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- Content: A clear outline of the content covered by the delivery guide;
- Thinking Conceptually: Expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject;
- Thinking Contextually: A range of suggested teaching activities using a variety of themes so that different activities can be selected which best suit particular classes, learning styles or teaching approaches.
Mathematical learning outcome:
CM3.4i arithmetic computation and ratio when determining empirical formulae, balancing equations
Assessable content statements:
C3.4a recall that metals (or hydrogen) are formed at the cathode and non-metals are formed at the anode in electrolysis using inert electrodes to include the terms cations and anions
C3.4b predict the products of electrolysis of binary ionic compounds in the molten state to include compounds such as NaCl
C3.4c describe competing reactions in the electrolysis of aqueous solutions of ionic compounds in terms of the different species present to include the electrolysis of aqueous NaCl and CuSO4 using inert electrodes
C3.4d describe electrolysis in terms of the ions present and reactions at the electrodes
C3.4e describe the technique of electrolysis using inert and non-inert electrodes
Learners often find electrolysis one of the most difficult areas of chemistry to understand thus the approach needs to be well constructed. Learners need to understand that electrolysis is the splitting of chemical reagents using electricity. That this is carried out using electrodes either in molten salts or their aqueous solutions. That the electrodes used might be inert electrodes such as graphite (carbon) or metal. That each electrode carries an electrical charge, which promotes the movement of ions within the reagent or its solution. That the cathode is negatively charged and attracts the movement of positive ions called cations towards it, while the positively charged anode attracts negatively charged ions called anions. Predominantly metals (and hydrogen) carry positive charges (Cu2+) thus these react at the cathode to produce the metal (or hydrogen) and non-metals carry negative charges (Cl ‾) and are released at the anode.
The products of electrolysis may be dependant on the state in which the reagent is electrolysed and/or the respective position of the elements in the reagent with respect to the Reactivity Series. Molten binary salts such as sodium chloride only release two ions, in this case Na+ and Cl ‾ thus the only possible products are sodium at the cathode and chlorine at the anode. However in aqueous solution the metal’s position in relation to hydrogen determines whether the metal or hydrogen are produced at the cathode, more reactive metals (Al Na Mg) will remain in solution as ions while hydrogen is released, less reactive metals (Cu and Ag) will be deposited on the cathode. Meanwhile the anions are ranked in terms of their stability with reference to that of hydroxide ions. Simple ions of the elements tend to produce the element at the anode, while oxyanions such as SO42- are very stable and the hydroxide is decomposed to produce oxygen and water at the anode.
Common misconceptions or difficulties learners may have
Learners find this area of chemistry very difficult in part due to the need to understand the competing reactions involved in aqueous electrolysis and as molten electrolysis is somewhat constrained they will probably only perform aqueous electrolysis. Learners also find it difficult to understand that ions are able to conduct electrical current, despite their being charged. They think that electrons must be released when ions are formed and these carry the current.
Another common misconception is that ionic solids do not conduct because there are no free electrons.
Conceptual links to other areas of the specification – useful ways to approach this topic to set learners up for topics later in the course
The electrolytic separation of metals from their ores is required knowledge for 6.1b. Understanding the electrolysis of a variety of materials such as copper sulphate solution at this point makes the later expansion more comprehensible.
Galvanising is the electrolytic deposition of zinc on steel objects to form an impermeable layer to protect the steel from corrosion 6.1o.
Approaches to teaching the content
The study of electrolysis has the capacity to provide good experience of practical work and given a reasonable knowledge of the terminology ('Electrolysis definitions') this can lead to an interesting learning experience for the learners. 'Electrolysis practical' is a hands-on look at electrolysis in solution and how ions compete to be discharged. While learners need an understanding of the electrolysis of molten solutions the temperatures involved and the level of hazard in the products usually mean that these are done as a demonstration. There are sufficient instructions available to mount something suitable. 'Electrolysis with metal electrodes' and 'Electrolysis with inert electrodes' look at the reactions involved at both metal electrodes and inert electrodes using copper sulphate as the electrolyte. 'Video on half equations' is a short piece that can be recommended for self-learning, dealing with half equations as seen at the electrodes.
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