NCEA

Chemistry - trial annotated exemplar Level 3 AS91393

Demonstrate understanding of oxidation-reduction processes (3.7)

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This annotated exemplar is intended for teacher use only. The student work shown does not always represent a complete sample of what is required. Selected extracts are used, focused on the grade boundaries, in order to assist assessors to make judgements at the national standard.

Excellence

Commentary

Student work extract
Student 1 (PDF, 113KB)

For Excellence, the student needs to demonstrate understanding of oxidation-reduction processes.

This involves producing evidence that shows justification of the spontaneity of electrochemical and electrolytic cells, supported by equations and calculations.

The student needs to produce evidence that links the balanced half and full equations and either cell potential calculations or using reduction potentials to justify the spontaneity of electrochemical and electrolytic cells.

This student has explained oxidation and reduction with supporting half and full equations (1). The student has also calculated the cell potential for both cells (2), and used the reduction potentials to compare the relative oxidant/reductant strength of the REDOX couples (3) for both electrochemical and electrolytic cells.

The student has provided evidence of using all their supporting evidence to justify the spontaneity (4) of both electrochemical and electrolytic cells.

High Merit

Commentary

Student work extract
Student 2 (PDF, 116KB)

For Merit, the student needs to undertake effective project management to support technological practice.

This involves making and explaining links between oxidation-reduction processes, observations, equations, and calculations. This requires explanations that use chemistry vocabulary, symbols, and conventions.

This student has identified what has been oxidised and reduced with reasons (1), and given balanced half equations for electrochemical and electrolytic cells (2). The student has also completed a correct reduction potential calculation for the electrochemical cell (3), explained the spontaneity of the reaction (4), and related species to the given observations (5).

This student begins to link the spontaneity of the electrochemical cell to the cell potential calculation. The student also includes balanced full equations.

This evidence does not reach Excellence because the student needs to justify the spontaneity of both electrochemical and electrolytic cells by using the cell potential calculation(s) or the standard reduction potentials.

Low Merit

Commentary

Student work extract
Student 3 (PDF, 98KB)

For Merit, the student needs to demonstrate in-depth understanding of oxidation-reduction processes.

This involves making and explaining links between oxidation-reduction processes, observations, equations and calculations. This requires explanations that use chemistry vocabulary, symbols, and conventions.

This student has identified what has been oxidised and reduced with reasons (1) and given balanced half equations for electrochemical and electrolytic cells (2). The student has also completed a reduction potential calculation for the electrochemical cell (3), and related some species to given observations (4).

For a more secure Merit, the student could relate all species to given observations, and included the correct unit for the calculation.

High Achieved

Commentary

Student work extract
Student 4 (PDF, 82KB)

For Achieved, the student needs to demonstrate understanding of oxidation-reduction processes.

This involves describing oxidation-reduction processes and may involve calculations. This requires the use of chemistry vocabulary, symbols, and conventions.

This student has identified what has been oxidised and reduced for electrochemical and electrolytic cells, with a description of either loss/gain of electrons or oxidation number (ON) changes (1). They have also made a reference to the requirement of energy for electrolytic cell (2) and reduction potentials for electrochemical cell (3).

To reach Merit, the student could include correct half equations for all reactions.

Low Achieved

Commentary

Student work extract
Student 5 (PDF, 82KB)

For Achieved, the student needs to demonstrate understanding of oxidation-reduction processes.

This involves describing oxidation-reduction processes and may involve calculations. This requires the use of chemistry vocabulary, symbols, and conventions.

This student has identified what has been oxidised and reduced for electrochemical and electrolytic cells, with descriptions of either loss/gain of electrons or oxidation number (ON) changes (1), some reference to the requirement of energy for the electrolytic cell (2), and the reduction potentials for the electrochemical cell (3).

For a more secure Achieved, the student could link oxidation number increase/decrease or loss/gain of electrons to oxidation and reduction processes.

High Not Achieved

Commentary

Student work extract
Student 6 (PDF, 51KB)

For Achieved, the student needs to demonstrate understanding of oxidation-reduction processes.

This involves describing oxidation-reduction processes and may involve calculations. This requires the use of chemistry vocabulary, symbols, and conventions.

This student has identified what has been oxidised and reduced for electrochemical and electrolytic cells, with some reference to either loss/gain of electrons or oxidation number (ON) changes (1).

To reach Achieved, the student could ensure that they have described all oxidation and reduction changes correctly, and made a reference to the requirement of energy for the electrolytic cell.