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Asia-Pacific Forum
on Science Learning and Teaching, Volume 11, Issue 2, Article 2 (Dec., 2010) |
In this study, a pre-test/post-test with control group experimental design was used. The study was conducted in the spring semester of the 2007-2008 academic year. Before treatment, the pre-test was applied to determine students’ prior knowledge about chemical equilibrium. The subject of chemical equilibrium was taught via using analogies in the experimental groups and at the same time control groups were taught via traditional method. After treatment; the post-test was administered to all subjects. Finally, semi-structed interviews were conducted with 24 students who were chosen according to their post-test scores. The instruction was accomplished in twelve course hours in all the classes. The experimental and control groups spent equal time for studying. However, the lessons in the experimental group focused on using the analogies that were designed to overcome students’ misconceptions about chemical equilibrium.
This study was conducted with the participation of 151, 11th grade students in three high schools in Izmir, Turkey. A cluster sampling technique was used to select the sample. Both schools had two classes that were randomly assigned to experimental and control groups. It was considered appropriate to name the high schools with codes of letters (such as A, B, C) instead of using their names. Distribution of groups with respect to the schools is presented in Table I.
Table I. Distribution of Groups with Respect to Schools
High School
Experimental Group
Control Group
A
26
27
B
18
20
C
30
30
In this study, although two measuring tools were used, the emphasis is on the data from semi- structed interviews.
• Chemical Equilibrium Misconceptions Test
• Semi- Structed Interviews
Chemical Equilibrium Misconceptions Test (CEMT)
Chemical equilibrium misconceptions test (CEMT) was developed in another research to diagnose students’ misconceptions and the level of understanding of students about chemical equilibrium. In this research, CEMT was used as a tool to select the interview sample. Test items were developed according to the objectives and the concepts in the curriculum. In this process, some written sources were used to write the items such as secondary level science textbooks, related literature and instruments developed by other researchers (Bilgin, 2006; Kousathana & Tsaparlis, 2002; Piquette & Heikkinen, 2005; Quilez, 2004; Solomonidou & Stavridou, 2001;Thomas & Schwenz, 1998; Voska & Heikkinen, 2000). First, a two tailed test which included multiple-choice questions that require students to explain their reasoning behind each multiple choice answers was developed. Then the CEMT, composed of 30 items, was designed and the test was piloted by participation of 45 11th grade students for reliability. After the data collection, item analysis was done. According to results of item analysis, 5 items whose discrimination value ranges from 0,19 to 0,14 were removed from the test. Finally, reliability coefficient (KR-20) of the test was found to be 0.79. The test included all aspects of chemical equilibrium concepts and was administered to all subjects of the study as a pre-test and post-test.
Each question has one correct answer and four distractors. Each item requires students to select a definition of a scientifically complete response and reason for the correct answer. Four different categories that help to classify scientifically acceptable and unacceptable explanations were determined. These categories are below:
Scientifically Correct: Scientifically complete responses and correct explanations are a part of this category.
Partially Correct: Scientifically complete responses and incorrect explanations or scientifically incorrect response and correct explanations match this category.
Incorrect: This level involves completely unacceptable scienfitic responses or explanations.
No Response: Students who do not choose any response and make any explanations are put in this category.
An example of question of test item is presented in Table II.
Table II. A Sample Question in CEMT
The equilibrium between A gas and B gas is as follows:
aA(g)
bB(g)
When the volume of a container was increased at a constant temperature, reaction shifted to the reactants’ side. According to this, what can we say about equilibrium?
A) a>b because when volume of the container was increased, equilibrium will proceed to make more moles of gases.
B) a>b because when volume of the container was increased, equilibrium will proceed to make fewer moles of gases.
C) b>a because when volume of the container was increased, equilibrium will proceed to make fewer moles of gases.
D) b>a because when volume of the container was increased, equilibrium will proceed to make more moles of gases.
E) It cannot be estimated because moles of gases don’t have any influence on equilibrium shift.
In order to determine whether there was any significant difference in constructing knowledge between experimental groups and control groups or not, interviews were carried out. The interview form consisted of five questions which were based on the reaction between carbon monoxide and chlorine forming carbonyl chloride.
CO(g) + Cl2 (g)
Table III summarizes the content of interview questions. As it is seen in the Table III, the interview questions were about the explanation of chemical equilibrium and the application of Le Chatelier’s principle.
Table III. Content of Questions in the Interview
Questions
Content
1st Question
Identification of chemical equilibrium
2nd Question
Changing equilibrium conditions ( effects of concentration)
3rd Question
Changing equilibrium conditions ( effects of temperature)
4th Question
Changing equilibrium conditions ( effects of pressure)
5th Question
Identification of Le Chatelier’s principle
The interview questions were selected mostly from the published research papers (Banerjee, 1991; Costu & Ünal, 2004; Hackling & Garnett, 1985). The interview form was submitted to two experts for checking its reliability and validity, and additionally it was applied to a small group (5 students) for piloting. Interviews were conducted to examine students’ deeply held ideas about chemical equilibrium. 24 students from each high school (12 students from experimental group, 12 students from control group) were interviewed. A stratified sample technique was used to select the interview participants. First, students were categorized by the scores they received from the post-test (CEMT) as high achievers, middle (average) achievers and low achievers. Secondly, four students from each of these groups were randomly selected. The interview sessions were conducted individually with each student and lasted an average of 10-15 minutes. The responses that emerged from the participants were classified and coded to search for common themes in their responses. The researchers and a subject-matter expert coded the answers separately, and then the two results were compared. In this research, the percentage agreement was used to calculate reliability (percentage agreement = 0.90). The interview questions are presented in the findings section.
As stated before, analogies are very important teaching tools that help students visualize abstract concepts (Heywood, 2002). In order to investigate the influences of analogies on preventing misconceptions about chemical equilibrium, nineteen analogies were developed in this research. The analogies are classified into two categories:
• Marble* models
• Molecular models.
The purpose of the using of marble models is to illustrate dynamic aspects of chemical equilibrium and applying of Le Chatelier’s principle. After all, molecular models account for making and breaking of bonds at a molecular level. While ten analogies are related to marble models, nine analogies are related to molecular models. These analogies and their targets are presented in Table IV.
Table IV. Analogies developed in this study and their targets
Analogy
Target
1. Marble Model Approach the equilibrium with reactants only
2. Marble Model Approach the equilibrium with different amounts of reactants and products
3. Marble Model The effects of increasing concentration of reactants on the equilibrium of the system
4. Marble Model The effects of decreasing concentration of reactants on the equilibrium of the system
5. Marble Model The effects of increasing concentration of products on the equilibrium of the system
6. Marble Model The effects of decreasing concentration of products on the equilibrium of the system
7. Marble Model Approach equilibrium as a heterogeneous system
8. Marble Model The effects of adding a solid substance to a heterogeneous equilibrium system
9. Marble Model The effects of increasing temperature on an endothermic equilibrium system
10. Marble Model The effects of decreasing temperature on an endothermic equilibrium system
1. Molecular Model Approach the equilibrium at a molecular level
2. Molecular Model The effects of increasing the concentration of reactants on a chemical equilibrium system
3. Molecular Model The effects of increasing concentration of products on a chemical equilibrium system
4. Molecular Model The effects of decreasing concentration of reactants on a chemical equilibrium system
5. Molecular Model The effects of decreasing concentration of products on a chemical equilibrium system
6. Molecular Model The effects of increasing pressure on a chemical equilibrium system
7. Molecular Model The effects of decreasing pressure on a chemical equilibrium system
8. Molecular Model The effects of increasing temperature on an exothermic equilibrium system
9. Molecular Model The effects of decreasing temperature on an exothermic equilibrium system
* A small colored glass ball, it isused to play a children’s game.
The second marble analogy is presented below.
- Students were grouped into 4. The students were given 60 marbles in total.
- In each group, while one student represented reactants, the other one represented the products. The other two students recorded the data in their data table.
- At the beginning, reactants were given 50 marbles, and the products were given 10 marbles.
- For each cycle, student reactants randomly turned over some of the marbles, at the same time as the other student, representing the products, turned over a few of marbles.
- The number of marbles that the reactants and products had visible at the end of each cycle was recorded in a table (Table V) against time. Students also recorded the number of marbles turned over in each cycle and determined the change in the total number of marbles at reactants and products from the end of the one cycle to the next. Students were told that the number of marbles as reactants represented the concentration of reactants. At the same time, it was stated that the number of marbles that the products had represented the concentration of the products.
- This process was repeated until a stage was reached where the number of the reactants marbles turned over in a cycle that was equal to the number of marbles the products turned over. As it is seen in the Table VI, after the reaching equilibrium, equilibrium constant (K) is calculated.
Table V. Marble Models Related to First Analogy
Time
(Min)Number of Marbles of the Reactant
Number of Marbles of the Products
The Rate of Forward Reaction
The Rate of Reverse Reaction
0-10
50
10
20
7
10-20
37
23
18
8
20-30
27
33
15
10
30-40
22
38
14
12
40-50
20
40
12
12
50-60
20 (Equilibrium)
40
12
12
60-70
20
40
12
12
70-80
20
40
12
12
80-90
20
40
12
12
Table VI. Equilibrium Constant
Number of Marbles of the Reactant
Number of Marbles of the Products
K (Equilibrium Constant)
Equilibrium
20
40
40/20 =2
The main purpose of this analogy was to explain how a system reaches equilibrium. The aspects of chemical equilibrium that this analogy illustrates are:
• The rates of reverse and forward reaction are equal at equilibrium.
• The rates of reverse reaction increase when equilibrium is reached.
• The rates of forward reaction decrease when equilibrium is reached.
• The dynamic aspect of chemical equilibrium.
• Reversibility of the reaction as the concept.
• Calculation of the equilibrium as a constant for the reaction.
Using this analogy, the researcher tried to prevent common students' misconceptions including:
• At equilibrium, no reaction occurs.
• When there is an equal mass or concentration of substances on both sides of equation, the reaction reaches equilibrium.
• T he rate of forward reaction is greater than the rate of reverse reaction at equilibrium.
The marble model could not explain the making and breaking of bonds at a molecular level. Therefore, molecular models which are based on formation of ammonia from H2 and N2 gases were developed.
N2(g) + 3H2(g)
One of the molecular models is presented below.
- Students are divided into four, like in the first analogy.
- First, one student is given 10 blue balls which represent N2, another student is given 20 grey balls which represent H2. The student who represents NH3 is not given any balls. The last student records the data in their data table (see Table VII).
- In the first stage 3 blue balls and 9 grey balls form 6 blue-grey balls which represent NH3.
- In the second stage, 2 blue-grey balls are converted back to reactants as 1 blue ball and 3 grey balls. At the end of tenth minute, there are 8 blue balls (N2), 14 grey balls (H2) on the reactant side and 4 blue-grey balls (NH3) on the product side.
- In the twentieth minute, while 2 blue balls and 6 grey balls produce blue-grey balls, 2 blue-grey balls decompose back to reactants as 1 blue ball and 3 grey balls. At the end of twenty minutes, there are 7 blue balls (N2) and 11 grey balls (H2) on the reactant side and 6 blue-grey balls (NH3) on the product side.
- In the fortieth minute, students see that the total numbers of balls on both the reactant and the product sides are constant. So, chemical reaction reaches equilibrium. Finally, the equilibrium constant (K ) is calculated (see Table VIII).
Table VII. Marble Models Related to First Analogy
Time
(Min)Balls represent reactants
Blue-Grey Balls represent product (NH3(g))
Blue Balls (N2(g) )
Grey Balls (H2(g))
0
10
20
----
0-10
8
14
4
10-20
7
11
6
20-30
7 (Equilibrium)
11
6
30-40
7
11
6
40-50
7
11
6
50-60
7
11
6
60-70
7
11
6
70-80
7
11
6
80-90
7
11
6
Table VIII. Equilibrium Constant
Balls Represent Reactants
Blue-Grey Balls Represent Product
(NH3(g))
K (Equilibrium Constant)
Blue Balls
(N2(g))Grey Balls
(H2(g))Equilibrium 7 11 6 (6)2 /7.(11)3 =0.004 The analysis of the quantitative data was done by using the SPSS packet program. A one-way ANOVA test was used to determine whether there was a statistically significant difference between the experimental and control groups’pre-test and post-test scores. The significant level of .05 was considered in comparing groups. For quantitative data analysis all audio-tapes were transcribed and then the responses were coded. During this process, it was considered appropriate to identify the participants with codes of numbers and letters (such as SA1, SB2, SC3...) instead of using their names. For example, expression of SA1 represents the first student who studies at school A.
