3 Phase maps The temperatures and pressures at which a given phase of a substance is stable (that is, from which the molecules have the lowest escaping tendency) is an important property of any substance. Because both the temperature and pressure are factors, it is customary to plot the regions of stability of the various phases in P T coordinates, coordinates, as in this generic phase diagram (or phase map) for a hypothetical substance.
Because pressures and temperatures can vary over very wide ranges, it is common practice to d raw phase diagrams with non-linear or distorted coordinates. This enables us to express a lot of information in a compact way and to visualize changes that could not be represented on a linearly-scaled plot. It is important that you be able to interpret a phase map, or alternatively, construct a rough one when given the appropriate data. Take special note of the following points: 1.
The three colored colored regions on the diagram are the ranges of pressure and temperature at which the corresponding phase is the only stable one.
2.
The three lines that bound these regions define all values of (P,T ( P,T ) at which two phases can coexist (i.e., be in equilibrium). Notice that one of these lines is the vapor pressure curve of curve of the liquid as as described above. The "sublimation curve" curve" is just a vapor pressure curve of the solid . The slope of the line depends on the difference in density of the two phases.
3.
In order to depict the important features features of a phase diagram over the very very wide range of pressures and temperatures they encompass, the axes are not usually drawn to scale, and are usually highly distorted. This is the reason that the "melting curve" looks like a straight line in most of these diagrams.
4.
Where the three named curves intersect, all three phases phases can coexist. This condition can only occur at a unique value of (P,T (P,T ) ) known as the triple point . Since all three phases are in equilibrium at the triple point, their vapor pressures will be identical at this temperature.
5.
The line that separates separates the the liquid and vapor regions ends at the critical point . At temperatures and pressures greater than the critical temperature and pressure, no separate liquid phase exists. We refer to this state simply as a fluid , although the term supercritical liquid is is also commonly used. The best way of making sure you understand a phase diagram is to imagine that you are starting at a certain temperature and pressure, and then change just one of these parameters, keeping the other constant. You will be traversing a horizontal or vertical path on the phase map, and there will be a change in state every time your path crosses a line. Of special importance is the h orizontal path (shown by the blue line on the diagram above) corresponding to a pressure of 1 atmosphere; this line defines the normal melting and boiling temperatures of temperatures of a substance.
Phase map of Water
Notice the following features of this very important phase diagram:
The slope of the line 2 separating the solid and liquid regions is negative; this reflects the unusual property that the density of the liquid is greater than that
of the solid, and it means that the melting point of ice decreases as the pressure increases. Thus if ice at 0°C is subjected to a high pressure, it will find itself above its melting point and it will melt. (Contrary to what is sometimes said, however, this is not the reason that ice melts under the pressure of ice skates or skis, providing a lubricating film which makes these modes of transportation so enjoyable. The melting in these cases arises from frictional heating.)
The dashed line 1 is the e xtension of the liquid vapor pressure line b elow the freezing point. This represents the vapor pressure of supercooled water water — a metastable state of water which can temporarily exist down to about – –20°C. 20°C. (If you live in a region subject to "freezing rain", you will have encountered supercooled water!)
3 The triple point (TP ( TP)) of water is just 0.0075° above the freezing point; only at this temperature and pressure can all three phases of water coexist indefinitely. indefinitely.
4 Above the critical point (CP ( CP)) temperature of 374°C, no separate liquid phase of water exists.
Phase map of Carbon dioxide
Dry ice, solid carbon dioxide, is widely used as a refrigerant. refrig erant. The phase diagram shows why it is ―dry‖. The triple point pressure is at 5.11 atm, so below this pressure, liquid CO 2 cannot exist; the solid can only sublime directly to vapor. Gaseous carbon dioxide at a partial pressure of 1 atm is in equilibrium with the solid at 195K (−79 °C, 1); this is the normal sublimation temperature of temperature of carbon dioxide. The surface temperature of dry ice will be slightly less than this, since the partial pressure of CO 2 in contact with the solid will usually be less than 1 atm. Notice also that the critical temperature of CO 2 is only 31°C. This means that on a very warm day, the CO 2 in a fire extinguisher will be entirely vaporized; the vessel must therefore be strong enough to withstand a pressure of 73 atm.
This view of the carbon dioxide phase map employs a logarithmic pressure scale and thus encompasses a much wider range of pressures, revealing the upper boundary of the fluid phase (liquid and supercritical). Supercritical carbon dioxide (CO dioxide (CO2 above its critical temperature) possesses the solvent properties of a liquid and the penetrating properties of a gas; one major use is to remove caffeine from co ff ee ee beans.
Phase diagram of iodine Elemental iodine, I 2, forms dark gray crystals that have an almost metallic appearance. It is often used in chemistry classes as an example of a solid that is easily sublimed; if you have seen such a demonstration or experimented with it in the lab, its phase diagram might be of interest. The most notable feature of iodine's phase behavior is the very small difference (less than a degree) between the temperatures of its triple point 1 and melting point 2. Contrary to the impression many people have, there is nothing really special about iodine's tendency to sublime, which is shared by many molecular crystals including ice and naphthalene ("moth crystals".) The vapor pressure of iodine at room temperature is really quite small — only about 0.3 torr (40 Pa).The fact that solid iodine has a strong odor and is surrounded by a purple vapor in a closed container is mainly a consequence of its strong ability to absorb green light (this leaves blue and red which make purple) and the high sensitivity of our noses to its vapor.
Phase diagram of sulfur Sulfur exhibits a very complicated phase behavior that has puzzled chemists for over a century; what you see here is the greatly simplified phase map shown in most textbooks. The difficulty arises from the tendency of S8 molecules to break up into chains (especially in the liquid above 159°C) or to rearrange into rings of various sizes (S 6 to S20). Even the vapor can contain a mixture of species S2 through S10. The phase diagram of sulfur contains a new feature: there are two solid
phases, rhombic and monoclinic . The names refer to the crystal structures in which the S 8 molecules arrange themselves. This gives rise to three triple points, indicated by the numbers on the diagram. [Question: which three phases can never coexist?] When rhombic sulfur (the stable low-temperature phase) is heated slowly, it changes to the monoclinic form at 114°C, which then melts at 119°. But if the monoclinic form is heated rapidly the molecules do not have time to rearrange themselves, so the rhombic arrangement persists as a metastable phase until it melts at 119-120°. Formation of more than one solid phase is not uncommon — in fact, if one explores into the very high pressures (see below), it seems to be the rule.
4 Phases at the extremes We tend to think of the properties of substances as they exist under the conditions we encounter in everyday life, forgetting that most of the matter that makes up our world is situated inside the Earth, where pressures are orders of magnitude higher. Geochemists and planetary scientists need to know about the phase behavior of substances at high temperatures and pressures in order to develop useful models to test their theories about the structure and evolution of the Earth and of the solar system. What ranges of temperatures and pressures are likely to be of interest — and more importantly, are experimentally accessible?
Kesetimbangan fasa dan diagram fasa Selama ini pembahasan perubahan mutual antara tiga wujud materi difokuskan pada keadaan cair. Dengan kata lain, perhatian telah difokuskan pada perubahan cairan dan padatan, dan antara cairan dan gas. Dalam membahas keadaan kritis zat, akan lebih tepat menangani tiga wujud zat secara simultan, bukan membahas dua dari tiga wujud zat.
Gambar 7.5 Diagram fasa. Tm adalah titik leleh normal air, , T3 dan P3 adalah titik tripel, Tb adalah titik didih normal, Tc adalah temperatur kritis, Pc adalah tekanan kritis.
Diagram fasa merupakan cara mudah untuk menampilkan wujud zat sebagai fungsi suhu dan tekanan. Sebagai contoh khas, diagram fasa air diberikan di Gambar 7.5. Dalam diagram fasa, diasumsikan bahwa zat tersebut diisolasi dengan baik dan tidak ada zat lain yang masuk atau keluar sistem. Pemahaman Anda tentang diagram fasa akan terbantu dengan pemahaman hukum fasa Gibbs, hubungan yang diturunkan oleh fisikawan-matematik Amerika Josiah Josiah Willard Gibbs (1839 -1903) di tahun 1876. Aturan ini menyatakan bahwa untuk kesetimbangan apapun dalam sistem tertutup, jumlah variabel bebas-disebut derajat kebebasan F- yang sama dengan jumlah komponen C ditambah 2 dikurangi jumlah fasa P, yakni, F=C+2-P F=C+2-P … (7.1) Jadi, dalam titik tertentu di diagram fasa, jumlah derajat kebebasan adalah 2 – 2 – yakni yakni suhu dan tekanan; bila dua fasa dalam kesetimbangan-sebagaimana ditunjukkan ditunjukkan dengan garis yang membatasi daerah dua fasa hanya ada satu derajat kebebasan-bisa suhu atau tekanan. Pada ttik tripel ketika terdapat tiga fasa tidak ada derajat kebebasan lagi. Dari diagram fasa, Anda dapat mengkonfirmasi apa yang telah diketahui, dan lebih lanjut, Anda dapat mempelajari apa yang belum diketahui. Misalnya, kemiringan yang negatif pada perbatasan padatan -cairan memiliki implikasi penting sebagaimana dinyatakan di bagian kanan diagram, yakni bila tekanan diberikan pada es, es akan meleleh dan membentuk air. Berdasarkan prinsip Le Chatelier, bila sistem pada kesetimbangan diberi tekanan, kesetimbangan akan bergeser ke arah yang akan mengurangi perubahan ini. Hal ini berarti air memiliki volume yang lebih kecil, kerapatan leb besar daripada es; dan semua kita telah hafal dengan fakta bahwa s mengapung di air.
Sebaliknya, air pada tekanan 0,0060 atm berada sebagai cairan pada suhu rendah, sementara pada suhu 0,0098 °C, tiga wujud air akan ada bersama. Titik ini disebut titik tripel air. Tidak ada titik lain di mana tiga wujud air ada bersama. Selain itu, titik kritis (untuk air, 218 atm, 374°C), yang telah Anda pelajari, juga ditunjukkan dalam diagram fasa. Bila cairan berubah menjadi fasa gas pada titik kritis, muncul keadaan antara (intermediate state), yakni keadaan antara cair dan gas. Dalam diagram fasa keadaan di atas titik kritis tidak didefinisikan.
