Nonideal Solutions

Consider a solid substancc dissolving in a liquid solvent. The solid is such that when melted, it is converted to a liquid that, in turn, can form an ideal solution with the solvent. The dissolution of the solid can be pictured as occurring in two hypothetical stages:

solid solute —» liquid solute —» solute in solution.

The second of these steps docs not involve any energy change, for the solution formed is ideal. In contrast, the first step does involve the absorption of energy in the amount per mole of solute. Consequently, while the tendency toward maximum entropy favours the dissolution of the solid, the tendency toward minimum energy favours the solid remaining undissolved. Therefore, the solubility of the solid is limited, and a saturated solution which represents the best compromise between maximizing entropy and minimizing energy is formed. Sincc AHfuJ is related to the strength of attractive forces between solute molecules, wc can deduce that the magnitudes of these same forces determine the solubility of the solid in ideal solutions.

By using some carc, wc can extend our aigumcnts to nonideal solutions. Two liquids which mix with the evolution of heat will be infinitely soluble in each other, for both energy and entropy cffccts favour their mixing. Two liquids which mix with the absorption of heat may have limited solubility in each other, for if the mixing proccss is energetically unfavourable, the tendency toward maximum molccular chaos may or may not be sufficient to allow the liquids to mix in all proportions. Likewise, the solubility of a solid is likely to be small if it enters the solution only with considerable absorption of heat. On the other hand, if the dissolution of the solid is accompanied by evolution of heat, the solubility of the solid may be quite high. Even with these generalizations it is difficult to predict or even rationalize qualitatively the solubilities of substanccs that form markedly nonideal solutions, for the energy and entropy changes that accompany the mixing of strongly irreacting molecules are subtle and difficult to anticipate.

Упр. 3. Передайте основное содержание текста в нескольких пред­ложениях.

Упр. 4. Какое слово в тексте означает «представить себе»?

Упр. 5. Найдите в тексте ответы на следующие вопросы:

I. What hypothetical steps can be pictured when a melted solid is dissolved? 2. At what stage docs the energy change take placc? 3. Why is the solubility of the solid limited? 4. What two liquids will be infinitely soluble in cach other? 5. When will the solubility of the solid be high?


Упр. 6. Закончите следующие предложения:

1. The magnitudes of the attractive forces determine... 2. Two liquids will be infinitely soluble in cach other if they mix with... 3. Two liquids mixing with the absorption of heat may have... 4. It is difficult to prcdict... 5. If the dissolution of the solid is accompanied by evolution of heat...

Scction III

Example:

Ex. 1. Make up short dialogues according to the example.

— Why didn't you come to the seminar yesterday?

— Bccausc I've missed Dr. N's lecture.

— I see. Having missed the lecture, you couldn't take part in the discussion.

Ex. 2. Respond to the following statements:

1. The proccss of dissolving a solid in a liquid is different from the proccss of dissolving a liquid in a liquid. 2. Solubility depends on a number of factors. 3. One can easily prcdict die solubility of a particular solute in a given solvent.

Ex. 3. Give detailed answers to the questions.

1. What is solubility? 2. What docs the solubility of a substancc depend on? 3. What is the difference between an ideal and nonidcal solution?

Ex. 4. Discuss the following topics:

1. Solubility of Liquids.

2. Solubility of Solids.

3. Factors Influencing Solubility.