Name: boiling point and vapor pressure
Uploaded: 2020-03-26T07:22:27.000Z
Duration: 4 min 25 s
Channel: JoVE

K12 Standards

Georgia Standards of Excellence

9-12

Physical Science

SPS2.

Physical Properties

Boiling Points

يتم تثبيت السوائل معا عن طريق التفاعلات الضعيفة بين جزيئاتها. للانتقال إلى الطور الغازي ، يجب أن يكتسب الجزيء طاقة كافية من الحرارة أو مصادر أخرى للتغلب على هذه التفاعلات. 
دعونا نفكر في حاوية مغلقة من سائل نقي في درجة حرارة الغرفة. تكتسب بعض جزيئات الطور السائل على السطح طاقة كافية للتبخر ، وبعض هذا البخار يفقد الطاقة ويتكثف مرة أخرى إلى سائل. عندما تكون معدلات التبخر والتكثيف متماثلة ، يصل السائل والبخار إلى التوازن. في هذه المرحلة ، تكون المساحة الموجودة فوق السائل مشبعة بالبخار ، ولا يوجد تغيير صاف في كمية السائل والبخار في الحاوية. يسمى الضغط الذي يمارسه البخار بضغط البخار. 
الآن ، ضع في اعتبارك حاوية مفتوحة من السائل. هنا يمكن للبخار الهروب ، مما يحافظ على المنطقة الموجودة فوق السائل من التشبع. وبالتالي ، يمكن أن يستمر التبخر حتى لا يتبقى سائل. مع زيادة درجة حرارة السائل ، يزداد معدل التبخر ، وبالتالي يزداد ضغط البخار. إذا تمت إضافة حرارة كافية ، تبدأ الجزيئات العميقة في السائل في التبخر ، والتي نراها على شكل فقاعات بخار تتشكل. 
وهذا ما يسمى الغليان ، ويبدأ عندما يكون ضغط بخار المركب مساويا للضغط الجوي. درجة الحرارة التي يحدث عندها هذا هي نقطة الغليان. نظرا لأن كل مادة نقية لها أنواع وقوة مختلفة من التفاعلات بين الجزيئات ، فإن المواد المختلفة لها نقاط غليان مختلفة. 
الآن ، دعونا نلقي نظرة على كيفية تأثير التفاعلات بين الجزيئات على درجة الغليان. تتضمن العديد من التفاعلات بين الجزيئات مناطق ذات كثافة إلكترون غير متساوية تسمى ثنائيات الأقطاب. جميع الجزيئات لها تقلبات قصيرة في توزيع الإلكترون ، لذلك حتى الجزيئات غير القطبية يمكن أن تحتوي مؤقتا على ثنائيات الأقطاب. ردا على ذلك ، تتشكل ثنائيات الأقطاب المتعارضة في الجزيئات القريبة. 
تسمى قوى الجذب بين ثنائيات الأقطاب المؤقتة والمستحثة قوى تشتت لندن ، وهي التفاعلات السائدة بين الجزيئات غير القطبية مثل الهيدروكربونات. تكون قوى تشتت لندن أقوى بين الجزيئات الكبيرة التي تحتوي على العديد من الإلكترونات لأن هذه الجزيئات تحتفظ بإلكتروناتها بشكل ضعيف ، مما يسهل على الجزيئات المجاورة التأثير على توزيع الإلكترونات. 
على سبيل المثال ، يحتوي الألكان غير المتفرع ذو الوزن الجزيئي العالي على نقطة غليان أعلى من الألكان غير المتفرع ذو الوزن الجزيئي المنخفض. بالإضافة إلى ذلك ، فإن الألكان غير المتفرع له نقطة غليان أعلى من الألكان المتفرع بوزن جزيئي مماثل بسبب مساحة سطحه الأعلى. 
تحتوي الجزيئات ذات الروابط القطبية على ثنائيات أقطاب دائمة ، مما يتيح تفاعلات ثنائية القطب وثنائية القطب بينها. التفاعلات ثنائية القطب وثنائية القطب أقوى من قوى تشتت لندن ، لذلك عادة ما يتطلب الأمر طاقة حرارية أكثر لغلي السوائل القطبية أكثر من السوائل غير القطبية. على سبيل المثال ، درجة غليان الكلوروبنتان ، وهو جزيء قطبي ، هي 108 درجة مئوية. 
الجزيئات التي تحتوي على كل من الهيدروجين المرتبط تساهميا بذرة أكثر كهرسلبية وذرة سحب الإلكترون مع زوج وحيد من الإلكترونات قادرة على الترابط الهيدروجيني. الروابط الهيدروجينية أقوى من التفاعلات ثنائية القطب ثنائية القطب وقوى تشتت لندن ، لذلك يتطلب الأمر المزيد من الطاقة الحرارية للتغلب عليها. على سبيل المثال ، البروبان والإيثانول لهما أوزان جزيئية ومساحات سطحية متشابهة. لكن درجة غليان البروبان أقل بكثير من الإيثانول لأن جزيئات الإيثانول يمكن أن تشكل روابط هيدروجينية مع بعضها البعض ، بينما لا يستطيع البروبان ذلك. 
في هذا المختبر ، ستستخدم الطريقة الشعرية لتحديد درجة غليان مذيبين عضويين.

نقاط الغليان

تحديد نقطة الغليان
على غرار نقطة الانصهار ، فإن نقطة الغليان هي خاصية فيزيائية. إذا كانت العينة مركبا نقيا ، فيمكن استخدام نقطة الغليان لتحديد هوية المركب. في النهاية ، يعد تحديد نقطة الغليان الدقيقة تجريبيا أمرا صعبا. مثل نقاط الانصهار ، يتم إعطاء نقاط الغليان التجريبية كنطاق وتختلف ببضع درجات عن قيمة الأدبيات الفعلية. 
ضغط البخار
لفهم سبب غليان المذيب ، والذي يتميز بالفقاعات المألوفة للمحلول ، من المهم فهم الديناميكيات بين المرحلتين السائلة والغازية. ضع في اعتبارك مركبا سائلا نقيا في حاوية محكمة الغلق. سيكون لبعض الجزيئات الموجودة على سطح السائل طاقة كافية للتغلب على القوى بين الجزيئات والدخول في المرحلة الغازية. ومع ذلك ، قد تفقد الجزيئات في الطور الغازي أيضا الطاقة وتتكثف مرة أخرى في سائل. لذلك ، هناك عمليتان متنافستان في هذا النظام: التبخر والتكثيف. 
عندما يكون معدل التبخر مساويا لمعدل التكثيف ، يكون النظام قد وصل إلى حالة التوازن. هذا يعني أنه لكل جزيء يدخل الطور الغازي ، يتكثف جزيء آخر في الطور السائل ، ولا يوجد ربح أو خسارة صافية لكمية السائل أو الغاز في الحاوية. بمجرد إنشاء التوازن ، يسمى الضغط الذي يمارسه البخار فوق السائل بضغط البخار. يسمى ميل السائل إلى التبخر بتقلبه. السائل الأكثر تقلبا له ضغط بخار أعلى ، بينما يكون للسائل الأقل تقلبا ضغط بخار أقل. 
يختلف ضغط البخار حسب درجة الحرارة. إذا قمت بزيادة درجة حرارة المحلول ، فإن المزيد من الجزيئات لديها طاقة كافية للهروب من الطور السائل ، وبالتالي يزداد ضغط البخار. في النهاية ، إذا تم تطبيق حرارة كافية ، فإن الجزيئات غير الموجودة في الواجهة بين السائل والغاز ستنتقل إلى طور الغاز وتشكل الفقاعات المألوفة التي نربطها بالغليان. 
يتم الوصول إلى درجة غليان السائل عندما يكون ضغط بخار السائل الكلي مكافئا للضغط الجوي. تسمى درجة الحرارة التي يحدث عندها هذا بنقطة الغليان. في الارتفاعات العالية ، وبالتالي الضغط الجوي المنخفض ، سوف يغلي السائل عند درجة حرارة منخفضة ، حيث تتطلب حرارة أقل لزيادة ضغط البخار إلى الضغط الجوي. بالإضافة إلى ذلك ، فإن التقلب ، أو قدرة المذيب على التبخر ، يؤثر أيضا على ضغط البخار. المذيبات ذات التقلب العالي لها ضغط بخار أعلى من المذيبات ذات التقلب المنخفض. 
العوامل التي تؤثر على نقطة الغليان
يعني التشابه بين نقاط الانصهار ونقاط الغليان أن نفس العوامل التي تؤثر على نقطة انصهار المركب ستؤثر أيضا على نقطة الغليان. لذلك ، فإن قوة وأنواع القوى بين الجزيئات الموجودة داخل المركب السائل ستؤثر على نقطة الغليان. تذكر أن هناك ثلاثة أنواع من القوى الجزيئية: الترابط الهيدروجيني ، والتفاعلات ثنائية القطب ثنائية القطب ، وقوى تشتت لندن. كل من هذه لها نقاط قوة جذب مختلفة وتتطلب كميات مختلفة من الطاقة للتغلب عليها. سيكون للمركبات التي يمكنها الرابطة الهيدروجينية نقاط غليان أعلى من المركبات التي لا يمكن أن تتفاعل إلا من خلال قوى تشتت لندن. يتضمن الاعتبار الإضافي لنقاط الغليان ضغط البخار وتقلب المركب. عادة ، كلما كان المركب أكثر تقلبا ، انخفضت درجة غليانه. 
طريقة الشعيرات الدموية لتحديد نقطة الغليان 
طريقة بسيطة لتحديد درجة غليان المركب العضوي هي استخدام طريقة الشعيرات الدموية. في هذا الإعداد ، يتم قلب أنبوب شعري زجاجي فارغ في وعاء من المركب النقي في الطور السائل. عندما يتم تسخين السائل ، يزداد ضغط بخار العينة ويبدأ البخار الغازي في دخول الأنبوب الشعري الزجاجي. هذا يجبر الهواء المحاصر في الداخل وينتج عنه فقاعات تخرج من قاع الأنبوب الشعري. في هذه المرحلة ، يسمح للسائل بالتبريد. بمجرد أن يكون ضغط بخار العينة هو نفسه الضغط الجوي داخل الأنبوب الشعري الزجاجي ، سيبدأ السائل في دخول الأنبوب. درجة حرارة المحلول عند حدوث هذه الظاهرة هي درجة غليان المركب السائل.

Boiling Point Determination
Similar to the melting point, the boiling point is a physical property. If the sample is a pure compound, then the boiling ...

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boiling point and vapor pressure

Boiling point and vapor pressure are key physical properties of a liquid. For a pure compound, the boiling point can help identify the substance. In experiments, the boiling point is usually recorded as a range because exact values are hard to measure and may differ by a few degrees from the literature value.

Vapor pressure explains why a liquid boils. In a sealed container, some molecules at the liquid surface gain enough energy to escape into the gas phase. At the same time, some gas molecules lose energy and condense back into the liquid. When evaporation and condensation happen at the same rate, the system is at equilibrium. The pressure from the vapor above the liquid is the vapor pressure.

Volatility is the tendency of a liquid to vaporize. A more volatile liquid has a higher vapor pressure, while a less volatile liquid has a lower vapor pressure. Vapor pressure also changes with temperature. As temperature rises, more molecules can escape the liquid, so vapor pressure increases. With enough heat, molecules inside the liquid also change to gas and form the bubbles seen during boiling.

A liquid boils when its total vapor pressure matches atmospheric pressure. That temperature is the boiling point. Because atmospheric pressure changes with elevation, a liquid boils at a lower temperature at higher elevations. Less atmospheric pressure means less heat is needed for the vapor pressure to reach that level.

Intermolecular forces also affect boiling point. These forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. They differ in strength, so different amounts of energy are needed to overcome them. Compounds that can hydrogen bond usually have higher boiling points than compounds that rely only on London dispersion forces. Volatility matters too. In general, more volatile compounds have lower boiling points.

One simple way to measure a boiling point is the capillary method. An empty glass capillary tube is placed upside down in the liquid sample. As the sample is heated, its vapor pressure rises and vapor enters the tube. This pushes out trapped air and causes bubbles to appear from the bottom of the capillary tube. The sample is then allowed to cool. When liquid begins to rise into the tube, the vapor pressure inside the tube matches the atmospheric pressure. The temperature at that moment is the boiling point of the compound.

Boiling Point and Vapor Pressure

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Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

hydrolysis of an ester

Soap Formation from Triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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Explore how vapor pressure, volatility, and intermolecular forces affect boiling point, and learn how the capillary method measures it in the lab.

Boiling Point and Vapor Pressure | JoVE K12

استعد لتدريس نقاط الغليان من خلال هذا الدليل الإرشادي للمدرسين لمدة 1 دقيقة. يغطي خطوات التحضير الأساسية لمختبر Chemistry على JoVE Lab Manual.

تحضير المدرّس لـ نقاط الغليان في Chemistry - أكثر من 4.9K مشاهدة | JoVE Lab Manual

اتبع إجراءات نقاط الغليان خطوة بخطوة في فيديو بروتوكول الطالب هذا لمدة 6 دقيقة. جزء من مجموعة Chemistry على JoVE Lab Manual.

بروتوكول الطالب لـ نقاط الغليان في Chemistry - أكثر من 90K مشاهدة | JoVE Lab Manual

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Chemical Bonds

Melting Points

Melting Points in Organic Chemistry
The melting point of a compound is the temperature at which the solid phase transitions into the liquid phase at a ...

Melting Point

Source: Vy M. Dong and Jan Riedel, Department of Chemistry, University of California, Irvine, CA
One of the most important properties of a crystalline ...

Electrolyte and Nonelectrolyte Solutions

Substances that undergo either a physical or a chemical change in solution to yield ions that can conduct electricity are called electrolytes. If a ...

ionic-compounds-formulas-and-nomenclature

Ionic Compounds: Formulas and Nomenclature

An element composed of atoms that readily lose electrons (a metal) can react with an element composed of atoms that readily gain electrons (a nonmetal) to ...

molecular-compounds-formulas-and-nomenclature

Molecular Compounds: Formulas and Nomenclature

Molecular compounds or covalent compounds result when atoms share electrons to form covalent bonds. Since there is no electron transfer, molecular ...

Chemical Symbols

A chemical symbol is an abbreviation that is used to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. We use the ...

Chemical Formulas

A chemical formula presents information about the proportions of atoms constituting a particular chemical compound or molecule, mainly using symbols of ...

Chemical Names and Chemical Formulas

Obtain, evaluate, and communicate information to explain how atoms bond to form stable compounds.

Boiling Point and Vapor Pressure

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