Expt7_electrolytes - Experiment 7 Electrolytes Ions and...

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109 Metals Ionic cpds Molecular cpds Solid Yes No No Liquid Yes Yes No Electrical Conductivity Experiment 7 Electrolytes Ions and Molecules in Aqueous Solution DISCUSSION Expt 7 Electrolytes.wpd Electrical Conductivities of Pure Substances The ability of any substance to conduct electricity often provides great insight into its chemical and physical character. Since electric current involves the flow of electrical charge, a conducting substance must provide a source of mobile, charged particles. The excellent conductivity of the metals, for example, involves the movement of the loosely held valence electrons of the metal atoms through the solid crystal lattice of the metal. In this case, there is no need for the atoms themselves to move. Ionic compounds are usually recognized by inspection of their chemical formulas which are likely to 32 contain both metals and nonmetals, e.g. , KCl and Mg(NO ) . Ionic solids consist of a highly ordered crystal lattice of positively and negatively charged ions, but these charged particles are locked into their positions within the crystal and are unable to transport charge. Ionic compounds do become conductors, however, in the molten or liquid state since the ions are now free to migrate. 2 Molecular compounds generally have chemical formulas that contain only nonmetals, e.g. , H S and 61 26 C H O . Molecular solids consist of discrete molecules which have no overall charge; that is, the molecules are neutral and cannot transfer charge even if they become mobile in the liquid state. If one were to investigate a series of solids by measuring their relative conductivities in both the solid and molten states, it may be possible to classify each into one of three broad categories: metals, ionic compounds, and molecular solids. Solution Electrolytes Solution electrolytes are compounds which dissolve or react in water to produce mobile ions in solution , and these solutions, therefore, conduct electricity . Many ionic compounds and some molecular compounds are capable of being solution electrolytes. The electrolytic behavior of five 2 2 3 3 representative compounds (CaBr , HBr , HCO H , NH , and CH OH) will be contrasted in the following discussion.
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Essentially, the collective ion-dipole solvation by the polar water molecules that are clustered around the individual ions 1 overcomes the strong, collective ion-ion forces of the solid crystal lattice. Note that the general acid here, HA , is written as having no overall charge. This is arbitrary, as acids may also have 2 positive or negative charge(s). Note that common bases may have no charge as well as negative charge(s) . 3 110 Strong Solution Electrolytes 2 Calcium bromide (CaBr ) is an ionic solid as indicated by the metal/nonmetal content of its chemical 2 formula. As expected, CaBr conducts electricity in the molten state but does not conduct as a solid.
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Expt7_electrolytes - Experiment 7 Electrolytes Ions and...

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