NCEA

Chemistry - 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.

Low Excellence

Commentary

Student work extract
Student 1 (PDF, 174KB)

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

This involves comparing and contrasting, and justifying, links between oxidation-reduction processes, observations, equations and calculations. This requires the consistent use of chemistry vocabulary, symbols, and conventions.

This student has completed fully balanced equations (1) with correct calculations (2) and justifies the spontaneity of the reactions (3). The student has also compared and contrasted the oxidation-reduction processes in both electrochemical and electrolytic cells (4).

For a more secure Excellence, the student could have elaborated further on the spontaneity of reactions, including the interpretation of E^o values. For example, linked the strengths of the reductants to the E^o values.

High Merit

Commentary

Student work extract
Student 2 (PDF, 176KB)

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 correct reduction potential calculation for the electrochemical cell (3), explained the spontaneity of the reaction (4), and related species to the given observations (5).

To reach Excellence, the student could ensure that both fully balanced equations are correct, and that they have elaborated on the spontaneity of reactions using E^o values.

Low Merit

Commentary

Student work extract
Student 3 (PDF, 144KB)

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

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, 114KB)

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, 114KB)

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, 85KB)

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.