TEXT 2

Chemical thermodynamics

The science of thermodynamics concerns the chemical and physical processes, which involve the interconversion of various forms of energy, and it is not confined to the relation between heat and mechanical energy. It is developed mathematically on the basis of a number of postulates, which have been supported by experiments.

Although its application to chemical processes quite general, thermodynamics is not at all concerned with either the rate of a process or the mechanism of it. Thermodynamics is based on two fundamental laws, called the first and second laws of thermodynamics. The two laws of thermodynamics constitute one of the most powerful tools of physical chemistry.

Of fundamental importance to thermodynamics is the concept of equilibrium state. Thermodynamic equilibrium in the true sense refers to a condition in which the properties of a system are absolutely unchanging with time so that, if the system is disturbed slightly in some way, it will return to essentially the same condition after the disturbing force is removed. This latter criterion may differentiate between a true state of equilibrium and a metastable one. If a metastable equilibrium is disturbed, as, for example, by introduction of a catalyst or by local heating, it may spontaneously undergo a drastic change to some new state.

Consider a container filled with chlorine gas. Provided that the container is sealed and thermally insulated from its surroundings a state of true thermodynamic equilibrium will be established in which the temperature and pressure are uniform. If we disturb the system by shining a light on it, some of the chlorine molecules will absorb radiation and dissociate into atoms. When we turn off the light, the chlorine atoms recombine and the system, except for the addition of small amount of energy from the light beam, returns to its original condition.

By way of contrast, a mixture of hydrogen and chlorine is metastable. Although chlorine and hydrogen react with one another at room temperature, the rate is so slow as to be virtually undetectable. Hence the system seems to behave in just the same way as the pure chlorine system, in which uniform temperature and pressure are established. However, if we direct a beam of light through this mixture, it explodes, forming hydrogen chloride and evolving a large amount of heat. After being disturbed in this way, the system can not revert spontaneously to its original condition. In fact, the change, which does occur (the explosion), is a state of true thermodynamic equilibrium.

Although thermodynamics can not deal with the rate at which reactions occur, it does establish the direction in which reaction can proceed. Metastable or unstable compounds can be treated by the methods of thermodynamics, provided that they have a lifetime sufficiently long for thermodynamic measurements to be made. This requirement may vary, depending on the type of experiment, from a minute fraction of a second to hours, or even days. In this regard, one can make the distinction between substances, which exits by virtue of its thermodynamic stability, or by reason only of its slow rate of reaction or decomposition.

There is a wide range in degree of inertness of unstable system. Diamond, on the one hand, is inert to the extent that there is no observable conversion (under ordinary condition) to the stable state of graphite. At the other extreme are such unstable substances as a supercooled liquid, or a sensitive explosive. In either of these cases only a slight perturbation is necessary to change these system drastically. Lewis and Ranall in their classic treatment of thermodynamic found that water and air, although inert, are thermodynamically unstable with respect to the formation of nitric acid.

Those substances, which are quite inert chemically, can generally be treated by the methods of thermodynamics.

As an example, both NO and NO2 are unstable with respect to decomposition into their elements, yet we can not only consider the reaction:

2NO2 =2NO + O2

but also study the equilibrium by a direct experiment.

Exercises

I. Answer the following question:

1. What does the science of thermodynamics deal with?

2. What are the main laws of thermodynamics?

3. What is meant under thermodynamic equilibrium?

4. Thermodynamics establishes the direction, in which a reaction proceeds, doesn't it?

5. What mixtures are considered to be metastable?

6. What conclusions did Lewis and Ranall draw?

II. Translate the following sentences

1. Water and air are thermodynamically unstable with respect to the formation of nitric acid.

2. The preparation of smokes has already been referred to in the scientific journal.

3. The inert dust presumably takes up in virtue of its heat capacity, which would otherwise be available for raising more coal dust to its ignition point.

4. The amino acids are amphoteric, i.e. can act either as acids by virtue of the carboxy, or as bases by virtue of the amino group.

5. With respect to catalytic activity, metals considerably exceed other catalytics for many reactions.

6. Oxide catalysts have provided interesting systems for the study of electronic factors in catalysis.

7. Provided the oxidation directly after chlorination is carried out at sufficiently high pH, little damage is done to the cellulose.

8. Synthetic rubbers possess a marked advantage over natural ones by reason of the diversity of properties they affect.

III. Translate the following derivatives into Russian:

1. Catalysant, catalysate, catalysis, catalyst, catalytic, catalytical, catalysator, catalyzed, catalyzer.

2. Stability, stabilization, stabilizator, stabilize, stabilizer, stabilizing, stable.

IV. Form nouns from the following verb:

behave

combine

convert

differentiate

dissociate

distinguish

disturb

direct

equilibrate establish

explode

fill

occur

perturb

provide

require

surround

IV. Translate the following sentences into Vietnamese paying attention to conditional sentences.

1. If a molecule with a lone pair of electrons is absorbed on a transition metal, it will donate a pair of electrons.

2. A stable of thermodynamic equilibrium will be established, provided that the container is sealed.

3. A mixture would explode, if one directed a beam of light through it.

4. Were gypsum heated to a much higher temperature than 1200 C, it would lose all its water of crystallization.

5. Had we disturbed a system by shining a light on it, some of the chlorine molecules would have absorbed radiation and dissociated into atoms.

6. If the fatty acid does not react chemically with the surface it is relatively ineffective as a lubricant.

7. It would be expected that the heat of adsorption would decrease with an increase of temperature if the thermodynamic state of the surface and adsorbed species remained the same.

V. Translate the following sentences into Vietnamese:

1. Lewis found water and air to be thermodynamically unstable with respect to the formation of nitric acid.

2. Metastable or unstable compounds can be treated by the methods of thermodynamics, provided that they have a lifetime sufficiently long for thermodynamic measurements to be made

3. Scientists consider adsorption from solution to be physical or chemical.

4. It is noteworthy that in the case of phosphate buffers, the increase of pH is accompanied by a drop of interface tension between the two phases.

VII. Translate into English:

1. §Þnh luËt thø nhÊt cña nhiÖt ®éng häc ®­îc xem lµ ®Þnh luËt b¶o toµn n¨ng l­îng.

2. C©n b»ng nhiÖt ®éng cã nghÜa lµ tr¹ng th¸i mµ ë ®ã tÝnh chÊt cña hÖ kh«ng bÞ thay ®æi theo thêi gian.

3. Nh­ ®• biÕt néi n¨ng cña mét chÊt phô thuéc vµo tr¹ng th¸i cña nã nghÜa lµ phô thuéc vµo nhiÖt ®é, ¸p suÊt, d¹ng tinh thÓ.

4. Theo ®Þnh luËt thø hai cña nhiÖt ®éng häc th× c©n b»ng kh«ng phô thuéc vµo c¸ch thøc mµ nã ®¹t tíi.