THEORY OF ELECTROLYTIC DISSOCIATION, ETC. 203 



Heat of neutralization. Heat is produced when an acid and a base neu- 

 tralize each other. If what is stated above is true, then the heat is due to the 

 formation of water molecules, and should be the same for all active acids and 

 bases in dilute solutions. This is exactly what is found by experiment to be 

 true. The heat of neutralization refers to such weights of acids and bases that 

 will furnish enough H* and (OH) X ions to form one molecular weight (17.88 

 gin.) of water, and is nearly 13,600 calories. 



Degree of dissociation. The degree of dissociation of electrolytes varies 

 with the nature of the substance and the concentration of the solution. Usu- 

 ally it is small in concentrated solutions, and increases rapidly with dilution. 

 In 62 per cent, nitric acid only about 9 per cent, of the molecules are ionized, 

 while in the 6.3 per cent, acid about 80 per cent, are ionized. Comparisons 

 for degree of dissociation must be made with solutions of the same relative con- 

 centrations. Usually normal solutions at 18 C. are used, except when the 

 substance is not sufficiently soluble. The following table gives the per cent. 

 of dissociation in normal solutions 1 at 18 C., except when otherwise specified, 

 of some acids, bases, and salts. 



PerCent ' 



Electrolyte. .. Electrolyte. / , 



dissociation. dissociation. 



Nitric acid ........... 82 Sodium phosphate, very dilute . . 83 



Hydrochloric acid ....... 78.4 Ammonium chloride ...... 74 



Sulphuric acid ......... 51 Sodium chloride ........ 67.5 



Hydrofluoric acid ...... .7 Potassium nitrate ........ 64 



Acetic acid ...... ..... 0.4 Potassium acetate ........ 64 



Carbonic acid (^) ....... 0.17 Silver nitrate . . . 



, n ^ Potassium sulphate ....... 53 



Hydrogen sulphide (jg) ..... 0.07 godium acetate ......... 53 



Boric acid (^\ ......... 0.01 Sodium bicarbonate ....... 52 



. , / n \ A A1 Potassium carbonate , ...... 49 



Hydrocyanic acid ( 1?) ) ..... 0.01 



Sodium sulphate ........ 44.5 



Phosphoric acid (~ at 25 C.) . . .17 Zinc sulphate .......... 24 



Potassium hydroxide ...... 77 Zinc chloride .......... 48 



Sodium hydroxide ....... 73 Copper sulphate ........ 22 



Calcium hydroxide, sat. sol. at 25 C. 90 Mercuric chloride, less than ... 1 



Ammonium hydroxide ...... 0.4 Mercuric cyanide ....... minute 



Mathematical formulas have been constructed for calculating the degree 

 of dissociation from the results obtained by four methods of work, namely, 

 freezing-point, boiling-point, osmotic pressure, and conductivity methods. 

 The results of calculation all agree. The simplest method in execution is the 

 conductivity method. 



QUESTIONS. State the theory of electrolytic dissociation or ionization. 

 Upon what experimental facts does it rest? What is an electrolyte? What 

 substances are found to be electrolytes ? What is a cation ; an anion ? Write 

 the ionic equation for the dissociation of sodium chloride in solution. Write 

 the ionic equation for the precipitation of silver chloride from solution. What 

 is electrolysis, and how is it explained? How are acids, bases, and salts defined 

 in terms of the ionic theory ? How is the activity or " strength " of acids and bases 

 related to dissociation ? How is the fact that some acid salts have an acid action 

 on litmus, while others are neutral, accounted for.? What is hydrolysis? 

 1 The concentration of normal solutions is denned in the section on Volumetric Analysis. 



