Electrolysis
Visão Geral
When Paris put on its Great Exhibition in 1855, one of the most carefully guarded exhibits was a small bar of a silvery metal that nobody could buy. Aluminium was then the most expensive metal in the world, and the Emperor of France is said to have kept a set of aluminium cutlery for the guests he most wanted to impress, leaving the merely important ones to eat with gold. The strange part is that aluminium was never rare. It is the most abundant metal in the Earth's crust and always has been. The problem was that no chemist could prise it away from the oxygen it was locked to. Within about thirty years the price had collapsed far enough for saucepans to be made of it, and the thing that changed was not a new reagent. It was a wire carrying a current.
This lesson is about that current, and about what it does to a compound. You will learn why a solid ionic compound is a hopeless conductor and a molten one is an excellent one, which electrode pulls which ion and why, and how to write the half equations that describe each electrode on its own, balanced for atoms and for charge. You will then use those ideas to predict the products of electrolysing a solution, where the water quietly supplies two extra ions and changes the answer, and to explain three processes industry could not run without: plating gold onto a cheap connector, extracting aluminium from its ore, and turning ordinary salt water into three of the most useful chemicals in the world.
Objetivos
- When an ionic substance is melted or dissolved in water, the ions are free to move about within the liquid or solution.
- Passing an electric current through ionic substances that are molten, eg lead bromide, or in solution breaks them down into elements. This process is called electrolysis and the substance broken down is called the electrolyte.
- During electrolysis, positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode).
- Oxidation and reduction can be defined as the loss and gain of electrons respectively.
- At the cathode, positively charged ions gain electrons; at the anode, negatively charged ions lose electrons.
- Reactions at electrodes can be represented by half equations, for example: 2 Cl- → Cl2 + 2 e- or 2 Cl- - 2 e- → Cl2 Students should be able to write half equations for the reactions occurring at the electrodes during electrolysis, and may be required to complete and balance supplied half equations.
- If there is a mixture of ions: at the cathode, the products formed depend on the reactivity of the elements involved; at the anode, the products formed also depend on the relative concentrations of the ions present.
- Electrolysis is used to electroplate objects. This may be for reasons such as appearance, durability and prevention of corrosion. It includes copper plating and silver plating.
- Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite. Aluminium forms at the negative electrode and oxygen at the positive electrode. The positive electrode is made of carbon, which reacts with the oxygen to produce carbon dioxide. Students should understand why cryolite is used in this process. Students should be aware that large amounts of energy are needed in the extraction process.
- The electrolysis of sodium chloride solution produces hydrogen and chlorine. Sodium hydroxide solution is also produced. These are important reagents for the chemical industry, eg sodium hydroxide for the production of soap and chlorine for the production of bleach and plastics. Students should be able to explain, using ideas related to reactivity, why each of these products is produced.
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- Why does solid sodium chloride not conduct electricity? A. It contains no charged particles B. Its ions are held in fixed positions in the lattice C. Its ions have lost their charges D. It contains no delocalised electrons in the liquid state Answer: B
- During the electrolysis of molten lead bromide, what happens at the positive electrode? A. Lead ions gain electrons B. Bromide ions gain electrons C. Bromide ions lose electrons D. Lead atoms lose electrons Answer: C
- Which of these half equations is balanced for both atoms and charge? A. Al3+ + 2e- -> Al B. Cu2+ + 2e- -> Cu C. Cl- -> Cl2 + e- D. 2O2- -> O2 + 2e- Answer: B
- Concentrated sodium chloride solution is electrolysed with inert electrodes. Which products form at the negative and positive electrodes? A. Sodium and chlorine B. Hydrogen and chlorine C. Hydrogen and oxygen D. Sodium and oxygen Answer: B
- Why is cryolite used in the extraction of aluminium? A. It lowers the temperature at which the mixture melts, saving energy B. It reacts with the oxygen released at the positive electrode C. It catalyses the breakdown of aluminium oxide D. It stops the carbon electrodes from conducting electricity Answer: A
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