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9.6 Shipwrecks, Corrosion and Conservation: 3. Electrolytic cells
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1. Electrolytic cells involve oxidation-reduction
reactions
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Students learn to: | Students: |
Extract from Chemistry Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit: 26 Jun08]
Prior learning: HSC module 9.2.4
Background: You have already seen that galvanic cells, where chemical energy is changed to electrical energy, are important in the corrosion of metals used in shipbuilding. Electrolytic cells which carry out the reverse process, changing electrical energy to chemical energy, are important in the conservation and restoration of artefacts from shipwrecks. Both galvanic cells and electrolytic cells can be explained using the concepts of oxidation-reduction studied in core module 9.2.4.
In both types of electrochemical cells:
- oxidation occurs at the anode - an ox
- reduction occurs at the cathode - red cat
In a galvanic cell, the source of electrons, the anode (where oxidation occurs), is negative.
In an electrolytic cell, the electrode to which electrons flow, the cathode, is negative.
The cell potentials measured in a laboratory can be different from the cell potentials calculated from tables of standard reduction potentials such as in the SI Data Book. The measured cell potentials can be different because they have not been measured under standard conditions (temperature, pressure and solution concentration) or the electrode material is not of high purity.
However, you should be familiar with using a table of standard reduction potentials, identifying the appropriate half equation, reversing the half equation where appropriate and using half equations to write balanced net ionic equations to describe the cell reaction. The table of standard potentials is still very useful in predicting likely reactions.
Remember that when the half equation is reversed the sign of the potential is changed. If it is necessary to multiply the half equation by some factor to balance the number of electrons, the half cell potential remains unaltered. The potential for the cell is obtained by adding the half cell potentials together.
plan and perform a first-hand investigation and gather first-hand data to identify the factors that affect the rate of an electrolysis reaction
- When planning your investigation you
may ask the questions, “What will happen if
……?” This will assist you in deciding
what data you will gather. You will need
to consider variables that need to be controlled or varied,
being sure to identify the dependent (the variable you
measure) and independent (the one you change to see the
effect) variables. Variables you might change (one at a
time) include voltage applied, electric current that flows,
temperature, concentration, surface area of electrode
covered by electrolyte solution and distance apart of the
electrodes.
- Gathering more data through repeat
trials will increase the reliability of the results.
- If you observe signs of electrolysis when performing the investigation you might ask, “What can I do to increase the rate of electrolysis?”
describe , using half equations, what happens at the anode and cathode during electrolysis of selected aqueous solutions
Background information
Half equations should be used to describe the anode and
cathode reactions observed. Students should be familiar
with and be proficient in using, the table of some standard
potentials in the HSC examination data sheet.
The chemistry data sheet for the HSC examinations 
.
In selecting the appropriate half equation to describe the reaction occurring at each electrode in aqueous solutions, the following guides should be considered:
- the higher the reduction potential (that is, the more positive the value) the harder it is to oxidise the anion
- before water molecules are oxidised to oxygen, bromide ions and iodide ions are oxidised
- nitrate ions, sulfate ions and fluoride ions are never oxidised, water molecules are oxidised to oxygen instead
- the lower the electrode potential (that is, the less positive or more negative) the harder it is to reduce a metal ion
- silver ions and copper ions are always reduced
- potassium ions, sodium ions, calcium ions and aluminium ions are never reduced.
From your observations of the electrolysis of aqueous solutions containing a variety of cations and anions, you should be able to make generalisations that enable you to make predictions about the electrolysis of solutions not previously encountered.
Some of the generalisations that you may have developed from your observations include:
- reduction occurs at the negative electrode, the cathode
- oxidation occurs at the positive electrode, the anode
- the charge on the electrodes is different for an electrolytic cell and a galvanic cell. The cathode is negative in an electrolytic cell while the cathode is positive in a galvanic cell
- anions carry charge towards the anode
- cations carry charge towards the cathode
- electrolysis only occurs when the voltage applied to the cell is greater than the calculated cell potential
- the reactions that occur at the anode and cathode can depend on the applied voltage.
describe factors that affect an electrolysis reaction
- effect of concentration
- nature of electrolyte
- nature of electrodes
The higher the current the greater the rate of electrolysis.
The higher the voltage the greater the current.
- Increased concentration of ions increases the current
and consequently the rate of electrolysis.
- Electrolytes that have more ions dissociated will have
a higher rate of electrolysis.
- Increased surface area of the electrodes used increases
the current and the rate of electrolysis.
- Decreased distance between the electrodes increases the current and the rate of electrolysis.
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