Students’ Reasoning of Redox Reactions in Three Different Situations
- a view of students’ conceptions of redox in different settings
Lise-Lotte Österlund, Department of Chemistry, Department of Mathematics Technology and
Science Education, University of Umeå, Sweden,
e-mail: lise-lotte.osterlund@chem.umu.se
Margareta Ekborg, Department of Mathematics Technology and Science Education,
University of Umeå, Sweden,
e-mail: margareta.ekborg@educ.umu.se
Paper presentation
Abstract
The aim of this study was to analyze upper secondary school students’ reasoning of
spontaneous redox reactions in three different situations, two situations in a school context
and one outdoor context. The study focused on students’ use of redox models, how they use
the activity series of metals and which conception they have of the particles participating in
the reaction. In the school context study 21 students participated in laboratory work about
corrosion and a demonstration of a single displacement reaction with zinc and a copper
sulphate solution. Ten of them volunteered for individual semi-structured interviews. The
same students were interviewed a second time in the outdoor context – a copper statue on a
corroded iron stand. Two individual and four two-group interviews were here collected. The
results indicate that the students used the activity series of metals in terms of noble and
ignoble metals to predict reducing agent. They exclusively used the electron model to explain
redox reactions but a few used also the oxygen model in their reasoning in some of the redox
reactions. The students showed an alternative conception in their explanation of the redox
reaction of the laboratory work, compared to the scientific model. In the outdoor situation the
reasoning of the redox reaction became more scientific. The students’ reasoning around the
displacement reaction seemed scientifically reinforced.
Background, Aims and Framework
The chemistry curricula for Swedish upper secondary school states that students should learn
the concepts oxidation and reduction (redox) and be able to apply them in an industrial- and
everyday context (Skolverket, 2000). Redox is perceived as one of the most difficult topics,
both to teach and learn. The reactions are not fully understood (De Jong & Treagust, 2002)
and students have difficulties to transfer their knowledge of redox between different
chemistry topics and life phenomena (Schmidt & Volke, 2003; Soudani, Sivade, &
Médimagh, 2000).
During history four models to explain redox reactions have developed. They are all used in
chemistry education of today. Rignes (1995) has described the models, summarized in Table1.
Table1. Four redox models.
Model
1. Oxygen model
2. Hydrogen model
3. Electron model
4. Oxidation number model
Reduction
loss of O
gain of H
gain of electrons
decrease in oxidation number
Oxidation
gain of O
loss of H
loss of electrons
increase in oxidation number
The activity series of metals (abbreviated hereafter as ASOM) is a series, arranging metals
according to their ability to act as a reducing agent. It includes hydrogen (H) which indicates
the metal’s ability to generate hydrogen gas from different sources. Ignoble metals are on the
left side of hydrogen while the noble ones are on the right (Silberberg, 1996).
A common oxidizing agent is oxygen. When iron corrodes, iron atoms lose electrons to the
dissolved oxygen. The oxygen molecules reduces, react further with water molecules and
hydroxide ions produces. The ions combine and form iron hydroxide, rust. Copper oxide
forms when the copper atoms oxidize and oxygen becomes reduced (Silberberg, 1996).
Chemistry is often taught without connection to everyday life. There is little or no learning
transfer of chemistry for life-long learning (Gilbert, 2006). For chemically similar tasks but a
shift in context, transfer of knowledge often fails (Blanchette & Dunbar, 2002; Soudani et al.,
2000).
The aim of this study was investigate secondary school students’ explanations of spontaneous
redox reactions in three different situations. The situations were: A laboratory work of
corrosion where iron and iron in combination with copper were investigated and a
demonstration experiment with zinc and cupper sulphate solution. These were classroom
situations. One situation was outdoors, a copper statue on a corroded stand.
The following research questions were posed:
How do students in upper secondary school reason about redox reactions in different
situations?
o What redox models do they use?
o How do they explain the reactions with reactants and products?
o How do they use the activity series of metals?
Methods and samples
Participants
This study was conducted in a middle size upper secondary school in northern Sweden. The
group was a second grade (17 yrs) of the natural science programme with 21 students.
Previous year, they had worked with electrochemistry, ASOM and oxides of copper. No
teaching had been carried out of the standard electrode potentials.
During the lesson the teacher made a revision of the topic redox reactions and gave a brief
overview of corrosion of iron. The lesson lasted about 45 minutes and included a lecture, a
demonstration of zinc in cupper sulphate solution and a dialogue with the students. A work
sheet with lab instructions was distributed to the students to prepare as homework.
Interviews
Ten students volunteered for the interviews, five females and five males. They were above
average in grade. The interviews were conducted on two separate occasions. The first
individual semi-structured interviews were performed in a small group room after the
laboratory work and lasted for 30-40 minutes. The topic was the laboratory work of corrosion
and the displacement reaction with zinc and copper sulphate solution. On the second occasion
two individual and four two-group semi-structured interviews were conducted outdoors at the
copper statue and lasted for 15 minutes. All interviews were audio recorded and transcribed
verbatim.
Data analyses
The data has been inductively analysed partly following Hatch (2002). My research questions,
the interview guide, concepts from prior research and the data itself set the frames of the
analyses. From this view the material was categorised.
Results
Redox model
All the students explained oxidation and reduction as an electron transfer. Some of the
students mixed up the meaning of oxidation and reduction, but they all indicated the use of the
electron model in their explanations in all three situations.
In the laboratory work of corrosion, four of the students supplemented the electron model
with stating oxygen as a substance always being a part in a redox reaction. I have considered
it as they used the oxygen model to some extent. On the other hand, in the displacement
reaction with zinc and copper sulphate solution, none of the students identified oxygen as a
potential electron acceptor.
The majority of the students explained oxidation and reduction as mutual reactions in all three
situations.
Reducing agent
In all the situations, the students had no difficulties in identifying the reducing agent in the
reactions. In the situations in which metals were in contact, they used ASOM and reasoned in
terms of noble and ignoble metals when identifying reducing agent. The students said that
ignoble metal lost electrons in the reaction and became oxidized.
Regarding the statue, almost all the students were aware that copper itself could act as a
reducing agent, being a part of other redox systems besides the contact area between copper
and iron.
Oxidizing agent
In the laboratory work of corrosion the majority of the students identified water as the
oxidizing agent. Only a few students described dissolved oxygen as the oxidizing agent which
is scientifically correct. In this situation ASOM seemed as an incomplete tool for reasoning
redox reactions.
In the single displacement reaction all students asserted copper or copper ions as the oxidizing
agent. Even if there was a mix between macroscopic (observable) and sub-microscopic (ions)
expressions my interpretation is that the students had an awareness of what substance acted as
the electron acceptor. In this situation ASOM seemed as an aid in reasoning about redox
reactions. In the outdoor situation the students could freely choose to reason about redox
reactions around copper or iron or both metals of the statue. The majority of the students
asserted oxygen as the oxidizing agent, in contrast to the isomorphic task of the laboratory
work of corrosion. Even here ASOM seemed as an incomplete tool for reasoning about redox
reactions.
Chemical product
In the laboratory work of corrosion the majority of the students identified some form of iron
oxid –rust, as the product formed during the reaction. Half of the students also declared that
an additional product, hydrogen- or oxygen gas was formed.
Regarding the statue, all students asserted iron oxide or copper oxide as the product. Here just
one student declared gas as an additional product.
In the single displacement reaction, the majority of the students identified copper as a
precipitate on the zinc plate and zinc ions as products, according to the scientific model. Some
had difficulties to determine the constitution of the precipitate. The overall chemical
reasoning in this redox reaction seemed to be unproblematic.
Conclusions and Implications
All students use the electron model in their reasoning of redox reactions in every situation.
Some students use the oxygen model in addition to the electron model in some redox
reactions i.e. in rust formation. The majority of the students asserted redox reactions as mutual
reactions.
None of them had problems with identification of the reducing agent in any of the situations.
The main problem for these students was to identify the oxidizing agent in the laboratory
experiment of corrosion. ASOM became no aid here. In this situation more than half of them
also identified gas formation as a product of the reaction, which I interpret as an attempt to
explain the reaction with the aid of the series.
Regarding the statue, most students reasoned from the copper metal and made a scientifically
correct explanation of the redox reaction. The oxidation of copper is a similar reaction as the
single displacement reaction, in comparison to rust, which is a complicated reaction.
The students’ explanations about the single displacement reaction of zinc and copper sulphate
solution seemed rather unproblematic and the use of ASOM seemed reinforced. As ASOM
became a limited tool in the students’ explanations of the laboratory work of corrosion, it
would be helpful for the students to develop the series with non metallic elements. This could
be an “in between” step if the teacher decide to work with the standard electrode potential. In
other cases this step could be clarifying for the students that redox reactions occur in
accordance with reactions explained from ASOM but with a non metallic electron acceptor. It
would be easier to hold on to the electron model even if oxygen is part of the reaction.
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