1991)
Aqueous solution of sodium acetate (ethanoate) would have equal concentration of H3O+ and OH- ions and would
consequently be neutral
25% to 28%
Greater concentration of OH- ions, so solution is basic.
Schmidt (1991)
From this research it is evident that some students have a poor understanding of what is hydrolysed, how it is hydrolysed, the consequences of the hydrolysis on the ions in solution and hence the acid-base nature of the resultant solution. The difficulties in Table 7.6 map to the following propositional knowledge:
• Ionic compounds dissociate into cations and anions when they dissolve in water. (8.2.5.1)
• If ions are stronger Brønsted acids or bases than water, they will react with water molecules.
(8.3.5)
• Hydrolysis is a chemical reaction between an ion or molecule and water (7.3.3.3.1)
• Brønsted acid-base reactions include hydrolysis. (7.3.3.3)
• Hydrolysis of anions or cations change the [H3O+] and [OH-] (8.3.5.2)
It is difficult to encapsulate this difficulty in terms of the original research findings, but the propositional statements reveal the missing idea of a reaction between ions are water, leading to the description as given above. The difficulty has been shown in different German and Spanish cohorts, but the description given here needs to be confirmed, it is classified at Level 3+, being only partially established. Schmidt’s data also shows that there was a close relationship between this conception and that for predicting neutrality (or otherwise) of the titrations between corresponding acids and bases, described as Difficulty P16 in Section 7.4.1.2.