Weak electrolyte A substance that conducts

Weak electrolyte A substance that conducts electricity poorly in dilute aqueous solution. 

Many reactions, particularly those that involve ionic compounds, take place in aqueous solution. Substances whose aqueous solutions contain ions and therefore conduct electricity are called electrolytes. Ionic compounds, such as NaCl, and molecular compounds that dissociate substantially into ions when dissolved in water are strong electrolytes. Substances that dissociate to only a small extent are weak electrolytes, and substances that do not produce ions in aqueous solution are nonelectrolytes. Acids dissociate in aqueous solutions to yield an anion and a hydronium ion, H30 +. Those acids that dissociate to a large extent are strong acids those acids that dissociate to a small extent are weak acids. 

The conductivity of a 0.1 M acetic acid solution is much lower, however, than that of a 0.1 M hydrogen chloride solution. This and other experiments show that only a small fraction of the dissolved acetic acid, CH3COOH, has formed ions. Such a substance that dissolves and dissociates to ions only to a limited extent is called a weak electrolyte. 

Substances that conduct electricity to a small extent in the molten state or in solution are called weak electrolytes. 

One key property of a solution is its electrical conductivity or ability to conduct electricity. When a substance, a solute, is dissolved is water, a solvent, ions may or may not be formed. A strong electrolyte is formed when the solute completely ionizes (the substance completely separates into ions), such as sodium chloride (a soluble salt), hydrochloric acid (strong acid), or sodium hydroxide (strong base). A weak electrolyte is formed when the solute partially ionizes, such as acetic acid (weak acid) or ammonia (weak base). A nonelectrolyte is a substance that dissolves in water but does not ionize, such as sugar or alcohol. Most soluble, nonacid organic molecules are nonelectrolytes. 

In Chapter 4, we classified solutes by their ability to conduct an electric current, which requires moving ions to be present. Recall that an electrolyte is a substance that dissociates into ions in aqueous solution strong electrolytes dissociate completely, and weak electrolytes dissociate very little. Nonelectrolytes do not dissociate into ions at all. To predict the magnitude of a colligative property, we refer to the solute formula to find the number of particles in solution. Each mole of nonelectrolyte yields 1 mol of particles in the solution. For example, 0.35 M glucose contains 0.35 mol of solute particles per liter. In principle, each mole of strong electrolyte dissociates into the number of moles of ions in the formula unit 0.4 M Na2S04 contains 0.8 mol of Na ions and 0.4 mol of S04 ions, or 1.2 mol of particles, per liter (see Sample Problem 4.1). 

Some substances (solutes) form aqueous solutions that weakly conduct electricity. Such substances are called weak electrolytes. This weak conduction of electricity is mainly because weak electrolytes only dissociate partially in solution. In other words, only a small fraction of the solute exists as ions, resulting in a solution that conducts electricity very weakly. Acetic acid (CH,COOH) and ammonia (NHj) are examples of weak electrolytes. 

Let ns snmmarize the main points in this section. Compounds that dissolve in water are soluble those that dissolve little, or not at all, are insoluble. Soluble substances are either electrolytes or nonelectrolytes. Nonelectrolytes form noncon-dncting aqneons solutions because they dissolve completely as molecules. Electrolytes form electrically conducting solutions in water because they dissolve to give ions in solntion. Electrolytes can be strong or weak. Almost all soluble ionic substances are strong electrolytes. Soluble molecular substances usually are nonelectrolytes or weak electrolytes the latter solution consists primarily of molecules, but has a small percentage of ions. Ammonia, NH3, is an example of a molecular substance that is a weak electrolyte. A few molecular substances (such as HCl) dissolve almost entirely as ions in the solution and are therefore strong electrolytes. The solubility rules can be used to predict the solubility of ionic compounds in water. 

Whether or not an aqueous solution is a conductor of electricity depends on the nature of the solute(s). Pure water contains so few ions that it does not conduct electric current. However, some solutes produce ions in solution, thereby making the solution an electrical conductor. Solutes that provide ions when dissolved in water are called electrolytes. Solutes that that do not provide ions in water are called nonelectrolytes. All electrolytes provide ions in water but not all electrolytes are equal in their tendencies for providing ions. A strong electrolyte is a substance that is essentially completely ionized in aqueous solution essentially all of the dissolved solute exists as ions. A weak electrolyte is only partially ionized in aqueous solution only some of the dissolved solute is converted into ions. One scheme for classifying solutes is summarized in Figure 5-3. 

Incomplete Dissociation into Free Ions. As is well known, there are many substances which behave as a strong electrolyte when dissolved in one solvent, but as a weak electrolyte when dissolved in another solvent. In any solvent the Debye-IIiickel-Onsager theory predicts how the ions of a solute should behave in an applied electric field, if the solute is completely dissociated into free ions. When we wish to survey the electrical conductivity of those solutes which (in certain solvents) behave as weak electrolytes, we have to ask, in each case, the question posed in Sec. 20 in this solution is it true that, at any moment, every ion responds to the applied electric field in the way predicted by the Debye-Hiickel theory, or does a certain fraction of the solute fail to respond to the field in this way In cases where it is true that, at any moment, a certain fraction of the solute fails to contribute to the conductivity, we have to ask the further question is this failure due to the presence of short-range forces of attraction, or can it be due merely to the presence of strong electrostatic forces ... 

Soluble ionic compounds tend to be strong electrolytes, while alcohols and organic compounds are nonelectrolytes. Remember that classification as a strong electrolyte, weak electrolyte, or nonelectrolyte is somewhat subjective. Freshwater can be either a weak electrolyte or a nonelectrolyte depending on its purity. The important consideration in classifying a substance is to what extent an aqueous solution of the substance will conduct electricity. 

Preparation of Solvent Conductance Water.—Distilled water is a poor conductor of electricity, but owing to the presence of impurities such as ammonia, carbon dioxide and traces of dissolved substances derived from containing vessels, air and dust, it has a conductance sufficiently large to have an appreciable effect on the results in accurate work. This source of error is of greatest importance with dilute solutions or weak electrolytes, because the conductance of the water is then of the same order as that of the electrolyte itself. If the conductance of the solvent were merely superimposed on that of the electrolyte the correction would be a comparatively simple matter. The conductance of the electrolyte would then be obtained by subtracting that of the solvent from the total this is possible, however, for a limited number of solutes. In most cases the impurities in the water can influence the ionization of the electrolyte, or vice versa, or chemical reaction may occur, and the observed conductance of the solution is not the sum of the values of the constituents. It is desirable, therefore, to use water which is as free as possible from impurities such water is called conductance water, or conductivity water. 

The water solutions of some substances conduct electricity, while the solutions of others do not. The conductivity of a solution depends on its solute. The more ions a solution contains, the greater its conductivity. Solutions that conduct electricity are called electrolytes. Solutions which are good conductors of electricity are known as strong electrolytes. Sodium chloride, hydrochloric acid, and potassium hydroxide solutions are examples of strong electrolytes. If solutions are poor conductors of electricity, they are called weak electrolytes. Vinegar, tap water, and lemon juice are examples of weak electrolytes. Solutions of substances such as sugar and alcohol solutions which do not conduct electricity are called nonelectrolytes. 

Because acids ionize to form ions in water, acidic solutions conduct electricity. As you learned in Chapter 4, substances that dissolve in water to form conducting solutions are called electrolytes. Figure 14.7 compares the electrical conductivities of water, a solution of an ionic compound, and a solution of a weak acid. 

Aqueous solutions of sodium chloride and cupric sulfate of, say, 1 M concentration conduct electricity much better than a 1 M solution of acetic acid. This suggests that the sodium chloride and cupric sulfate exist in solution to a greater extent as ions than does acetic acid. More detailed investigations have indicated that certain substances, including sodium chloride and cupric sulfate, occur almost entirely as ions when in aqueous solution. Such substances are known as strong electrolytes. Other substances are present only partially as ions. Acetic acid, for example, exists in solution partly as CH3COOH and partly as CHaCOO"" -f These are known as weak electrolytes. 

The importance of molar conductivity is that it gives us information about the conductivity of ions produced in solution by 1 mol of a substance. Studies of the variation of molar conductivity have revealed important results, such as those shown schematically in Figure 6.2. In all cases, molar conductivity diminishes as the concentration is raised, but two patterns of behavior can be distinguished. The magnitudes of conductivities suggest that there are two extreme classes of electrolytes the strong, which produce many ions, and the weak, which produce few. 

Conduction of electric current in conductors can be electronic or ionic, depending on the type of charges involved. Electronic conduction is found in all metals and also in certain other nonmetals. Ionic conductors are also known as electrolytes. Substances that ordinarily are not conducting can produce ionic conduction after being dissolved in water or another solvent (e.g., electrolyte solution and weak electrolyte ). The relative amount of substance present in a solution or a mixture is known as its concentration. The different concentration units used mostly are molarity, molality, normality, and mole fraction. The acidity or basicity of a solution is measured by a relative measurement called the pH of solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions (H" "). Pure water is said to be neutral. The pH for pure water at 25 °C (77 °F) is close to 7.0. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are said to be basic or alkaline. 

Introduction to conductance of electrolytes. Because a compound or mixture of compounds is electrically neutral, a solution made by dissolving a compound or mixture in any solvent must be neutral also. This means that the total positive charge must equal the total negative charge. This statement is known as the law of electroneutrality for electrolyte solutions. When a substance dissociates into ions in solution and the dissociation is essentially complete, the substance is called a strong electrolyte. Incomplete dissociation is found for weak electrolytes. 

The question as to this is answered by Ostwald s law of dissociation for the weak acids and bases, whose electrolytic dissociation is slight for ordinary concentrations, and which may therefore be spoken of as half-electrolytes. Assuming that the electrolytic conductivity is effected by the free ions present in the solution only, then the conductivity calculated for a fixed mass of dissolved substance is a measure of the fraction of it dissociated into ions, and that is the case too with the so-called molecular conductivity referred to the normal solution. The latter increases with the dilution, on account of increasing dissociation, and reaches the limiting value which corresponds to complete dissociation, so that the fraction —... 

---