CHAPTER 5 RESULTS SHOWING SCOPE AND QUALITY OF RESEARCH ON
7.3 DIFFICULTIES WITH ACID-BASE CHEMICAL
7.4.1 Difficulties with macroscopic aspects of neutralization reactions
The difficulties described in this sub-section reflect inappropriate empirical observations of a product of neutralization reactions, the neutrality (or otherwise) of the end-point and heat of reaction
7.4.1.1 Difficulty P15 Neutralization reactions between alkalis and acids produce insoluble salts
The conception above has been reported by Tan et al. (2002) but not pursued further. As a result, the difficulty is classified as Level 2, or Emergent. At this stage, not knowing the cause of the conception, the difficulty maps to very general propositional knowledge, given as:
• During neutralization reactions, cations from the base and anions from the acid form a salt.
(7.2.2)
• The solubility of salts depends on the particular ions involved. (7.2.2.1)
7.4.1.2 Difficulty P16: Every neutralization reaction produces a neutral solution
The description of the misconception published as: “Every neutralization reaction yields a neutral solution” comes from comprehensive research described is sufficient detail to show that the misconception exists in different chemical contexts, and among several different student cohorts (Schmidt, 1991; 1995). Consequently it is possible for a single comprehensive research study to ‘move’ a description of a student conception right through the classification framework up to Level 4. This research was evaluated as being of high quality (see Section 5.4.1) as it started appropriately with free-response interviews, only later becoming more focused, and furthermore it involved large numbers of students from different educational cohorts. Schmidt (1991) attributes the conception to the word ‘neutralization’ becoming firmly entrenched before students are introduced to weak acids and bases, and so it was termed a “hidden persuader”. In this regard, the propositional knowledge given by Nakhleh and Krajcik (1994) is of concern.
They state that: “Acids react with bases to form a salt; this is called a neutralization reaction. In aqueous solutions, water is often formed”. There is no problem with these statements but then they continue: “this occurs at pH 7”. Their research context included both weak and strong acids, so the pH of the end point would not always have been 7. Such propositional knowledge is misleading. I sought appropriate propositional statements in the macroscopic domain (as given below) because students are likely to encounter neutralization reactions before they are familiar with ions.
• Titrations use neutralization reactions between equivalent amounts of acids and bases (7.1.3), which in principle, react completely. (7.1.3.1)
• Neutralization reaction results in a solution that may be acidic, basic or neutral. (Schmidt, 1991) (7.2.3)
• When equivalent amounts of a strong acid and an equally strong base react, the resulting solution will be neutral (Schmidt, 1995). (7.2.3.1)
• When equivalent amounts of an acid and base of unequal strength react, the resulting solution will not be neutral (Schmidt, 1995). (7.2.3.2)
• Neutralization reactions between equivalent amounts of weak acids and strong bases result in basic solutions. (Drechsler & Schmidt, 2005b). (7.2.3.2.2)
In describing his work, Schmidt gives propositional knowledge in more abstract terms of hydrogen and hydroxide ions suitable for more advanced students. Accordingly, the statements given below, which are based on Nakhleh and Krajcik (1994) and Schmidt (1997) reflect these more sophisticated ideas.
• Brønsted neutralization in water is the reaction between H3O+ and OH– ions (7.3.3.1) represented by H3O+(aq) + OH–(aq) 2 H2O(l) (10.3.2.1)
• If neutralization reactions involves weak acid or base molecules, there will be at least two competing equilibria (7.3.3.3.1)
• As a base, acetate ion, Ac- , is stronger than its conjugate HAc is an acid.(8.3.3.1)
• A strong conjugate base will compete for H3O+ ions (7.3.3.3.1.1) as given by:
H3O+(aq) + Ac–(aq) H2O(l) + HAc(aq) (10.3.4.2)
7.4.1.3 Difficulty P17 No heat is evolved (OR Heat is absorbed) during neutralization reactions
Results from two research groups indicate difficulty among senior secondary and tertiary students with the observation that heat is released during an acid-base reaction (see Table 7.4).
Table 7.4 Research concerning heat of reaction for acid-base reaction
Assertion Percentage of
students in agreement with assertion
Educational level of students
Authors
There is a temperature change when a solution of a base is added to an acid
41 1st year university Cros et al. (1986) There is a temperature change when a
solution of a base is added to an acid
47 After 1year at university
Cros et al. (1988) In the acid-base reaction there is
evolution of heat
29 Senior secondary Vidyapati &
Seetharamappa (1995) In the acid-base reaction there is
absorption of heat
15 Senior secondary Vidyapati &
Seetharamappa (1995)
All three publications mentioned above report on highly focused research probes in a true/false format. Furthermore, not one report gives qualitative evidence to substantiate the authors’
descriptions. In particular, Cros et al. (1986) do not show how the student response of ‘false’ to the statement about temperature change (that is increase or decrease) can be interpreted to show that students believed “no heat was evolved” (that is only temperature increase). By contrast,
the two possibilities (evolved and absorbed) are reported by Vidyapati and Seetharamappa (1995). Correspondingly, the difficulty description given above allows for either alternative.
Due to the narrowness of the research probes and the questionable interpretation by Cros et al.
there needs to be more investigation into this conception and the difficulty can only be given a low classification of Level 2 or Emergent. In the interim, the propositional statement below indicates the knowledge which I have introduced to students though practical work with relatively concentrated solutions of 1 mol.dm-3.
• The acid-base neutralization reaction will cause a temperature rise. (7.1.4)
7.4.2 Difficulty interpreting observations of neutralization reactions