Exemplar for internal assessment resource Chemistry 3.7A for Achievement Standard 91393

© Crown 2012

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Exemplar for Internal Assessment Resource

Chemistry Level 3

Resource title: Making Copper

This exemplar supports assessment against:

Achievement Standard 91393

Demonstrate understanding of oxidation-reduction processes

Student and grade boundary specific exemplar

The material has been gathered from student material specific to an A or B assessment resource.

Date version published by Ministry of Education

December 2012 To support internal assessment from 2013

Exemplar for internal assessment resource Chemistry 3.7A for Achievement Standard 91393

© Crown 2012

Grade Boundary: Low Excellence

1. To achieve at Excellence level students are expected to demonstrate a 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. Links between the spontaneity of the reactions using reduction potentials and the changes in colour and state are made. Reactions of both cells are compared and contrasted (1). Chemistry vocabulary is used consistently except for the calculation of a cell potential for the electrolytic cell (2). However, there is part explanation (electrolytic cell) of why the reaction is spontaneous/non-spontaneous by referring to relative strengths of oxidants and reductants (3). To meet Excellence more securely, comprehensive understanding must ensure that a full explanation (for both cells) of why the reaction is spontaneous/non-spontaneous by referring to relative strengths of oxidants and reductants.

Exemplar for internal assessment resource Chemistry 3.7A for Achievement Standard 91393

© Crown 2012

Grade Boundary: High Merit

2. To achieve at Merit level students are expected to demonstrate an 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. There is sufficient explanation as to what has been oxidised and reduced in terms of oxidation number or electron loss/gain (1). Full, completely balanced equations have been written for oxidation-reduction reactions occurring (2). Explanations of oxidation-reduction processes for the electrochemical cell are supported by a conventional cell diagram and cell potential calculation. There is justification as to why the reactions occurring in the electrochemical cell are spontaneous (3). To meet Excellence, justification as to why the electrolysis of copper chloride to produce copper is non-spontaneous is required.

Exemplar for internal assessment resource Chemistry 3.7A for Achievement Standard 91393

© Crown 2012

Grade Boundary: Low Merit

3. To achieve at Merit level students are expected to demonstrate an 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. There is sufficient explanation as to what has been oxidised and reduced in terms of oxidation number or electron loss/gain (1). Half equations have been written (2). A cell diagram has been written for the electrochemical cell and the cell potential has been calculated (3). There is an attempt to make and explain links between oxidation-reduction processes, observations and calculations. However, there is no full equation for the electrolytic cell and there are some inaccuracies in the use of chemistry vocabulary and conventions. To meet Merit more securely, links between oxidation-reduction processes must be explained using correct chemistry vocabulary and conventions, and a full equation for the electrolytic cell must be provided.

Exemplar for internal assessment resource Chemistry 3.7A for Achievement Standard 91393

© Crown 2012

Grade Boundary: High Achieved

4. To Achieve this standard students must 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. Reactants and products of oxidation-reduction have been named and half equations have been written for electrolysis and electrochemical reactions (1). Explanations for oxidation-reduction have been provided and supported by a cell calculation for the electrochemical cell, but there is no conventional cell diagram for the electrochemical cell (2). Also, there is no making of and explanation of links between the oxidation-reduction processes, observations and equations. To meet Merit, making and explaining links between oxidation-reduction processes is required.

Exemplar for internal assessment resource Chemistry 3.7A for Achievement Standard 91393

© Crown 2012

Grade Boundary: Low Achieved

5. To Achieve this standard students must 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. Reactants and products have been named. Half equations have been written which show electron loss/gain (1). However, the oxidation number of the chloride ion has been incorrectly identified as -2 and the description of change of oxidation number is not correct for the oxidation process in the electrolytic cell (2). For a more secure Achievement, the correct description of oxidation number change for all couples is required.

Exemplar for internal assessment resource Chemistry 3.7A for Achievement Standard 91393

© Crown 2012

Grade Boundary: High Not Achieved

6. To Achieve this standard students must 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. Reactants and products of the reduction couple only have been correctly named for electrolysis of CuCl2 (1). For the oxidation process (in the electrolytic cell), oxygen gas (instead of chlorine gas) has been incorrectly identified as the product from the oxidation of water (2). Oxidation-reduction processes for the electrochemical cell have been correctly described (3). Half equations have been written correctly for the electrochemical reaction only. Oxidation number change has been described and the oxidation-reduction process in terms of electron loss/gain is described for the electrochemical cell only (4). To meet Achievement, oxidation-reduction processes in the electrolytic cell must be described correctly.