Teacher Resource 1: Extending ideas of
electrochemical cells
Introduction
To extend learners’ investigations into electrochemistry, an investigation of electrochemical cells
formed using half-cells that do not contain a metal and require the use of a platinum or graphite
electrode to connect to the voltmeter can be carried out. Halogen/halide half-cells are good
examples of these. (Specification statement - 5.2.3(g)).
Chemicals and apparatus
•
chemicals and apparatus relevant to investigating metal/metal ion half cells – e.g. OCR
Practical Activity Group 8 Activity 2 – available from OCR Interchange
•
potassium chloride solution in chlorine water – dissolve 7.5g potassium chloride in 100cm3
chlorine water
•
potassium bromide solution in bromine water – dissolve 12g potassium bromide in 100cm3
bromine water
•
iron(II)/iron(III) solution - dissolving 81g of iron(III) nitrate-9-water together with 56g of
iron(II) sulfate-7-water in 200cm3 of water.
•
platinum or graphite electrode
•
if learners are creating their own halogen/halide half cells, a power pack, variable resister,
ammeter and 1 mol dm-3 potassium halide solutions will additionally by required.
A risk assessment for carrying out this activity will need to be completed.
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Redox chemistry and electrode potentials
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© OCR 2015
The activity
The following should allow you to extend you investigation of electrode potentials to include
halogen/halide half cells and a half-cell with both oxidised and reduced species in the aqueous
state.
1.
Construct a halogen/halide half cell using the apparatus and method from the OCR PAG 8
Activity 2. The electrolyte will be a solution of potassium halide in halogen water supplied by
your teacher. You will be using a graphite or platinum half cell.
2.
Alternatively, you can create your own halogen/halide half cell by electrolysing approximately
100cm3 of a 1 mol dm-3 solution of potassium halide with graphite electrodes for one minute.
The electrolysis circuit will need a variable resistor to limit the current and an ammeter to
ensure that the current does not exceed 0.5 A. The electrolysis should be done in a fume
hood as it will liberate toxic halogen gases. At the end of the one minute of electrolysis, the
solution will contain residual halide ions together with the halogen, which will be partially in
solution as well as adsorbed onto the surface of the positive graphite electrode. This
electrode then forms the connection to the voltmeter in the construction of the full
electrochemical cell.
3.
Connect this halogen/halide half cell to a metal/metal ion half cell (e.g. copper or iron) with a
salt bridge and voltmeter. How does your measured Ecell compare to the predicted Ecell?
Write out the half equations and full redox equation for each cell you construct.
4.
To investigate the iron(II)/iron(III) half-cell in which Fe3+ acts as an oxidising agent, construct
electrochemical cells with iron(II)/iron(III) half cells and halogen/halide half-cells. Determine
the ability of Fe3+ to oxidise the halides.
OCR Resources: the small print
OCR’s resources are provided to support the teaching of OCR specifications, but in no way constitute an endorsed teaching method that is required by the Board, and the decision to
use them lies with the individual teacher. Whilst every effort is made to ensure the accuracy of the content, OCR cannot be held responsible for any errors or omissions within these
resources.
© OCR 2016 - This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work.
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Version 1
Redox chemistry and electrode potentials
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© OCR 2015