Electrolyte Solutions Dissolved Ionic Solids

A Rock candy is composed of sugar crystals that form through recrystallization. 

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When an ionic solid such as NaCl dissolves in water the solution formed contains Na+ and Cl- ions. Since ions are charged particles, the solution conducts an electric current (Figure 2.12) and we say that NaCl is a strong electrolyte. In contrast, a water solution of sugar, which is a molecular solid, does not conduct electricity. Sugar and other molecular solutes are nonelectrolytes. 

In Chapter 6 we saw that the chemistry of sodium can be understood in terms of the special stability of the inert gas electron population of neon. An electron can be pulled away from a sodium atom relatively easily to form a sodium ion, Na+. Chlorine, on the other hand, readily accepts an electron to form chloride ion, Cl-, achieving the inert gas population of argon. When sodium and chlorine react, the product, sodium chloride, is an ionic solid, made up of Na+ ions and Cl- ions packed in a regular lattice. Sodium chloride dissolves in water to give Na+(aq) and C (aq) ions. Sodium chloride is an electrolyte it forms a conducting solution in water. 

An electrolyte is a substance that, in solution, is present as ions. Ionic solids that are soluble in water are electrolytes because the ions become free to move when the solid dissolves (Fig. 1.2). Some electrolytes, however (such as acids), form... 

Many of the reactions that you will study occur in aqueous solution. Water readily dissolves many ionic compounds as well as some covalent compounds. Ionic compounds that dissolve in water (dissociate) form electrolyte solutions— solutions that conduct electrical current due to the presence of ions. We may classify electrolytes as either strong or weak. Strong electrolytes dissociate (break apart or ionize) completely in solution, while weak electrolytes only partially dissociate. Even though many ionic compounds dissolve in water, many do not. If the attraction of the oppositely charged ions in the solid is greater than the attraction of the water molecules to the ions, then the salt will not dissolve to an appreciable amount. 

A solute may be present as ions or as molecules. We can identify the form of the solute by noting whether the solution conducts an electric current. Because a current is a flow of electric charge, only solutions that contain ions conduct electricity. There is such a tiny concentration of ions in pure water (about 10-7 m) that water alone does not conduct electricity. A substance that dissolves to give a solution that conducts electricity is called an electrolyte. Electrolyte solutions (solutions of electrolytes), which conduct electricity because they contain ions, include aqueous solutions of ionic compounds, such as sodium chloride and potassium nitrate. The ions are not formed when an ionic solid dissolves they exist as separate ions in the solid but become free to move apart in the presence of water (Fig. 1.1). Acids also are electrolytes. Unlike salts, they are molecular compounds in the pure state but form ions when they dissolve. One example is hydrogen chloride, which exists as gaseous HC1 molecules. In solution, however, HCl is called hydrochloric acid and is present as hydrogen ions and chloride ions. 

A strong electrolyte in aqueous solution may be represented as separate ions because the ions of each type are free to move about independently of the ions of the other type. However, an ionic solid that is not dissolved in water is not written as separate ions the oppositely charged ions in the solid lattice of an ionic compound are not independent of each other (Figure 9.2). 

Many ionic solids dissolve in water, dissociating into positive and negative ions (an electrolytic solution). Because these ions can carry (conduct) a current of electricity, they are called electrolytes. Covalent solids in solution usually retain their neutral character and are nonelectrolytes. 

The situation changes dramatically, however, when an ionic solid melts to become a liquid or is dissolved in solution. The ions—previously locked in position—are now free to move and conduct an electric current. Both ionic compounds in solution and in the liquid state are excellent conductors of electricity. An ionic compound whose aqueous solution conducts an electric current is called an electrolyte. You will learn more about solutions of electrolytes in Chapter 14. 

We have learned in this chapter that many ionic solids dissolve in water as strong electrolytes, that is, as separated ions in solution. What properties of water facilitate this process Would you expect ionic compounds to be soluble in elemental Uquids like bromine or mercury, just as they are in water Explain. 

In electrochemistry an electrode is an electronic conductor in contact with an ionic conductor. The electronic conductor can be a metal, or a semiconductor, or a mixed electronic and ionic conductor. The ionic conductor is usually an electrolyte solution however, solid electrolytes and ionic melts can be used as well. The term electrode is also used in a technical sense, meaning the electronic conductor only. If not specified otherwise, this meaning of the term electrode is the subject of the present chapter. In the simplest case the electrode is a metallic conductor immersed in an electrolyte solution. At the surface of the electrode, dissolved electroactive ions change their charges by exchanging one or more electrons with the conductor. In this electrochemical reaction both the reduced and oxidized ions remain in solution, while the conductor is chemically inert and serves only as a source and sink of electrons. The technical term electrode usually also includes all mechanical parts supporting the conductor (e.g., a rotating disk electrode or a static mercury drop electrode). Furthermore, it includes all chemical and physical modifications of the conductor, or its surface (e.g., a mercury film electrode, an enzyme electrode, and a carbon paste electrode). However, this term does not cover the electrolyte solution and the ionic part of a double layer at the electrode/solution interface. Ion-selective electrodes, which are used in potentiometry, will not be considered in this chapter. Theoretical and practical aspects of electrodes are covered in various books and reviews [1-9]. 

Teachers need to be aware of two different uses of the term electrolyte . In the strict sense, an electrolyte is a liquid that cm undergo electrolysis. This can be a single substance, as in the case of a molten salt, or a solution. The most typical electrolytes are the aqueous solutions of salts (in general of ionic compounds), of acids, and of bases. By extension, we also call electrolytes the pure substances (solid, liquid, or gaseous) that, when dissolved in water, provide liquid electrolytes. Some biological substances (such as DNA or polypeptides) and synthetic polymers (such as polystyrene sulfonate) contain multiple charged functional groups and their dissolution leads to electrolyte solutions these are termed polyelectrolytes. 

In its modern form, the model assmnes that solid electrolytes, such as the salts, are composed of ions which are held together by electrostatic forces of attraction. In other words, the ions do not form during the dissolution process, but pre-exist in the solid state (Chapter 3). Remind students that sodium chloride and calcium chloride are ionic compounds. When such an electrolyte is dissolved in a solvent, these forces are overcome and the electrolyte undergoes dissociation into ions. In the solution, the ions are moving about, each surrormded and followed in its movement by a number of solvent molecules, and so we say that in the solution the ions are solvated (hydrated in the case of aqueous solutions). 

Ions are commonly found in a class of compounds called salts, or ionic solids. Salts, when melted or dissolved in water, yield solutions that conduct electricity. A substance that conducts electricity when melted or dissolved in water is called an electrolyte. Table salt — sodium chloride — is a good example. On the other hand, when table sugar (sucrose) is dissolved in water, it becomes a solution that doesn t conduct electricity. So sucrose is a nonelectrolyte. Whether a substance is an electrolyte or a nonelectrolyte gives clues to the type of bonding in the compound. If the substance is an electrolyte, the compound is probably ionically bonded (see Chapter 6). If it s a nonelectrolyte, it s probably covalently bonded (see Chapter 7). 

Solubility product, Ksp n. The equilibrium constant for ionic solubility equilibrium. When a solid electrolyte (MA) dissolves at least two kinds (M and A) of particles (ions) are released to the solution (e.g., NaCl in water), then... 

The solution process [Figure 8.5(c)] usually leads to an increase in entropy, too. When a sugar crystal (a nonelectrolyte) dissolves in water, the highly ordered structure of the solid and part of the ordered structure of water break down. Thus, the solution has a greater number of microstates than the pure solute and pure solvent combined. When an ionic solid (an electrolyte) such as NaCl dissolves in water, two factors contribute to an iuCTease in entropy the solution process (the mixing of the solute with solvent) and the dissociation of the compound into ions ... 

We can divide the substances that dissolve in water into two broad classes, electrolytes and nonelectrolytes. An electrolyte is a substance that dissolves in water to give an electrically conducting solution. Sodium chloride, table salt, is an example of an electrolyte. When most ionic substances dissolve in water, ions that were in fixed sites in the crystalline solid go into the surrounding aqueous solution, where they are liee to move about. The resulting solution is conducting because the moving ions form an electric current Thus, in genraal, ionic solids that dissolve in water are electrolytes. 

When electrolytes dissolve in water they produce ions, bnt to varying extents. A strong electrolyte is an electrolyte that exists in solution almost entirely as ions. Most ionic solids that dissolve in water do so by going into the solution almost completely as ions, so they are strong electrolytes. An example is sodium chloride. We can represent the dissolution of sodium chloride in water by the following equation ... 

Now let us look at the product side of the equation. One product is the precipitate CaC03(.s). According to the solubility rules, this is an insoluble ionic compound so it will exist in water as a solid. We leave the formula as CaC03(s) to convey this information in the equation. On the othCT hand, NaOH is a soluble ionic substance and therefore a strong electrolyte it dissolves in aqueous solution to give the freely moving ions, whieh we denote by writing Na (a ) + OH (a ). The complete equation is... 

Ions are commonly found in a class of compounds called salts, or ionic solids. Salts, when melted or dissolved in water, yield solutions that conduct electricity. A substance that conducts electricity when melted or dissolved in water is called an electrolyte. Table salt — sodium chloride — is a good example. 

Electrolyte a substance, liquid or solid, which conducts electrical current by movement of ions (not of electrons). In corrosion science, an electrolyte is usually a liquid solution of salts dissolved in a solvent, or a molten salt. The term also applies to polymers and ceramics which are ionically conductive. 

The great importance of the solubility product concept lies in its bearing upon precipitation from solution, which is, of course, one of the important operations of quantitative analysis. The solubility product is the ultimate value which is attained by the ionic concentration product when equilibrium has been established between the solid phase of a difficultly soluble salt and the solution. If the experimental conditions are such that the ionic concentration product is different from the solubility product, then the system will attempt to adjust itself in such a manner that the ionic and solubility products are equal in value. Thus if, for a given electrolyte, the product of the concentrations of the ions in solution is arbitrarily made to exceed the solubility product, as for example by the addition of a salt with a common ion, the adjustment of the system to equilibrium results in precipitation of the solid salt, provided supersaturation conditions are excluded. If the ionic concentration product is less than the solubility product or can arbitrarily be made so, as (for example) by complex salt formation or by the formation of weak electrolytes, then a further quantity of solute can pass into solution until the solubility product is attained, or, if this is not possible, until all the solute has dissolved. 

All cells comprise half-cells, electrodes and a conductive electrolytethe latter component separates the electrodes and conducts ions. It is usually, although not always, a liquid and normally has an ionic substance dissolved within it, the solid dissociating in solution to form ions. Aqueous electrolytes are a favourite choice because the high dielectric constant e of water imparts a high ionic conductivity k to the solution. 

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