Target group: Secondary 3/4 (express) students.
Context: Teaching
Problem:
Problem:
Students often fail to understand which chemical species are reduced and which are oxidised in a given equation. In addition, they often hold the misconception that reduction and oxidation reactions occur independently of each other. This is usually not addressed by didactic teaching strategies within a constrained time. Students also tend to find this topic boring and meaningless.
Relevant prior knowledge: Ion formation, ionic and covalent bonding, qualitative analysis.
Suggested teaching strategies to counter problem:
- Keep it short and simple, focusing on key concepts.
- Show a clear progression from basics to advance.
- Link this topic to all related topics to offer a coherent and significant picture of redox.
- Use clear visuals.
- Relate this topic to everyday life.
- Maintain a more casual tone to keep students engaged.
- Address the 5 W's and 1 H where possible.
Application of teaching strategies:
1. So why does redox still occur if atoms have already achieved the stable duplet or octet configurations after losing, gaining or sharing electrons in their original compounds?
- Use an analogy:
State a few reasons as to why people still change jobs even if they are stable?
3 possibilities:
- Poor working conditions (eg. no breaks) in the original job.
- More attractive prospects (eg. higher salaries, promotions) in the new job.
- Forced to leave job due to incompetency or replacement by workers of better quality.
- Make a comparision between the analogy and the actual concepts to be taught:
Similarly, an atom/ion still moves from one stable compound to another due to:
- (1) conditions (eg. heating, the presence of a catalyst) which drive its transfer,
- (2) a stronger force of attraction for it by another atom/ion in the latter compound (higher electronegativity of specific atoms relative to others) and
- (3) it being displaced from its original compound by a more reactive atom/ion (reactivity series of metals - survival of the fittest?).
2. What occurs during redox reactions? (Provide the 3 main definitions).
- Gain of O2 (oxidation) :
Eg: Zn (s) + CuO (s) ----> ZnO (s) + Cu (s)
- Loss of O2 (reduction) :
- Eg: Zn (s) + CuO (s) ----> ZnO (s) + Cu (s)
- Loss of H2 (oxidation):
Eg: H2S (g) + Cl2 (g) ----> 2 HCl (g) + S (s)
- Gain of H2 (reduction):
Eg: H2S (g) + Cl2 (g) ----> 2 HCl (g) + S (s)
- Loss (oxidation) and gain (reduction) of electrons (oxidation):
- Relate to half-equations.
- Oxidising agent: Chemical species which undergoes reduction itself and simultaneously causes another to gain O2 and glose H2/electrons.
- Reducing agent: Chemical species which undergoes oxidation itself and simultaneously causes another to lose O2 and gain H2/electrons.
3. So when do we use each definition? And which is the most reliable definition?
4. How do we know if a redox reaction has occurred? (Relate to observations form qualitative analysis experiments).
- Colour changes.
- Formation of precipitates.
- Production of gases.
- Change in states?
- Test for cations.
- Colour changes of pH indicators.
- Colour change of oxidising and reducing agents (in titrations).
- Colour change of litmus paper.
- Acid-base reactions.
6. How is redox applicable to everyday life? (Include familiar and authentic examples)
- Rusting.
- "Browning" of apples.
- Electroplating.
- Transport of oxygen by red blood cells.
- Mixing of paints or dyes?
8. Are redox reactions reversible? (Thinking question for students to discuss in class).
9. Since almost all compounds can undergo redox reactions, can they still be considered as chemically stable even though their constituent atoms/ions have attained the stable duplet or octet configuration after their formation? (Topic for students to post their comments on this blog).
