EXPERIMENTAL: AN INVESTIGATION INTO SOME OF THE VARIABLES INVOLVED IN THE CONDUCTIVITY AND ELECTROLYSIS OF AQUEOUS SOLUTIONS OF METAL SALTS
Introduction Studies have shown that, in order to conduct electricity, a substance must contain either free-moving electrons or free-moving ions; an ion is either a charged atom or a charged group of atoms (e.g., ammonium and hydrogencarbonate). In the solid state, an ionic compound consists of a lattice of oppositely charged ions which are not free to move because they are held together by strong electrostatic attractions. However, when an ionic compound is either melted or dissolved in water, these ions are free to move, and so the electrolyte formed does conduct an electric current.
Directly or indirectly, conductivity is important in a wide variety of scientific contexts; e.g., nerve impulses, electroplating, electrical cells, and the extraction of metals by electrolytic reduction.
Sea water is an important natural resource because it is rich in water- soluble metal salts (e.g., LiI, NaCl, and MgBr2). The extraction of pure substances from sea water involves a series of physical processes (e.g., evaporation and fractional crystallization), as well as chemical processes (in particular, electrolysis; i.e., the decomposition of an electrolyte by an electric current, as a result of redox reactions which occur at electrodes). So, quite reasonably, a knowledge of the effects of the variables involved in the electrolysis of metal salts is important (not least because the process uses vast amounts of expensive electrical energy).
A few semi-quantitative relationships between some variables have been determined (see the three examples summarized in Note 1). Nevertheless, purely quantitative relationships for a wide range of variables are not readily available ...
In this investigation, you are required to examine at least four variables involved in the conductivity and/or electrolysis of aqueous solutions of metal salts; at least two of these variables must be quantitative, and at least one must be qualitative.
Notes 1. The three examples are as follows. One, at room temperature (23°C), using carbon-graphite electrodes at a depth of 46 mm and a distance of 7 mm, the conductance (C) of 100 cm³ of NaCl(aq), when a small voltage is applied (~6.0 V), is in inverse proportion to concentration (M; within the range 0.20 - 0.80 mol dm-³); i.e., C = k × M-¹ + c. Two, the resistance (R) of 0.45 mm diameter nichrome wire, when a small voltage is applied (~5.0 V), is in linear proportion to length (L; within the range 0.15 - 0.40 m); i.e., R = k × L + c. And three, the rate of energy transfer (P) in 0.20 m of nichrome wire, when a small voltage is applied (~5.0 V), is in inverse proportion to diameter (D; within the range 0.35 - 0.55 mm); i.e., P = k × D-¹ + c.
2. Apart from your notes and standard textbooks, sources of scientific knowledge include encyclopaedias in libraries, on CD-ROMs, and on the Web; it is good practice to include a bibliography in your write-up.
3. You are provided with different types of electrodes (e.g., carbon- graphite, copper, and iron), various aqueous solutions of metal salts and acids (each with an initial concentration of 1.00 mol dm-³), a low- voltage d.c. supply, and a range of electrical components. In addition, you will need to use - within reason - other suitable apparatus.
4. The proposed plans of your investigation should be presented in detail; these plans may be modified as the investigation proceeds.
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