# Redox reactions and redox equilibria

The concept of redox reactions is first introduced at GCSE where students will have met the idea of defining oxidation and reduction in terms of electrons. This is extended and refined in the first year of A level by the introduction of the concept of oxidation state (or number). Students need to learn to correctly assign oxidation state to atoms in compounds and ions. This then leads to redefining **oxidation** as involving an **increase **in oxidation number and **reduction** as involving a **decrease.**

Redox reactions are then revisited in the second year and a much more rigorous approach is taken, leading to the concept of electrochemical cells and standard electrode potentials. Students need to learn a significant amount of terminology and to be able to apply this to a discussion of electrohemical cells, and to use data on standard electode potentials to calculate the overall potential for an electrochemical cell. This is an area that students often find difficult and practice is needed to become comfortable and proficient with the various mathematical concepts and manipulations.

One particular area of difficulty is assigning the electrodes in an electrochemical cell, as the definition of anode and cathode seems to conflict with that learned at GCSE in electrolysis. At GCSE students will have been taught that the cathode is negative, yet in an electrochemical cell the cathode is positive. This problem arises because at GCSE the electrodes have been assigned based on polarity, and this is a fundamental mistake. The electrodes in **any **electrochemical process are correctly assigned based on **process** and not **polarity**. Thus, if we define the anode as the electrode where oxidation takes place, and the cathode as being where reduction takes place, the conflict disappears. The incorrect approach to assigning anode and cathode based on polarity is unfortunately very common in text books and other otherwise reliable resources.

Cell potential is actually a thermodynamic property, and so the link is made between this and feasibility of reaction. In addition, students should appreciate that the reactions occuring in electrochemical cells are equilibrium processes. Some syllabuses include the quantitative relationships between Gibbs function and cell potential, and Gibbs function and equilibrium constant, whereas others leave the relationship at a qualitative level.

The application of the concepts developed is then applied to understanding the function of energy cells.

Redox titrations may appear is this section, or in the section on transition element chemistry, depending on the syllabus. They are included in **this **list, as from a pedagogical point of view, as this seems the most sensible.

The first year resources are to be found at the beginning of the list followed by resources appropriate for second year studies.

Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or other recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.’

## Links and Resources

### 9. Redox *suitable for home teaching*

This resource provides useful worksheets on oxidation numbers and half equations and the application of ideas on redox reactions to metal extraction and reactions of halogens.The material could be useful to reinforce learning either in class or as homework exercises.

### Oxidation Numbers

This interesting resource extends the concept of oxidation number and could be used as an extension activity for more able students. It shows how electonegativity values for atoms within molecules can allow assignment of oxidation number without having to apply the usual list of remembered rules. Once developed, the idea is applied to a number of problems.

Student sheets and a teachers' guide with solutions and explanations are included.

### Redox Reactions

This resource is a simple microscale investigation of the redox reactions between a number of different ions and would be a nice resource to use in support of theory sessions on oxidation states and redox reactions. This is suitable for students in year one of the A level course.

### Problem 3: Cleaning Solutions

This activity enables students to apply ideas related to oxidation states and redox reactions to a real life scenario, that of best value in commercial cleaning products. Students are asked to design and conduct an experiment to find out how much active ingredient is in each type of bleach and from this to determine which is best value for money. Consideration of the redox reaction taking place is part of the analysis, but note that this activity is **not** a redox titration and is therefore suitable for students in year one of the A level course.

Useful summary notes for the teacher, and answer sheets are included.

### Unit P6: Equilibrium III: Redox Reactions *suitable for home teaching*

This comprehensive resource provides much detailed background material on the topics covered in the section on redox equilibria (electrochemistry). The resource would be useful for teachers when planning lessons, but also many of the exercises, with a few dated exceptions, are still useful and pertinent to current syllabuses and can be scanned or retyped and edited to provide class or homework material. An invaluable resource for planning, and for setting example problems to support teaching and learning.

Experiment two on page eight of the text outlines the simple construction of an electrochemical cell and its use to measure the standard cell potential (EMF) produced with different electrode combinations. This would be a logical follow up to the experiment described in the next resource in this list (electricity from chemicals).

**This resource is the first of those included to support teaching in year two of the A level course.**

### Electricity from Chemicals

This resource describes a simple experiment related to electrochemical cells, which enables students to measure the potential between pairs of metals and therefore place the metals in order of reactivity.

The practical allows electrochemical cells to be related to the reactivity series, which students are already familiar with, and could be used early on in the development of ideas of electochemical cells as a bridging activity between previous knowledge and new material.

### Rusting

This resource, aimed at gifted and talented students, is an activity on rusting in which students use their knowledge and understanding of electrochemistry to answer a number of questions related to corrosion and corrosion protection.

Students need to be familiar with manipulation of data on standard reduction potentials in order to predict reaction feasibilty. This could be used as a stretching activity for the more able or as a whole class problem with guidance from the teacher.

### Accumulator

This resource describes how to set up a simple, rechargeable, lead-acid cell. The cell is then used to investigate the relationship between length of charging time and the time taken to discharge the cell when using it to power a light bulb. A simple construction of a rechargeable cell that can be used as an introductory activity to a topic on energy cells.

### Hydrogen Powered Cars

This is a short magazine article of hydrogen powered vehicles and hydrofgen fuel cells, taken from the December 2012 issue of Catalyst.

### Burning Water 2

This is a project based activity giving students an opportunity to look at hydrogen fuel cells in more detail. The resource includes student task and resource sheets (bundled together as Activity Sheet) and quite comprehensive teacher notes with suggestions on how to present the activity.

The suggested time for the activity is two hours.

The activity has been created by the Insitution of Engineering and Technology.

### Vintage Titrations: Sulphur Dioxide in Wine

This activity gives students the opportunity to carry out a redox titration in the interesting context of measuring the amount of sulphur dioxide in wine. The student work sheet presents the activity as an open ended task which the students need to research in order to find a suitable method. Students will need to be familiar with redox titrations involving iodine using starch as the indicator.

Alternatively the students could be given a method and simply carry out the procedure providing practice of the techniques and calculations.

A teachers' guide is included.

### Redox Titration *suitable for home teaching*

This resource contains a student worksheet and teachers notes, with solutions, to enable students to practice calculations relating to redox titrations.

The resource is suitable for ues in class or as a homework exercise.

Subject(s) | Science |
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Tags | n.a |

Age | 16-19 |

Last updated | 12 July 2016 |

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URL | https://www.stem.org.uk/lxq2p |