Phase transition

The degree of dispersal of matter and energy changes when a substance freezes or boils as a result of changes in the order with which the molecules pack together and the extent to which the energy is localized or dispersed. Therefore, we should expect the transition to be accompanied by a change in entropy. For example, when a substance vaporizes, a compact condensed phase changes into a widely dispersed gas and we can expect the entropy of the substance to increase considerably. The entropy of a solid also increases when it melts to a liquid and when that liquid turns into a gas. Consider a system and its surroundings at the normal transition temperature, T_trs , the temperature at which two phases are in equilibrium at 1 atm. This temper ature is 0°C (273 K) for ice in equilibrium with liquid water at 1 atm, and 100°C (373 K) for water in equilibrium with its vapour at 1 atm. At the transition temperature, any transfer of energy as heat between the system and its surroundings is reversible

because the two phases in the system are in equilibrium. Because at constant pressure q =∆trsH, the change in molar entropy of the system is⁴

If the phase transition is exothermic (∆_trsH < 0, as in freezing or condensing), then the entropy change is negative. This decrease in entropy is consistent with localization of matter and energy that accompanies the formation of a solid from a liquid or a liquid from a gas. If the transition is endothermic (∆_trsH > 0, as in melting and vaporization), then the entropy change is positive, which is consistent with dispersal of energy and matter in the system. Table 3.1 lists some experimental entropies of transition. Table 3.2 lists in more detail the standard entropies of vaporization of several liquids at their boiling points. An interesting feature of the data is that a wide range of liquids give approximately the same standard entropy of vaporization (about 85 J K⁻¹ mol−1): this empirical observation is called Trouton’s rule.

اخر الاخبار

اخبار العتبة العباسية المقدسة

انعقاد الجلسة البحثية باللغة الإنكليزية ضمن فعاليات المؤتمر التمهيدي لألفية حوزة النجف الأشرف

مقالة علمية تناقش الأهمية البالغة في معرفة مشايخ الشيخ الطوسي (قدس سره)

دراسة علمية تثبت تاريخ تأليف الشيخ الطوسي لكتاب الفهرست

باحث يقدم دراسة علمية بعنوان (الشيخ الطوسي والكبرى الرجالية تأصيلًا وتطبيقًا)

المزيد

الآخبار الصحية

الشاي والقهوة وتدهور الذاكرة.. دراسة تكشف نتائج جديدة

هل يساعد الزنك على تحسين النوم والطاقة؟

علامات يجب عدم تجاهلها بعد تناول مكملات "فيتامين د"

لتجنب ارتفاع سكر الدم.. ما أفضل وقت لتناول الحلوى؟

المزيد

الآخبار التكنلوجية

الإمارات ترسخ مكانتها كأحد أبرز قادة الذكاء الاصطناعي عالميا !

"أوبن إيه آي" تحذر المشرعين الأميركيين من تحايل "ديب سيك" ؟

تصاعد التوتر بين "أبل" وإدارة ترامب.. ما السبب؟

سلاح طبي في ساحات الحرب.. رذاذ يوقف النزيف خلال ثانية !

المزيد

مواضيع ذات صلة

Units

Names of quantities

Working galvanic cells

Electrolysis

Experimental techniques

The rate laws

The electric potential at the interface

The structure of the interface

Catalytic activity at surfaces

Mechanisms of heterogeneous catalysis

Mobility on surfaces

The rate of desorption