11 January 2019
TIME OF IMPLEMENTATION:
1445 - 1645
VENUE OF IMPLEMENTATION:
Gallery Science, Library KMS
IMPLEMENTATION:
Students divided into a group of four. All students were given a set of worksheets and each group was given a set of B-B B-F card.
STEP 1: Identify
B-B/ simultaneous B-B B-F/B-F
Students arranged species using B-B B-F card as given in the worksheet 1. Based
on the arrangement, students were stimulated to identify whether each of the species
given in the worksheet 1 would undergo bond breaking or bond forming
or simultaneous bond breaking and bond forming (Figure 12, 13 and 14). After
completing the worksheet 1, students deduced conclusions on free radical
substitution mechanism of alkanes with the guidance of lecturers.
- In initiation step, species (neutral molecule) is expected to undergo bond breaking (homolytic cleavage) after gaining energy from UV light or heating.
- In propagation step, species (neutral molecule and free radical intermediate) are expected to undergo simultaneous bond breaking and bond forming.
- In termination step, species (free radical intermediates) are expected to undergo bond forming.
Figure
12: B-B B-F
card of X-X species in initiation step used to stimulate students so that they
realized that X-X species would tend to undergo bond breaking in initiation step.
In worksheet 3, students were requested to draw the product or/and intermediate formed for each of the species after undergoing bond breaking or bond forming or simultaneous bond breaking and bond forming. Hence, the B-B B-F card was used to show the bond breaking or bond forming or simultaneous bond breaking and bond forming to allow students to see what was/were the product or/ and intermediate(s) formed (Figure 15, 16 and 17). Students with the lecturers’ guidance, then deduced conclusions on the product or/and intermediate(s) formed in each step of free radical substitution reaction of alkanes.
Conclusions of product/intermediate formed from free radical substitution reaction of alkanes include:
- Initiation step results in formation of free radical intermediates.
- Propagation step results in formation of free radical intermediate and neutral molecule.
- Termination step results in formation of neutral molecule.
Figure 15: B-B B-F card used to allow
students to see clearly what the intermediates formed after bond breaking in
initiation step.
Figure 17: B-B B-F card used to create
observations that are tangible to students on product formed in each step of
termination.
STEP 3: Draw the arrows for B-B/simultaneous B-B B-F/B-F.
On the whiteboard with the
aid of the B-B B-F card, students were stimulated to draw the arrows to show
bond breaking for the initiation step, which produced free radical
intermediates as shown in the model. For propagation step, students were asked
to identify which bond to break and form simultaneously. Then, they were
encouraged to draw the arrows to show simultaneous bond breaking and bond
forming with lecturers’ guidance. Students were prompted on drawing the arrows
to show bond forming for termination step involving same and different species
of free radical intermediates. With lecturers’ guidance, conclusions on how to
draw the arrow for initiation, propagation and termination are made.
Conclusions on arrow drawing for initiation, propagation and termination include:
- Initiation step involves bond breaking. Thus, the arrows should be pointing outward from the bond (Figure 18).
- Propagation step involves simultaneous bond breaking and bond forming. Thus, the arrows should point outward for bond breaking and point together (Figure 19) to show bond forming.
- Termination step involves bond forming. This, the arrows should point together to show bond is being formed (Figure 20).
Figure 18: Illustration of arrows to
show bond breaking in initiation step.
Figure 19: Illustration of arrows to
show simultaneous bond breaking and bond forming in propagation step. From the
B-B B-F card of initiation step, students should realize that bond breaking
should occur before bond forming in both steps in propagation.
Figure 20: Illustration of arrows to
show bond forming in termination step.
STEP 4: Combine IDEAS
The question in worksheet 4 was
the same question used for pre-test. In this step, students practiced all the
skills from arrow drawing to show bond breaking, simultaneous bond breaking
bond forming and bond forming, deducing correct product or/and intermediate(s)
formed and selecting the correct reactant or/ and intermediate for each step of
mechanism. The prior knowledge for the whole free radical substitution
mechanism was also enhanced. For example, students were reminded again why
halogen is the reactant used in initiation step. As the bond between halogen
atoms is the weakest bond and is easiest to be broken via homolytic cleavage
when energy is supplied by sunlight or heating.
Halogen radical then will
react with another reactant alkane to form alkyl radical and by-product (H-X).
Alkyl radical will continue the reaction by reacting with reactant halogen to
form product and halogen radical, which then continues to propagate the
reaction to form more products in propagation step.
Termination step will slow
down the reaction first and kill the reaction eventually when all the radicals
needed to propagate the reaction in propagation step are being wiped out.
To evaluate whether students’ understanding improves after this intervention, a post test will be conducted. A questionnaire is also carried out to assess the effectiveness of this method and whether it is appropriate to be employed in future teaching and learning session for this learning outcome.
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