The methods for liquefying air have been simplified greatly in that the low temperature required is obtained by allowing a portion of the compressed air to expand. The expansion of a gas is always attended by the absorption of heat. In liquefying air the apparatus is so constructed that the heat absorbed is withdrawn from air already under great pressure. This process is continued until the temperature is lowered to the point of liquefaction.
The methods for liquefying air have been simplified greatly in that the low temperature required is obtained by allowing a portion of the compressed air to expand. The expansion of a gas is always attended by the absorption of heat. In liquefying air the apparatus is so constructed that the heat absorbed is withdrawn from air already under great pressure. This process is continued until the temperature is lowered to the point of liquefaction.
Fig. 29Fig. 29
The Dewar bulb.It is not possible to preserve air in the liquid state in a closed vessel, on account of the enormous pressure exerted by it in its tendency to pass into the gaseous state. It may however be preserved for some hours or even days before it will completely evaporate, by simply placing it in an open vessel surrounded by a nonconducting material. The most efficient vessel for this purpose is theDewar bulbshown in Fig. 29.The air is withdrawn from the space between the two walls, thus making it nonconducting.
Properties and uses of liquid air.When first prepared, liquid air is cloudy because of the presence of particles of solid carbon dioxide. These may be filtered off, leaving a liquid of slightly bluish color. It begins to boil at about -190°, the nitrogen passing off first, gradually followed by the oxygen, the last portions being nearly pure oxygen. To a certain extent oxygen is now prepared in this way for commercial purposes.
The extremely low temperature of liquid air may be inferred from the fact that mercury when cooled by it is frozen to a mass so hard that it may be used for driving nails.
Liquid air is used in the preparation of oxygen and as a cooling agent in the study of the properties of matter at low temperatures. It has thus been found that elements at extremely low temperatures largely lose their chemical activity.
1.When oxygen and nitrogen are mixed in the proportion in which they exist in the atmosphere, heat is neither evolved nor absorbed by the process. What important point does this suggest?
2.What essential constituent of the air is found in larger amount in manufacturing districts than in the open country?
3.Can you suggest any reason why the growth of clover in a field improves the soil?
4.Why are the inner walls of a Dewar bulb sometimes coated with a film of silver?
5.To what is the blue color of liquid air due? Does this color increase in intensity on standing?
6.When ice is placed in a vessel containing liquid air, the latter boils violently. Explain.
7.Taking the volumes of the oxygen and nitrogen in 100 volumes of air as 21 and 78 respectively, calculate the percentages of these elements present by weight.
8.Would combustion be more intense in liquid air than in the gaseous substance?
9.A tube containing calcium chloride was found to weigh 30.1293 g. A volume of air which weighed 15.2134 g. was passed through, after which the weight of the tube was found to be 30.3405 g. What was the percentage amount of moisture present in the air?
10.10 l. of air measured at 20° and 740 mm. passed through lime water caused the precipitation of 0.0102 g. of CaCO3. Find the number of volumes of carbon dioxide in 10,000 volumes of the air.
Definitions.When a substance disappears in a liquid in such a way as to thoroughly mix with it and to be lost to sight as an individual body, the resulting liquid is called asolution. The liquid in which the substance dissolves is called thesolvent, while the dissolved substance is called thesolute.
Classes of solutions.Matter in any one of its physical states may dissolve in a liquid, so that we may have solutions of gases, of liquids, and of solids. Solutions of liquids in liquids are not often mentioned in the following pages, but the other two classes will become very familiar in the course of our study, and deserve special attention.
Fig. 30Fig. 30
It has already been stated that oxygen, hydrogen, and nitrogen are slightly soluble in water. Accurate study has led to the conclusion that all gases are soluble to some extent not only in water but in many other liquids. The amount of a gas which will dissolve in a liquid depends upon a number of conditions, and these can best be understood bysupposing a vesselB(Fig. 30), to be filled with the gas and inverted over the liquid. Under these circumstances the gas cannot escape or become mixed with another gas.
Circumstances affecting the solubility of gases.A number of circumstances affect the solubility of a gas in a liquid.
1.Nature of the gas.Other conditions being equal, each gas has its own peculiar solubility, just as it has its own special taste or odor. The solubility of gases varies between wide limits, as will be seen from the following table, but as a rule a given volume of a liquid will not dissolve more than two or three times its own volume of a gas.
Ammonia1148.00 l.Hydrochloric acid503.00Sulphur dioxide79.79Carbon dioxide1.80Oxygen41.14 cc.Hydrogen21.15Nitrogen20.03
In the case of very soluble gases, such as the first three in the table, it is probable that chemical combination between the liquid and the gas takes place.
2.Nature of the liquid.The character of the liquid has much influence upon the solubility of a gas. Water, alcohol, and ether have each its own peculiar solvent power. From the solubility of a gas in water, no prediction can be made as to its solubility in other liquids.
3.Influence of pressure.It has been found that the weight of gas which dissolves in a given case is proportional to the pressure exerted upon the gas. If thepressure is doubled, the weight of gas going into solution is doubled; if the pressure is diminished to one half of its original value, half of the dissolved gas will escape. Under high pressure, large quantities of gas can be dissolved in a liquid, and when the pressure is removed the gas escapes, causing the liquid to foam oreffervesce.
4.Influence of temperature.In general, the lower the temperature of the liquid, the larger the quantity of gas which it can dissolve. 1000 volumes of water at 0° will dissolve 41.14 volumes of oxygen; at 50°, 18.37 volumes; at 100° none at all. While most gases can be expelled from a liquid by boiling the solution, some cannot. For example, it is not possible to expel hydrochloric acid gas completely from its solution by boiling.
This is the most familiar class of solutions, since in the laboratory substances are much more frequently used in the form of solutions than in the solid state.
Circumstances affecting the solubility of a solid.The solubility of a solid in a liquid depends upon several factors.
1.Nature of the solid.Other conditions being the same, solids vary greatly in their solubility in liquids. This is illustrated in the following table:
100 cc. of water will dissolve:Calcium chloride71.0 g.Sodium chloride35.9Potassium nitrate29.1Copper sulphate21.4Calcium sulphate0.207
No solids are absolutely insoluble, but the amount dissolved may be so small as to be of no significance for most purposes. Thus barium sulphate, one of the most insoluble of common substances, dissolves in water to the extent of 1 part in 400,000.
2.Nature of the solvent.Liquids vary much in their power to dissolve solids. Some are said to be good solvents, since they dissolve a great variety of substances and considerable quantities of them. Others have small solvent power, dissolving few substances, and those to a slight extent only. Broadly speaking, water is the most general solvent, and alcohol is perhaps second in solvent power.
3.Temperature.The weight of a solid which a given liquid can dissolve varies with the temperature. Usually it increases rapidly as the temperature rises, so that the boiling liquid dissolves several times the weight which the cold liquid will dissolve. In some instances, as in the case of common salt dissolved in water, the temperature has little influence upon the solubility, and a few solids are more soluble in cold water than in hot. The following examples will serve as illustrations:
At 0°At 100°Calcium chloride49.6 g.155.0 g.Sodium chloride35.739.8Potassium nitrate13.3247.0Copper sulphate15.573.5Calcium sulphate0.2050.217Calcium hydroxide0.1730.079
Saturated solutions.A liquid will not dissolve an unlimited quantity of a solid. On adding the solid to the liquid in small portions ata time, it will be found that a point is reached at which the liquid will not dissolve more of the solid at that temperature. The solid and the solution remain in contact with each other unchanged. This condition may be described by saying that they are in equilibrium with each other. A solution is said to besaturatedwhen it remains unchanged in concentration in contact with some of the solid. The weight of the solid which will completely saturate a definite volume of a liquid at a given temperature is called thesolubilityof the substance at that temperature.
Supersaturated solutions.When a solution, saturated at a given temperature, is allowed to cool it sometimes happens that no solid crystallizes out. This is very likely to occur when the vessel used is perfectly smooth and the solution is not disturbed in any way. Such a solution is said to besupersaturated. That this condition is unstable can be shown by adding a crystal of the solid to the solution. All of the solid in excess of the quantity required to saturate the solution at this temperature will at once crystallize out, leaving the solution saturated. Supersaturation may also be overcome in many cases by vigorously shaking or stirring the solution.
General physical properties of solutions.A few general statements may be made in reference to the physical properties of solutions.
1.Distribution of the solid in the liquid.A solid, when dissolved, tends to distribute itself uniformly through the liquid, so that every part of the solution has the same concentration. The process goes on very slowly unless hastened by stirring or shaking the solution. Thus, if a few crystals of a highly colored substance such as copper sulphate are placed in the bottom of a tall vessel full of water, it will take weeks for the solution to become uniformly colored.
2.Boiling points of solutions.The boiling point of a liquid is raised by the presence of a substance dissolved in it. In general the extent to which the boiling point of a solvent is raised by a given substance is proportional to theconcentration of the solution, that is, to the weight of the substance dissolved in a definite weight of the solvent.
3.Freezing points of solutions.A solution freezes at a lower temperature than the pure solvent. The lowering of the freezing point obeys the same law which holds for the raising of the boiling point: the extent of lowering is proportional to the weight of dissolved substance, that is, to the concentration of the solution.
Electrolysis of solutions.Pure water does not appreciably conduct the electric current. If, however, certain substances such as common salt are dissolved in the water, the resulting solutions are found to be conductors of electricity. Such solutions are calledelectrolytes. When the current passes through an electrolyte some chemical change always takes place. This change is calledelectrolysis.
Fig. 31Fig. 31
The general method used in the electrolysis of a solution is illustrated in Fig. 31. The vesselDcontains the electrolyte. Two plates or rods,AandB, made of suitable material, are connected with the wires from a battery (or dynamo) and dipped into the electrolyte, as shown in the figure. These plates or rods are calledelectrodes. The electrode connected with the zinc plate of the battery is the negative electrode orcathode, while that connected with the carbon plate is the positive electrode oranode.
Theory of electrolytic dissociation.The facts which have just been described in connection with solutions, together with many others, have led chemists to adopt a theory of solutions calledthe theory of electrolytic dissociation. The main assumptions in this theory are the following.
1.Formation of ions.Many compounds when dissolved in water undergo an important change. A portion of their molecules fall apart, ordissociate, into two or more parts, calledions. Thus sodium nitrate (NaNO3) dissociates into the ions Na and NO3; sodium chloride, into the ions Na and Cl. These ions are free to move about in the solution independently of each other like independent molecules, and for this reason were given the name ion, which signifies a wanderer.
2.The electrical charge of ions.Each ion carries a heavy electrical charge, and in this respect differs from an atom or molecule. It is evident that the sodium in the form of an ion must differ in some important way from ordinary sodium, for sodium ions, formed from sodium nitrate, give no visible evidence of their presence in water, whereas metallic sodium at once decomposes the water. The electrical charge, therefore, greatly modifies the usual chemical properties of the element.
3.The positive charges equal the negative charges.The ions formed by the dissociation of any molecule are of two kinds. One kind is charged with positive electricity and the other with negative electricity; moreover the sum of all the positive charges is always equal to the sum of all the negative charges. The solution as a whole is therefore electrically neutral. If we represent dissociation by the usual chemical equations, with the electrical charges indicated by + and - signs following the symbols, the dissociation of sodium chloride molecules is represented thus:
NaCl --> Na+, Cl-.
NaCl --> Na+, Cl-.
The positive charge on each sodium ion exactly equals the negative charge on each chlorine ion.Sodium sulphate dissociates, as shown in the equation
Na2SO4--> 2Na+, SO4-.
Na2SO4--> 2Na+, SO4-.
Here the positive charge on the two sodium ions equals the double negative charge on the SO4ion.
4.Not all compounds dissociate.Only those compounds dissociate whose solutions form electrolytes. Thus salt dissociates when dissolved in water, the resulting solution being an electrolyte. Sugar, on the other hand, does not dissociate and its solution is not a conductor of the electric current.
5.Extent of dissociation differs in different liquids.While compounds most readily undergo dissociation in water, yet dissociation often occurs to a limited extent when solution takes place in liquids other than water. In the discussion of solutions it will be understood that the solvent is water unless otherwise noted.
The theory of electrolytic dissociation and the properties of solutions.In order to be of value, this theory must give a reasonable explanation of the properties of solutions. Let us now see if the theory is in harmony with certain of these properties.
The theory of electrolytic dissociation and the boiling and freezing points of solutions.We have seen that the boiling point of a solution of a substance is raised in proportion to the concentration of the dissolved substance. This is but another way of saying that the change in the boiling point of the solution is proportional to the number of molecules of the dissolved substance present in the solution.
It has been found, however, that in the case of electrolytes the boiling point is raised more than it should be toconform to this law. If the solute dissociates into ions, the reason for this becomes clear. Each ion has the same effect on the boiling point as a molecule, and since their number is greater than the number of molecules from which they were formed, the effect on the boiling point is abnormally great.
In a similar way, the theory furnishes an explanation of the abnormal lowering of the freezing point of electrolytes.
The theory of electrolytic dissociation and electrolysis.The changes taking place during electrolysis harmonize very completely with the theory of dissociation. This will become clear from a study of the following examples.
Fig. 32Fig. 32
1.Electrolysis of sodium chloride.Fig. 32 represents a vessel in which the electrolyte is a solution of sodium chloride (NaCl). According to the dissociation theory the molecules of sodium chloride dissociate into the ions Na+and Cl-. The Na+ions are attracted to the cathode owing to its large negative charge. On coming into contact with the cathode, the Na+ions give up their positive charge and are then ordinary sodium atoms. They immediately decompose the water according to the equation
Na + H2O = NaOH + H,
Na + H2O = NaOH + H,
and hydrogen is evolved about the cathode.
The chlorine ions on being discharged at the anode in similar manner may either be given off as chlorine gas, or may attack the water, as represented in the equation
2Cl + H2O = 2HCl + O.
2Cl + H2O = 2HCl + O.
2.Electrolysis of water.The reason for the addition of sulphuric acid to water in the preparation of oxygen and hydrogen by electrolysis will now be clear. Water itself is not an electrolyte to an appreciable extent; that is, it does not form enough ions to carry a current. Sulphuric acid dissolved in water is an electrolyte, and dissociates into the ions 2 H+and SO4—. In the process of electrolysis of the solution, the hydrogen ions travel to the cathode, and on being discharged escape as hydrogen gas. The SO4ions, when discharged at the anode, act upon water, setting free oxygen and once more forming sulphuric acid:
SO4+ H2O = H2SO4+ O.
SO4+ H2O = H2SO4+ O.
The sulphuric acid can again dissociate and the process repeat itself as long as any water is left. Hence the hydrogen and oxygen set free in the electrolysis of water really come directly from the acid but indirectly from the water.
3.Electrolysis of sodium sulphate.In a similar way, sodium sulphate (Na2SO4), when in solution, gives the ions 2 Na+and SO4—. On being discharged, the sodium atoms decompose water about the cathode, as in the case of sodium chloride, while the SO4ions when discharged at the anode decompose the water, as represented in the equation
SO4+ H2O = H2SO4+ O
SO4+ H2O = H2SO4+ O
Fig. 33Fig. 33
That new substances are formed at the cathode and anode may be shown in the following way. A U-tube, such as is represented in Fig. 33, is partially filled with a solution of sodium sulphate, and the liquid in one arm is colored with red litmus, that in the otherwith blue litmus. An electrode placed in the red solution is made to serve as cathode, while one in the blue solution is made the anode. On allowing the current to pass, the blue solution turns red, while the red solution turns blue. These are exactly the changes which would take place if sodium hydroxide and sulphuric acid were to be set free at the electrodes, as required by the theory.
The properties of electrolytes depend upon the ions present.When a substance capable of dissociating into ions is dissolved in water, the properties of the solution will depend upon two factors: (1) the ions formed from the substance; (2) the undissociated molecules. Since the ions are usually more active chemically than the molecules, most of the chemical properties of an electrolyte are due to the ions rather than to the molecules.
The solutions of any two substances which give the same ion will have certain properties in common. Thus all solutions containing the copper ion (Cu++) are blue, unless the color is modified by the presence of ions or molecules having some other color.
1.Distinguish clearly between the following terms: electrolysis, electrolyte, electrolytic dissociation, ions, solute, solvent, solution, saturated solution, and supersaturated solution.
2.Why does the water from some natural springs effervesce?
3.(a) Why does not the water of the ocean freeze? (b) Why will ice and salt produce a lower temperature than ice alone?
4.Why does shaking or stirring make a solid dissolve more rapidly in a liquid?
5.By experiment it was found that a certain volume of water was saturated at 100° with 114 g. of potassium nitrate. On cooling to 0° a portion of the substance crystallized. (a) How many grams of the substance remained in solution? (b) What was the strengthof the solution at 18°? (c) How much water had been used in the experiment?
6.(a) 10 g. of common salt were dissolved in water and the solution evaporated to dryness; what weight of solid was left? (b) 10 g. of zinc were dissolved in hydrochloric acid and the solution evaporated to dryness; what weight of solid was left?
7.Account for the fact that sugar sometimes deposits from molasses, even when no evaporation has taken place.
8.(a) From the standpoint of the theory of electrolytic dissociation, write the simple equation for a dilute solution of copper sulphate (CuSO4); this solution is blue. (b) In the same manner, write one for sodium sulphate; this solution is colorless. (c) How would you account for the color of the copper sulphate solution?
9.(a) As in the preceding exercise, write a simple equation for a dilute solution of copper chloride (CuCl2); this solution is blue. (b) In the same manner, write one for sodium chloride; this solution is colorless. To what is the blue color due?
10.What component is present in concentrated sulphuric acid that is almost wanting in very dilute sulphuric acid?
11.Why will vegetables cook faster when boiled in strong salt water than when boiled in pure water?
12.How do you explain the foaming of soda water?
Acids, bases, and salts.The three classes of compounds known respectively as acids, bases, and salts include the great majority of the compounds with which we shall have to deal. It is important, therefore, for us to consider each of these classes in a systematic way. The individual members belonging to each class will be discussed in detail in the appropriate places, but a few representatives of each class will be described in this chapter with special reference to the common properties in accordance with which they are classified.
The familiar acids.Hydrochloric acidis a gas composed of hydrogen and chlorine, and has the formula HCl. The substance is very soluble in water, and it is this solution which is usually called hydrochloric acid.Nitric acidis a liquid composed of hydrogen, nitrogen, and oxygen, having the formula HNO3. As sold commercially it is mixed with about 32% of water.Sulphuric acid, whose composition is represented by the formula H2SO4, is an oily liquid nearly twice as heavy as water, and is commonly calledoil of vitriol.
Characteristics of acids.(1) All acids contain hydrogen. (2) When dissolved in water the molecules of the acid dissociate into two kinds of ions. One of these is always hydrogen and is the cation (+), while the other consists of the remainder of the molecule and is the anion (-). (3) The solution tastes sour. (4) It has the power to change thecolor of certain substances calledindicators. Thus blue litmus is changed to red, and yellow methyl orange is changed to red. Since all acids produce hydrogen cations, while the anions of each are different, the properties which all acids have in common when in solution, such as taste and action on indicators, must be attributed to the hydrogen ions.
DEFINITION:An acid is a substance which produces hydrogen ions when dissolved in water or other dissociating liquids.
Undissociated acids.When acids are perfectly free from water, or are dissolved in liquids like benzene which do not have the power of dissociating them into ions, they should have no real acid properties. This is found to be the case. Under these circumstances they do not affect the color of indicators or have any of the properties characteristic of acids.
The familiar bases. The bases most used in the laboratory are sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2). These are white solids, soluble in water, the latter sparingly so. Some bases are very difficultly soluble in water. The very soluble ones with most pronounced basic properties are sometimes called thealkalis.
Characteristics of bases.(1) All bases contain hydrogen and oxygen. (2) When dissolved in water the molecules of the base dissociate into two kinds of ions. One of these is always composed of oxygen and hydrogen and is the anion. It has the formula OH and is called thehydroxyl ion. The remainder of the molecule, which usually consists of a single atom, is the cation. (3) The solution of a base hasa soapy feel and a brackish taste. (4) It reverses the color change produced in indicators by acids, turning red litmus blue, and red methyl orange yellow. Since all bases produce hydroxyl anions, while the cations of each are different, the properties which all bases have in common when in solution must be due to the hydroxyl ions.
DEFINITION:A base is a substance which produces hydroxyl ions when dissolved in water or other dissociating liquids.
Undissociated bases.Bases, in the absence of water or when dissolved in liquids which do not dissociate them, should have none of the properties characteristic of this class of substances. This has been found to be the case. For example, they have no effect upon indicators under these circumstances.
Neutralization.When an acid and a base are brought together in solution in proper proportion, the characteristic properties of each disappear. The solution tastes neither sour nor brackish; it has no effect upon indicators. There can therefore be neither hydrogen nor hydroxyl ions present in the solution. A study of reactions of this kind has shown that the hydrogen ions of the acid combine with the hydroxyl ions of the base to form molecules of water, water being a substance which is not appreciably dissociated into ions. This action of an acid on a base is calledneutralization. The following equations express the neutralization of the three acids by three bases, water being formed in each case.
Na+, OH-+ H+, Cl-= Na+, Cl-+ H2O.K+, OH-+ H+, NO3-= K+, NO3-+ H2O.Ca++, (OH)2—+ H2++, SO4-= Ca++, SO4—+ 2H2O.
Na+, OH-+ H+, Cl-= Na+, Cl-+ H2O.
K+, OH-+ H+, NO3-= K+, NO3-+ H2O.
Ca++, (OH)2—+ H2++, SO4-= Ca++, SO4—+ 2H2O.
DEFINITION:Neutralization consists in the union of the hydrogen ion of an acid with the hydroxyl ion of a base to form water.
Salts.It will be noticed that in neutralization the anion of the acid and the cation of the base are not changed. If, however, the water is expelled by evaporation, these two ions slowly unite, and when the water becomes saturated with the substance so produced, it separates in the form of a solid called asalt.
DEFINITION:A salt is a substance formed by the union of the anion of an acid with the cation of a base.
Characteristics of salts.(1) From the definition of a salt it will be seen that there is no element or group of elements which characterize salts. (2) Salts as a class have no peculiar taste. (3) In the absence of all other substances they are without action on indicators. (4) When dissolved in water they form two kinds of ions.
Heat of neutralization.If neutralization is due to the union of hydrogen ions with hydroxyl ions, and nothing more, it follows that when a given weight of water is formed in neutralization, the heat set free should always be the same, no matter from what acid and base the two kinds of ions have been supplied. Careful experiments have shown that this is the case, provided no other reactions take place at the same time. When 18g. of water are formed in neutralization, 13,700 cal. of heat are set free. This is represented in the equations
Heat of neutralization.If neutralization is due to the union of hydrogen ions with hydroxyl ions, and nothing more, it follows that when a given weight of water is formed in neutralization, the heat set free should always be the same, no matter from what acid and base the two kinds of ions have been supplied. Careful experiments have shown that this is the case, provided no other reactions take place at the same time. When 18g. of water are formed in neutralization, 13,700 cal. of heat are set free. This is represented in the equations
Na+, OH-+ H+, Cl-= Na+, Cl-+ H2O + 13,700 cal.K+, OH-+ H+, NO3-= K+, NO3-+ H2O + 13,700 cal.Ca++, (OH)2-+ H2++, SO4-= Ca++, SO4-+ 2H2O + 2 × 13,700 cal.
Na+, OH-+ H+, Cl-= Na+, Cl-+ H2O + 13,700 cal.
K+, OH-+ H+, NO3-= K+, NO3-+ H2O + 13,700 cal.
Ca++, (OH)2-+ H2++, SO4-= Ca++, SO4-+ 2H2O + 2 × 13,700 cal.
Neutralization a quantitative act.Since neutralization is a definite chemical act, each acid will require a perfectly definite weight of each base for its neutralization. Forexample, a given weight of sulphuric acid will always require a definite weight of sodium hydroxide, in accordance with the equation
H2, SO4+ 2Na, OH = Na2, SO4+ 2H2O.
H2, SO4+ 2Na, OH = Na2, SO4+ 2H2O.
Determination of the ratio in neutralization.The quantities of acid and base required in neutralization may be determined in the following way. Dilute solutions of the two substances are prepared, the sulphuric acid being placed in one of the burettes (Fig. 34) and the sodium hydroxide in the other. The levels of the two liquids are then brought to the zero marks of the burettes by means of the stopcocks. A measured volume of the acid is drawn off into a beaker, a few drops of litmus solution added, and the sodium hydroxide is run in drop by drop until the red litmus just turns blue. The volume of the sodium hydroxide consumed is then noted. If the concentrations of the two solutions are known, it is easy to calculate what weight of sodium hydroxide is required to neutralize a given weight of sulphuric acid. By evaporating the neutralized solution to dryness, the weight of the sodium sulphate formed can be determined directly. Experiment shows that the weights are always in accordance with the equation in the preceding paragraph.
Determination of the ratio in neutralization.The quantities of acid and base required in neutralization may be determined in the following way. Dilute solutions of the two substances are prepared, the sulphuric acid being placed in one of the burettes (Fig. 34) and the sodium hydroxide in the other. The levels of the two liquids are then brought to the zero marks of the burettes by means of the stopcocks. A measured volume of the acid is drawn off into a beaker, a few drops of litmus solution added, and the sodium hydroxide is run in drop by drop until the red litmus just turns blue. The volume of the sodium hydroxide consumed is then noted. If the concentrations of the two solutions are known, it is easy to calculate what weight of sodium hydroxide is required to neutralize a given weight of sulphuric acid. By evaporating the neutralized solution to dryness, the weight of the sodium sulphate formed can be determined directly. Experiment shows that the weights are always in accordance with the equation in the preceding paragraph.
Fig. 34Fig. 34
Extent of dissociation.The question will naturally arise, When an acid, base, or salt dissolves in water, do all the molecules dissociate into ions, or only a part of them? The experiments by which this question can be answered cannot be described here. It has been found, however, that only a fraction of the molecules dissociate. The percentage which will dissociate in a given case depends upon several conditions, the chief of which are: (1) The concentration of the solution. In concentrated solutions only a very smallpercentage of dissociation occurs. As the solution is diluted the percentage increases, and in very dilute solutions it may be very large, though it is never complete in any ordinary solution. (2) The nature of the dissolved compound. At equal concentrations substances differ much among themselves in the percentage of dissociation. The great majority of salts are about equally dissociated. Acids and bases, on the contrary, show great differences. Some are freely dissociated, while others are dissociated to but a slight extent.
Strength of acids and bases.Since acid and basic properties are due to hydrogen and hydroxyl ions respectively, the acid or base which will produce the greatest percentage of these ions at a given concentration must be regarded as the strongest representative of its class. The acids and bases described in the foregoing paragraphs are all quite strong. In 10% solutions they are dissociated to about 50%, and this is also approximately the extent to which most salts are dissociated at this same concentration.
Partial neutralization.1.Basic salts.The chemical action between an acid and a base is not always as complete as has been represented in the foregoing paragraphs. For example, if the base magnesium hydroxide (Mg(OH)2) and hydrochloric acid (HCl) are brought together in the ratio of an equal number of molecules of each, there will be only half enough hydrogen ions for the hydroxyl ions present.Mg, (OH)2+ H, Cl = Mg, OH, Cl + H2O.Magnesium, hydroxyl, and chlorine ions are left at the close of the reaction, and under the proper conditions unite to form molecules of the compound Mg(OH)Cl. This compound, when dissolved, can form hydroxyl ions and therefore possesses basic properties; it can also form the ions of a salt (Mg and Cl), and has properties characteristic of salts. Substances of this kind are calledbasic salts.DEFINITION:A basic salt is a substance which can give the ions both of a base and of a salt when dissolved in water.2.Acid salts.In a similar way, when sulphuric acid and sodium hydroxide are brought together in the ratio of equal numbers of the molecules of each, it is possible to have a reaction expressed by the equationNa, OH + H2, SO4= Na, H, SO4+ H2O.The ions remaining after all the hydroxyl ions have been used up are those of an acid (H) and those of a salt (Na and SO4). These unite to form the substance NaHSO4, and as the solution becomes saturated with this substance through evaporation, it separates in the form of crystals. In solution this substance can give hydrogen ions, and therefore possesses acid properties; it can also give the ions characteristic of a salt. It is therefore called anacid salt.DEFINITION:An acid salt is one which can give the ions of an acid and of a salt when in solution.3.Normal salts.Salts which are the products of complete neutralization, such as Na2SO4, and which in solution can give neither hydrogen nor hydroxyl ions, but only the ions of a salt, are callednormal saltsto distinguish them from acid and basic salts.
Partial neutralization.1.Basic salts.The chemical action between an acid and a base is not always as complete as has been represented in the foregoing paragraphs. For example, if the base magnesium hydroxide (Mg(OH)2) and hydrochloric acid (HCl) are brought together in the ratio of an equal number of molecules of each, there will be only half enough hydrogen ions for the hydroxyl ions present.
Mg, (OH)2+ H, Cl = Mg, OH, Cl + H2O.
Magnesium, hydroxyl, and chlorine ions are left at the close of the reaction, and under the proper conditions unite to form molecules of the compound Mg(OH)Cl. This compound, when dissolved, can form hydroxyl ions and therefore possesses basic properties; it can also form the ions of a salt (Mg and Cl), and has properties characteristic of salts. Substances of this kind are calledbasic salts.
DEFINITION:A basic salt is a substance which can give the ions both of a base and of a salt when dissolved in water.
2.Acid salts.In a similar way, when sulphuric acid and sodium hydroxide are brought together in the ratio of equal numbers of the molecules of each, it is possible to have a reaction expressed by the equation
Na, OH + H2, SO4= Na, H, SO4+ H2O.
Na, OH + H2, SO4= Na, H, SO4+ H2O.
The ions remaining after all the hydroxyl ions have been used up are those of an acid (H) and those of a salt (Na and SO4). These unite to form the substance NaHSO4, and as the solution becomes saturated with this substance through evaporation, it separates in the form of crystals. In solution this substance can give hydrogen ions, and therefore possesses acid properties; it can also give the ions characteristic of a salt. It is therefore called anacid salt.
DEFINITION:An acid salt is one which can give the ions of an acid and of a salt when in solution.
3.Normal salts.Salts which are the products of complete neutralization, such as Na2SO4, and which in solution can give neither hydrogen nor hydroxyl ions, but only the ions of a salt, are callednormal saltsto distinguish them from acid and basic salts.
Methods of expressing reactions between compounds in solution.Chemical equations representing reactions between substances in solution may represent the details of the reaction, or they may simply indicate the final products formed. In the latter case the formation of ions is not indicated. Thus, if we wish to call attention to the details of the reaction between sodium hydroxide and hydrochloric acid in solution, the equation is written as follows:
Na+, OH-+ H+, Cl-= Na+, Cl-+ H2O.
Na+, OH-+ H+, Cl-= Na+, Cl-+ H2O.
On the other hand, if we wish simply to represent the final products formed, the following is used.
NaOH + HCl = NaCl + H2O.
NaOH + HCl = NaCl + H2O.
Both of these methods will therefore be used:
Radicals.It has been emphasized that the hydroxyl group (OH) alwaysforms the anion of a base, while the group NO3forms the anion of nitric acid and sodium nitrate; the group SO4, the anion of sulphuric acid and calcium sulphate. A group of elements which in this way constitutes a part of a molecule, acting as a unit in a chemical change, or forming ions in solution, is called aradical. Some of these radicals have been given special names, the names signifying the elements present in the radical. Thus we have the hydroxyl radical (OH) and the nitrate radical (NO3).
DEFINITION:A radical is a group of elements forming part of a molecule, and acting as a unit in chemical reactions.
Names of acids, bases, and salts.Since acids, bases, and salts are so intimately related to each other, it is very advantageous to give names to the three classes in accordance with some fixed system. The system universally adopted is as follows:
Naming of bases.All bases are calledhydroxides. They are distinguished from each other by prefixing the name of the element which is in combination with the hydroxyl group. Examples: sodium hydroxide (NaOH); calcium hydroxide (Ca(OH)2); copper hydroxide (Cu(OH)2).
Naming of acids.The method of naming acids depends upon whether the acid consists of two elements or three.
1.Binary acids.Acids containing only one element in addition to hydrogen are calledbinary acids. They are given names consisting of the prefixhydro-, the name of the second element present, and the termination-ic. Examples: hydrochloric acid (HCl); hydrosulphuric acid (H2S).
2.Ternary acids.In addition to the two elements present in binary acids, the great majority of acids also contain oxygen. They therefore consist of three elements andare calledternary acids. It usually happens that the same three elements can unite in different proportions to make several different acids. The most familiar one of these is given a name ending in the suffix-ic, while the one with less oxygen is given a similar name, but ending in the suffix-ous. Examples: nitric acid (HNO3); nitrous acid (HNO2). In cases where more than two acids are known, use is made of prefixes in addition to the two suffixes-icand-ous. Thus the prefixper-signifies an acid still richer in oxygen; the prefixhypo-signifies one with less oxygen.
Naming of salts.A salt derived from a binary acid is given a name consisting of the names of the two elements composing it, with the termination-ide. Example: sodium chloride (NaCl). All other binary compounds are named in the same way.
A salt of a ternary acid is named in accordance with the acid from which it is derived. A ternary acid with the termination-icgives a salt with the name ending in-ate, while an acid with termination-ousgives a salt with the name ending in-ite. The following table will make the application of these principles clear: