An Elementary Study of Chemistry, William McPherson [icecream ebook reader txt] 📗
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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 the concentration 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 called electrolytes. When the current passes through an electrolyte some chemical change always takes place. This change is called electrolysis.
The general method used in the electrolysis of a solution is illustrated in Fig. 31. The vessel D contains the electrolyte. Two plates or rods, A and B, 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 called electrodes. The electrode connected with the zinc plate of the battery is the negative electrode or cathode, while that connected with the carbon plate is the positive electrode or anode.
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 called the 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, or dissociate, into two or more parts, called ions. 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:
The positive charge on each sodium ion exactly equals the negative charge on each chlorine ion. Sodium sulphate dissociates, as shown in the equation
Here the positive charge on the two sodium ions equals the double negative charge on the SO4 ion.
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 to conform 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.
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
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
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 SO4 ions, when discharged at the anode, act upon water, setting free oxygen and once more forming sulphuric acid:
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 SO4 ions when discharged at the anode decompose the water, as represented in the equation
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 other with 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.
EXERCISES1. 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 strength of 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
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