CHAPTER 5 RESULTS SHOWING SCOPE AND QUALITY OF RESEARCH ON
6.2 DIFFICULTIES WITH ACID-BASE MODELS
6.2.2 Difficulties with the Brønsted model
Five sub-difficulties, all with similar implications in terms of students understanding and propositional knowledge are discussed in this section.
6.2.2.1 Difficulty S2: Acids and bases are substances not particles
Student conceptions of acids and bases may have advanced to the theoretical Arrhenius model of substances in aqueous solution, but in some cases students do not yet accommodate the Brønsted model of particles as proton donors or acceptors, despite having studied the later model at senior secondary or tertiary level. For example Kousathana et al. (2005) reported that students who were asked to chose an option that: “is not a Brønsted-Lowry acid” from a selection of four species still justified their choice by reference instead to the Arrhenius model.
Similar ideas are reported from Cros et al. (1986). More specifically, the student difficulty of distinguishing the essential attributes of the Brønsted model (that it concerns particles such as molecules or ions rather than substances, see Section 3.3.3.1) was identified among students (Sumfleth, 1987) and teachers (Drechsler & Schmidt, 2005a). This essential attribute of the model is indicated in the propositional knowledge statements given below.
• Different theoretical models conceive acids and bases as substances or as particles (1.1.3.3)
• Brønsted acids are molecules or ions that can release a proton (hydrogen ion) (2.3.1.1)
• Brønsted bases are molecules or ions that can accept a proton (hydrogen ion). (3.3.1.1)
As in the previous difficulty, less research has been reported on the student conception for bases (only Drechsler & Schmidt, 2005b) than for acids. Accordingly, the difficulties have been separated so that classifications levels can indicate this disparity, as follow:
Difficulty S2.1: Acids are substances not particles Level 4 Difficulty S2.2: Bases are substances not particles Level 2
The Difficulty S2 can also show itself in other ways, which are discussed as sub-difficulties in the following two sections.
6.2.2.2 Sub-difficulties of S2: Examples of acids and bases are limited to the Arrhenius model
Some students think that only compounds with OH groups are bases and in this way limit themselves to Arrhenius bases. Similarly they also do not recognise Brønsted acids that are not also Arrhenius acids. Table 6.2 below shows a summary of the relevant research for this sub- difficulty. In this research, some probes were open ended (Cros et al., 1986), others asked students to classify examples as acids, bases or neutral species (Furió-Más et al., 2007;
Ouertatani et al., 2007) or probes were in a multiple-choice format along the lines of: Which of the following is not a Brønsted acid or Brønsted base (Kousathana et al., 2005).
Table 6.2 The formulae for species not recognised as Brønsted bases or acids by students
Formula investigated
Educational level of students
Country of Cohort Authors
Bases NH3 Senior secondary Greece Kousathana et al. (2005)
NH3 Senior secondary Spain Furió-Más et al. (2007)
NH3 Senior secondary Tunisia Ouertatani et al. (2007)
CN– Senior secondary Greece Kousathana et al. (2005)
S2– Senior secondary Tunisia Ouertatani et al. (2007)
CH3COO– Tertiary France Cros et al. (1986)
CH3COO– Senior secondary Tunisia Ouertatani et al. (2007)
HCOO– Senior secondary Greece Kousathana et al. (2005)
Acid NH4+
Senior secondary Greece Kousathana et al. (2005)
The research summarised in the table was conducted among many cohorts of senior students in a variety of countries to give a coherent picture of the student difficulty which can be described directly from the data as given in the section heading above. The research has covered a wide variety of Brønsted base species, including both molecules such as ammonia, NH3, and ions such as CN–. Furthermore similar conclusions regarding bases are reported from other research (Schmidt & Volke, 2003; Drechsler & Schmidt, 2005b). However, only one Brønsted acid was included (ammonium ion, NH4
+) and this research concerned only one student cohort.
Consequently, for the purpose of classification the difficulty is separated into two sub- difficulties, as follows:
Sub-difficulty S2.3 Examples of acids are limited to the Arrhenius model. Level 2 Sub-difficulty S2.4 Examples of bases are limited to the Arrhenius model. Level 4
The research described above shows that students have limited knowledge of Brønsted bases, and also need examples of Brønsted acids that are not also Arrhenius acids. Research by Drechsler and Schmidt (2005b) suggests that the student conception could be directly caused by limited examples introduced during instruction. To address the problem, teachers need to be
aware that propositional knowledge should include a variety of examples to indicate both the extent of the Brønsted model and where it differs from the Arrhenius model. I attempted to take cognisance of these aspects in the three propositional statements below.
• Arrhenius bases are limited to substances containing OH groups. (3.2.2.0)
• Brønsted bases include molecule NH3, and ions HCOO–, CH3COO–, CN– and S2– (3.3.2.1)
• Brønsted acids include the ion NH4
+ (2.3.2.2)
Discussion of individual substances which pose particular problems for students follows. These substances represent specific contexts for Difficulties S2 and accordingly, they are not considered to be separate difficulties.
6.2.2.3 Specific contexts of Difficulty S2
Familiarity with water makes it especially difficult for students to accommodate into the more abstract Brønsted model. Analysis of examination board answers showed that students avoided options where water was described as a base, or where it acted as a proton acceptor (Drechsler
& Schmidt, 2005a) which is confirmed by Kousathana et al. (2005). The quotation: “I can't imagine drinking an acid but you drink water” (Drechsler & Schmidt, 2005b) suggests that students have the substance water in mind, rather than water molecules. Schmidt and Volke (2003) report similarly concerning water as a base. Consequently education practitioners need to be especially careful to clarify that the Brønsted model refers to water molecules, as shown by the propositional knowledge given below.
The second substance presenting particular difficulty is sodium hydroxide, a prototypic Arrhenius base. In this regard, Drechsler and Schmidt (2005b) quote a student who claimed to be using the Brønsted model but said: “HCl is the acid and NaOH is the base”. For both these substances, students were superimposing their limited conception of acids or bases as substances onto the Brønsted model. Teachers (and textbook authors) need to be particularly aware of the difficulties encountered with these substances. Accordingly, the propositional statements below are modified from those given earlier. They now address the specific contexts of the difficulty and clarify the boundaries of the models as recommended by Herron (1996), through a range of both examples and non-examples.
• Arrhenius acids do not include water (2.2.2.2.1.1)
• Brønsted acids include the molecule H2O and ion NH4
+ (2.3.2.2)
• Arrhenius bases include sodium hydroxide, NaOH. (3.2.2.1.1)
• Brønsted bases include the molecules H2O, NH3, and ions OH–, HCOO–, CH3COO–, CN– and S2– (3.3.2.1)
• Brønsted base: examples do not include Arrhenius bases (3.3.2.2) such as NaOH (3.3.2.2.1) The next section shows how limited conception of acids and bases impacts on student conceptions of their reactions.
6.2.2.4 Sub-Difficulty S2.5: Neutralization is limited to and always occurs between compounds having H and OH in the formula
From the Difficulty S2, it follows that students would have a limited conception of species involved in acid-base reactions, as shown by the sub-difficulty described above. Evidence for this sub-difficulty, reported by Furió-Más et al. (2007), is summarised in Table 6.3 below.
Ouertatani et al. (2007) also reported similar observations.
Table 6.3 Some of the reaction equations investigated by Furió-Más et al. (2007)
Reaction equation investigated Classification by students
Incidence Acceptable classification
1 SiO2 + CaO CaSiO3 Not
neutralization
11 Neutralization in Operational model or Lewis model (Theoretical framework) 2 HCl + CH3OH CH3Cl + H2O Neutralization 91 Nucleophilic substitution (Morrison &
Boyd, 1966)
3 NH3 + CH3OH CH3NH2 + H2O Neutralization 97 Nucleophilic substitution (Morrison &
Boyd, 1966)
The first reaction between silicon dioxide and calcium oxide shown by equation 1 in the table above could fit the operational model for a non-aqueous system or even a Lewis acid-base reaction (see Section 3.3.4), but was not recognised as such by some students. The reaction between hydrogen chloride and methanol (equation 2) and that between ammonia and methanol (equation 3) were both overwhelmingly identified as neutralization, which the authors consider incorrect. Morrison & Boyd (1966) confirm that the reactions shown as (2) and (3) above are nucleophilic substitution of alcohols rather than neutralization. Consequently, it appears that not only do students limit their idea of neutralization reactions to species with H atoms and OH groups but also they consider all reactions shown in this format as neutralization, that is, an acid-base reaction. Accordingly, the description for the difficulty: Neutralization is shown by reactants with H and OH in the formulae is classified at Level 3 because it has been identified through two investigations using a variety of open-ended methods and different chemical contexts. The corresponding propositional knowledge shows that the operational model can accommodate non-aqueous systems. Students need to integrate the propositional knowledge below, which includes a modified statement 3.1.2.4 to account for calcium oxide.
• Neutralization is a process whereby acidic substances and basic substances react chemically to produce new substances (7.1) including water, if in aqueous solution. (7.1.2.2)
• CO2 and SO2 are acidic gases found in the atmosphere. (2.1.2.2)
• Basic substances found in the laboratory include metal oxides or hydroxides such as limewater Ca(OH)2 (3.1.2.4)
• Acidic substances and basic oxides or hydroxides react chemically but produce no gases except water vapour. (2.1.1.6.1)
• Arrhenius bases do not include alcohols. (3.2.2.2.2)