CHAPTER 5 RESULTS SHOWING SCOPE AND QUALITY OF RESEARCH ON
7.3 DIFFICULTIES WITH ACID-BASE CHEMICAL
7.4.3 Difficulties with the nature of reactions in acid-base chemistry
The conceptions discussed in this section represent student difficulties in explaining the nature of a chemical reaction. Some of the difficulties reflect conceptions about fundamental principles in chemistry and for this reason, it is inappropriate to classify them only in the acid- base context.
7.4.3.1 Difficulty P19: Neutralization is mixing, not a chemical reaction.
The conception described above had been shown by Sheppard (2006) where 37.5% of the Grade 10 students held the idea that a neutralization reaction was a physical mixing rather than a chemical reaction due to interaction between particles. These students could neither name the new product nor give any equations; furthermore, they made particulate drawings showing unreacted chemical species. Nakhleh and Krajcik (1993) report a similar conception. The evidence in the two reports above, for the acid-base context, comes from students towards the end of high school but Talanquer (2008) reports the persistence of such naïve understanding of chemical reactions even after a semester of university chemistry. Consequently, the propositional statements concerning chemical reactions that were given earlier for P11 (see Section 7.3.1.5 e.g.: “When acidic substances react with carbonates, carbon dioxide is
produced”) have been subtly rephrased, as given below, to indicate that a mutual reaction between two substances produces the new substance. Because the difficulty is not unique to acid-base chemistry, it is inappropriate to classify the difficulty only in that context.
• Neutralization is a process whereby acids and bases react chemically to produce new substances. (7.1)
• Acids and carbonates react chemically to produce carbon dioxide. (2.1.1.6.2)
• Acids and some metals react chemically to produce a salt and hydrogen. (2.1.1.7)
• Acids and basic oxides react chemically, but produce no gases except water vapour.
(2.1.1.6.1)
7.4.3.2 Difficulty P20: Acid-base reactions are additive
Sheppard (2006) reports a conception in which some students “described the formation of new products by the addition of an acid species to a base species”. Moreover, students’ drawings of sub-microscopic representations frequently showed base particles simply attached to acid particles. Nakhleh and Krajcik (1993) identified similar conceptions from student interviews.
Furthermore, in the light of Talanquer’s (2008) research, the difficulty is probably closely aligned to students using an additive, rather than an emergent framework. In such a case, students conceive the properties of the reactants to be the sum of the properties of the reactants, rather than new emergent properties. Accordingly, the difficulty is not classified only in the acid-base context here. The following propositional knowledge from Nakhleh & Krajcik (1993) is nevertheless useful:
• Neutralization is a double decomposition (or metathesis) reaction. (7.1.1)
The following discussion shows two sub-difficulties that arise from the difficulty which are particular to acid-base chemistry.
7.4.3.3 Sub-Difficulty P20.1: Indicators are necessary for or assist with neutralization Within an additive framework for chemical reactions students will not accept the production of new substances with new properties, as might be detected by means of indicators. Instead they assign another role to indicators, which is that they assist with neutralization. This conception has been shown among junior secondary students (Botton, 1995), senior secondary students (Nakhleh & Krajcik, 1994) and pre-service teachers (Bradley & Mosimege, 1998). The consistency behind the three reports and corroborations from Demircioğlu et al. (2005) together allow me to classify the difficulty at Level 4, so it is now Established. These students may have been taught that an indicator is a weak acid or base (McNaught & Wilkinson, 1997), which in molecular or ionic form shows a different colour (Szabadváry, 1964). If this is the case, students
also need to know that, in practice, negligible amounts of indicators are used. In this regard, Demircioğlu et al. (2005) report that the incidence of the difficulty reduced from 34 to 14%
through conceptual conflict strategies and practical exercises. Based on the argument above, I include the following specific propositional statement:
• Indicators are substances which change colour at certain pH values. (6.1.2)
• Indicators are added in very small amounts, about 8 drops per 100 ml (Vogel, 1961).
(6.1.1.2)
7.4.3.4 Sub-difficulty P20.2: Acid-base neutralization is neutralization of oppositely charged ions.
When investigating student conceptions of conjugate acid-base pairs, Schmidt (1995) found a common idea that neutralization involves positive and negative ionic charges neutralizing each other, as for example in the student quotation: “NH4
+ has protons in excess and HSO4 – has electrons in excess, that means it lacks protons. ... HSO4
– and NH4
+ seem to belong together, as if they somehow neutralized each other.” From his 1997 interpretation of the set of data, Schmidt argues: “Apparently they looked for positively and negatively charged ions ...
assuming that they could somehow neutralize each other.” The research presented here is sufficient in itself to classify the difficulty description at Level 4 as it has been established in multiple chemical contexts through research among many different students groups in Germany.
Schmidt’s description is consistent with later research into the same conception (Lin et al., 2004; Drechsler & Schmidt, 2005b; Lin & Chiu, 2007; Furió-Más et al., 2007). Brønsted (1923) appears to have already anticipated such a difficulty, as he clarifies: “Electric charge is irrelevant to the acid-base function.” Students, therefore, need to integrate the following propositional knowledge (see also Schmidt, 1991; Nakhleh & Krajcik, 1994; Ouertatani et al., 2007):
• Electric charge is irrelevant to the acid-base function. (10.2.0.1)
• Arrhenius neutralization is the reaction between hydrogen ions and hydroxide ions (7.2) to produce water. (7.2.1)
• During neutralization reactions, cations from the base and anions from the acid form a salt.
(7.2.2)
• Brønsted neutralization in water: H3O+ and OH– ions tend to neutralize each other (7.3.3.1) which may be represented as: H3O+ + OH– H2O + H2O. (10.3.2.1)
7.4.3.5 Difficulty P21: Acids are stronger than bases
Research shows that students believe bases are inherently weaker than acids, as shown by the following quotation from a Grade 11 student: “Bases are not strong” (Nakhleh & Krajcik, 1994). Very similar evidence is also reported by Ross and Munby (1991), Toplis (1998) and Sheppard (2006). All this evidence shows that students are sometimes unaware that both acids and bases can be strong. Consequently, without further analysis, the description of the difficulty given above can be classified at Level 3++ or Partially Established in more than one context.
The conception maps directly to the following propositional knowledge:
• Acid or base strength depends on the chemical nature of the acid or base. (8.1) (Furió-Más et al., 2007)
• Strong Arrhenius acids and bases are fully dissociated in solution. (8.2.2.1) (Ouertatani et al., 2007)
• Strong Brønsted acids are good proton donors. (8.3.1) (Carr, 1984)
• Strong Brønsted bases are good proton acceptors. (8.3.2) (Carr, 1984)
A possible source of the difficulty is students’ dichotomous view of acids and bases (See P4 in Section 7.2.2) and accordingly the difficulty is not given a separate classification. Alternatively, the source may lie within the teaching curriculum where the operational model defines an acid in terms of its ability to release hydrogen from particular metals; while a base is almost an adjunct with no character of its own – it is simply something that an acid tends to neutralize. In this regard, Solomonidou and Stavridou (2000) have shown that students think that substances have ‘relative’ properties and that the stronger substance would act on the weaker, without itself being affected. In the context of acids and bases, students may think that acids act on bases, rather than a reaction being a mutual interaction, as shown in the sub-difficulty below.
7.4.3.6 Sub-difficulty P21.1: The product of neutralization is acidic
As reported by Sheppard (2006) 10th grade students (12.5% incidence) considered acids as inherently more powerful than bases, leading always to an acidic product of neutralization reactions. The context he used was a titration between the strong acid HCl and the strong base NaOH where the product would be neutral. The difficulty maps to the following propositional knowledge:
• Neutralization reaction results in a solution that may be acidic, basic or neutral. (7.2.3)
• When equivalent amounts of a strong acid and an equally strong base react, the resulting solution will be neutral (7.2.3.1).
With evidence from only one controlled study, the sub-difficulty is described at Level 3. It needs to be confirmed in other chemical contexts, perhaps varying the relative strengths of acid and base.
7.4.3.7 Difficulty P22: Acid-base reactions proceed to completion Two reports show the following similar student conceptions:
“The base took over the acid” (Erduran, 2003).
“In all neutralization reactions, acid and base consume each other completely”
(Demircioğlu et al., 2005).
Is this in fact a problematic conception? Neutralization had originally meant that acid and base consumed each other so that neither acidic not basic property remained (Kauffman, 1988) and this remains acceptable today within an operational model (Drechsler & Schmidt, 2005b), as used for titrations (de Vos & Pilot, 2001). With modern knowledge of reversible reactions, Schmidt (1997) clarifies that because some H+ and OH- ions remain, “Acid and base do not consume each other completely; they react to a great extent.” Moreover, in the Brønsted reaction scheme (Equation 3.9 in Figure 3.1, page 47) “acids and bases never disappear. An acid reacts with a base forming another acid and base” (Drechsler & Schmidt, 2005a). The following propositional knowledge reflects the argument above:
• For acid-base titrations, in principle, equivalent amounts react completely (7.1.3.1)
• Brønsted acid and base react to form Brønsted base and acid (7.3)
• Brønsted reactions are in principle reversible. (7.3.2)
Furthermore, a statement given for a previous difficulty (P20.2) is modified:
• Brønsted neutralization in water: H3O+ and OH– ions tend to neutralize each other to a large extent, but not completely (7.3.3.1)
By reversing the propositional statements, the difficulty description as given above can be derived. However, there are certain problems with the two research reports on which this description is based. In the first report, Erduran (2003) makes no claims about the unanticipated student quotation representing a typical conception, and in the second Demircioğlu et al. (2005) do not substantiate their claim with qualitative data. Consequently, the difficulty can only be classified as Level 1, or Suspected. Future research needs to find out whether students carry the assumptions from an operational model through to equilibrium systems.
7.5 DIFFICULTIES WITH OTHER ACID-BASE REACTIONS