Chemistry Reference
In-Depth Information
transfer through redox reaction.” In addition this group was able to arrive at
chemical equations for demonstrated precipitation reactions. For the questionnaire
the following experiments were carried out:
1. The reaction of nickel oxide with aluminium and the corresponding observable
bright flash of light was shown to the students. The students were asked to
explain their observations, to draw a model of the structure of nickel oxide and
aluminium crystals and to write down chemical equations in words, structures
and formulae.
2. Concentrated solutions of calcium chloride and sodium sulfate were mixed and
a white precipitate could be observed. The students were asked to make draw
models of both solutions and write the reaction equations in words, structures
and formulae. The students knew from previous examples that “structures”
require labeling with ionic symbols in the case of ions, while in case of
molecules it means labeling molecular structures.
Only a few students were able to completely solve these problems. Statistics
show that nearly 100% of all students wrote down right reaction symbols in words,
but only 20% formulated correct structures or formulae [
4
]. Up to 80% have mental
models as illustrated in Fig.
7.11
. The main result of this study is that students often
do not write down ions of familiar metal oxides, but switch to molecules or mix ions
and molecules (marked with a dashed box). Concepts of the formation of ions from
corresponding atoms appear in cases, where ions already exist in salt solutions
and do not have to be formed anymore. These misconceptions are “school made”
and caused by deficits in the teaching processes [
14
]. It seems to be desirable
to hand out a list of “atoms and ions as basic units of matter” to the students,
as shown in Fig.
7.7
.
Concepts of stoichiometry.
With an empirical study on stoichiometric
calculations Schmidt [
20
] could show that only a small part of the students acquires
this ability. He noticed the following misconceptions: “No differentiation between
equation coefficient and formula indices; for example between 2 O and O
2
.No
differentiation between amount-of-substance ratio and mass ratio, equal amounts of
substance of reagent and product in gas reactions, equal volumes of reagent and
product, and others” [
20
]. Schmidt found these misconceptions through construc-
tion and analysis of specific multiple choice questions with suitable distractors:
“The distractors were built in a way that the students had to deal with numbers that
fit to the correct as well as to the wrong answers, to solve the problems. For
example: 2 g of a compound contain 1 g of copper and the rest is sulfur. Which
chemical formula fits to this information - CuS, CuS
2
,Cu
2
SorCu
2
S
2
?” [
20
].
Schmidt tried to find strategies, which led the students to make typical mistakes
and get to the wrong answers: “You cannot avoid misconceptions in chemistry
lessons. They should not be suppressed, but students should be aware of them: the
mistakes in their strategies should be discussed together. If chemistry lessons are
not successful in solving stoichiometric exercises, students might turn their back to
chemistry: stoichiometry might be the crossroads, where the student's decision is
