Name: how impurities change melting points
Uploaded: 2020-03-26T07:22:27.000Z
Duration: 5 min 12 s
Channel: JoVE

K12 Standards

Alabama Course of Study Science

9-12

Chemistry

Matter and Its Interactions

9-12.C.1.5.e

Melting Points

Bir maddenin erime noktası, o maddenin katı fazdan sıvı faza geçmeye başladığı sıcaklıktır. Bu sıcaklıkta sıvı ve katı fazlar denge halindedir. Ek ısı ile madde tamamen eriyecektir. Fakat bir maddenin erime noktasını ne belirler? Katıları ve sıvıları düşünelim. Bir katının molekülleri birbirlerini kafes adı verilen katı, düzenli bir yapıda tutarken, bir sıvının molekülleri daha zayıf etkileşimlere sahiptir ve hareket eder.
Bir katının ısıtılması, enerjiyi moleküllere aktarır. Yeterli enerji ile moleküller kendilerini kafes içinde tutan kuvvetlerin üstesinden gelir ve hareket etmeye başlarlar. Başka bir deyişle, bir katıyı yeterince ısıtırsak, bir sıvıya erir. Bu nedenle, erime noktası, moleküller arasındaki kuvvetlerin veya onları kafes içinde tutan moleküller arası kuvvetlerin üstesinden gelmek için gereken enerjiye bağlıdır. Moleküller arası kuvvetler ne kadar güçlüyse, o kadar fazla enerji gerekir, bu nedenle erime noktası o kadar yüksek olur.
Birçok moleküller arası kuvvet, moleküldeki atomların elektronları ne kadar güçlü çektiğine veya elektronegatifliklerine bağlıdır. Azot, oksijen, flor ve klor oldukça elektronegatifken, karbon, hidrojen ve kükürt sadece orta derecede elektronegatiftir. Önemli ölçüde farklı elektronegatifliklere sahip atomlar arasındaki bağlar polardır. Örneğin, tipik bir karbon-oksijen bağı polardır, ancak tipik bir karbon-hidrojen bağı polar değildir.
Bir molekülün elektronları, en elektronegatif atomlarının etrafında daha fazla zaman harcar, bu da ona o tarafta hafif bir negatif yük ve diğer tarafta hafif bir pozitif yük verir. Buna dipol denir. Dipol, aynı moleküldeki eşit ve zıt bir dipol tarafından iptal edilmezse, molekülün kalıcı bir dipolü vardır ve polardır.
Şimdi, üç önemli moleküller arası kuvveti tartışalım: hidrojen bağı, dipol-dipol etkileşimleri ve London dağılım kuvvetleri. Hidrojen bağı, yalnız bir elektron çiftine sahip elektron çeken bir atom ile daha elektronegatif bir atoma bağlı bir hidrojen arasında meydana gelir. Hidrojen bağları en güçlü moleküller arası kuvvetler arasındadır.
Dipol-dipol etkileşimleri polar moleküller arasında meydana gelir. Çekici bir dipol-dipol etkileşiminde, bir dipolün negatif tarafı başka bir dipolün pozitif tarafı ile hizalanır. Dipol-dipol etkileşimleri genellikle hidrojen bağlarından daha zayıftır.
London dağılım kuvvetleri, bir molekülün elektron dağılımındaki kısa, rastgele kaymalardan gelir ve bu da yakındaki moleküllerde karşılık gelen kaymalara neden olur. Bu rastgele kaymalar her molekülde meydana gelir, bu nedenle bu, polar olmayan moleküller için mevcut olan birkaç etkileşimden biridir. Londra dağılma kuvvetleri, en zayıf moleküller arası kuvvetler arasındadır.
Daha önce, daha güçlü moleküller arası kuvvetlerin daha yüksek erime noktalarına karşılık geldiğini tahmin etmiştik. Bunu hekzadekan, 2-heksadekanon ve hekzadekanoik asit ile hareket halinde görebiliriz. Her molekül için mevcut olan moleküller arası etkileşimlerin gücü arttıkça erime noktası da artar.
Moleküller arası kuvvetler, bir maddenin erime noktasını belirleyen tek faktör değildir. Saflığı, 'donma noktası depresyonu' adı verilen bir etkiyle erime ve donma noktalarını önemli ölçüde etkiler. Bu etki, bir çözeltinin saf çözücüden daha düşük bir donma noktasına sahip olduğu anlamına gelir. Bu yüzden sokaklar çok soğuduğunda tuz serpilir. Sokakta herhangi bir su toplanırsa, tuz hızla çözülür ve saf sudan çok daha düşük donma noktasına sahip bir çözelti elde edilir.
Bir katıda, safsızlıklar kafes yapısına dahil edilir. Bu alanlar genellikle daha zayıf moleküller arası etkileşimlere sahiptir ve bu da yapının bazı kısımlarının bozulmasını kolaylaştırır. Bu nedenle, saf bir katı ile karşılaştırıldığında, erime daha düşük bir sıcaklıkta başlar ve daha geniş bir sıcaklık aralığında gerçekleşir.
Bu laboratuvarda, bilinen iki organik bileşiğin erime noktalarını ölçecek ve ardından safsızlıkların erime noktası aralığını nasıl etkilediğini keşfetmek için bir karışımı analiz edeceksiniz.

erime noktaları

<h4>Organik Kimyada Erime Noktaları</h4>
Bir bileşiğin erime noktası, katı fazın 1 atmosferlik standart bir basınçta sıvı faza geçtiği sıcaklıktır. Bir bileşiğin erime noktası, bir bileşiği tanımlamak için kullanılabilecek çözünürlük, yoğunluk, renk ve elektronegatiflik gibi fiziksel bir özelliktir. Bir bileşiğin erimeye başladığı tam sıcaklığı belirlemek zorlu bir iştir; Bu nedenle, bileşiklerin erime noktası bir aralık olarak rapor edilir. Erime noktası aralığının alt sınırı, sıvının ilk damlalarının gözlemlendiği sıcaklıktır. Aralığın üst sınırı, katı fazın tamamının sıvı faza geçtiği sıcaklıktır. Literatürde, bileşikleri tanımlamak için kullanılan kabul edilmiş değerlere sahip referans kılavuzları bulunmaktadır.
<h4>Moleküller Arası Kuvvetlerin Erime Noktaları Üzerindeki Etkisi</h4>
Bileşiğin erime noktasını etkileyen önemli bir faktör, bileşik içinde var olan moleküller arası kuvvetlerin türüdür. Moleküller arası kuvvetler, bir bileşiğin molekülleri arasında çekici veya iticidir. Katı fazda, bir bileşiğin molekülleri, moleküller birbirine yakın paketlendikçe organize bir kafes yapısı oluşturacaktır. Moleküller arası kuvvetin üç ana türü vardır:
<ol>
	<li style="font-size:17px">Hidrojen bağı – Hidrojen bağı, bir elektronegatif oksijen ile bir hidrojen atomu arasındaki çekim kuvvetleri nedeniyle oluşan bir tür moleküller arası kuvvettir. Bu nedenle, bu tür moleküller arası kuvvetin mevcut olması için bileşiğin oksijen ve hidrojen içermesi gerekir. Bu nedenle, alkoller gibi hidroksil grupları içeren bileşikler kolayca hidrojen bağları oluşturur. Hidroksil grubu içinde, daha elektronegatif oksijen elektron yoğunluğunu kendisine doğru çektiğinde bir dipol oluşur ve oksijenin kısmi bir negatif yüke sahip olmasını sağlar. Bu aynı zamanda hidrojeni kısmi bir pozitif yükle bırakır. Yakındaki elektronegatif oksijenler, bir hidrojen bağı oluşturarak kısmi pozitif yüke çekilir. Üç tür moleküller arası kuvvetten hidrojen bağı en güçlü olanıdır.</li>
	<li style="font-size:17px">Dipol-Dipol Etkileşimleri – Moleküller arası kuvvetin ikinci en güçlü türü olan dipol-dipol etkileşimleri, oksijen, nitrojen gibi elektronegatif atomlar ve klor ve flor gibi halojenürlerden herhangi birini içeren moleküllerde oluşur. Örneğin, flor içeren bir hidrokarbon molekülü, dipol-dipol etkileşimleri oluşturacaktır. Nasıl? Elektronegatif flor atomu, elektron yoğunluğunu kendisine doğru çekerek kısmi bir negatif yüke sahip olmasını sağlar. Bağlanan atom, karbon, bu elektron yoğunluğunun bir kısmını kaybeder ve böylece kısmi bir negatif yük kazanır. Bu, flor-karbon bağında geçici bir dipol oluşturur. Zıt yükler birbirini çekerken, kısmen negatif flor, başka bir komşu molekülün kısmen pozitif karbonuna çekilir ve bir dipol-dipol etkileşimi oluşturur.</li>
	<li style="font-size:17px">London dağılma kuvvetleri – Bu tür bir etkileşim, van der Waals kuvvetlerinin bir şeklidir ve tüm bileşiklerde bulunur. London dağılma kuvvetleri, moleküller arası kuvvetlerin en zayıf türüdür. Dipol-dipol etkileşimleri gibi, molekül etrafında elektron yoğunluğunun yeniden dağılımı olur ve bu da geçici yüklerin oluşumuna neden olur. Dipol-dipol etkileşimlerinin aksine, Londra dispersiyon kuvvetlerinde oluşan dipoller çok zayıf ve minimaldir. Örneğin, metan, etan, pentan ve oktan gibi polar olmayan bileşikler, Londra dağılma kuvvetleri aracılığıyla etkileşime girer. Molekülün yüzey alanı ve uzunluğu, çekici kuvvetlerin gücünü belirler, öyle ki daha fazla yüzey alanına sahip bileşikler, daha küçük bileşiklerden daha büyük London dispersiyon kuvvetlerine sahiptir. Bu nedenle, oktan metandan daha güçlü Londra dağılma kuvvetlerine sahip olacaktır.</li>
</ol>
Her tür moleküller arası kuvvetin farklı bir çekim gücü vardır. Bu nedenle, hidrojen bağları içeren bileşikler, moleküller arasındaki çekimi kırmak için yalnızca London dispersiyon kuvvetlerine sahip polar olmayan bir bileşikten daha fazla enerji gerektirir. Böylece, hidrojen bağlarının varlığı, bir bileşiğin erime noktasını arttırır.
<h4>Safsızlıkların erime noktaları üzerindeki etkisi</h4>
Erime noktalarının bildirilen literatür değerleri, söz konusu bileşiğin saf bir örneğine sahip olduğunuzu varsayar. Genellikle laboratuvarda veya bilinmeyen numunelerde, test edilen numuneler saf bileşikler değildir. Safsızlıklar, bir karışımın gözlemlenen erime noktasının, saf bileşiğin gerçek erime sıcaklığından daha düşük olmasına neden olur. Gözlemlenebilir aralık, saf maddeninkinden daha büyüktür.
Saf bir bileşikte katı, düzgün ve düzenli bir yapıdan oluşur ve bileşiğin sıvı faza geçmesi için yapıyı parçalamak için belirli bir miktarda sıcaklık gerektirir. Safsızlık içeren bir karışımda, katı faz düzensiz bir yapıdan oluşur. Bu, sıvı faza geçmek için çok daha az enerji gerektirir, böylece erime noktasını düşürür. Bu fenomen erime noktası depresyonu olarak bilinir. Numunedeki safsızlıklar ne kadar fazlaysa, erime noktası aralığı o kadar geniş ve erime sıcaklığı o kadar düşük olur.

Erime Noktaları

Organik Kimyada Erime Noktaları
Bir bileşiğin erime noktası, katı fazın bir anda sıvı faza geçtiği sıcaklıktır.

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how impurities change melting points

Melting point helps identify an organic compound by showing the temperature where a solid turns into a liquid at 1 atmosphere of pressure. It is a physical property, like solubility, density, color, and electronegativity. Because the exact start of melting is hard to measure, the result is reported as a range. The lower end is when the first drops of liquid appear. The upper end is when the last solid has melted. Scientists compare this range with literature values to help identify a compound.

Intermolecular forces have a strong effect on melting point. In a solid, molecules pack into an ordered lattice structure. A compound with stronger attractions between molecules needs more energy to break that structure apart. Hydrogen bonding is the strongest of the three main intermolecular forces. It forms when an electronegative oxygen attracts a hydrogen atom, as in compounds with hydroxyl groups such as alcohols. The oxygen pulls electron density toward itself, so oxygen becomes partially negative and hydrogen becomes partially positive. Nearby electronegative oxygens are attracted to that positive hydrogen and form a hydrogen bond.

Dipole-dipole interactions are the second-strongest type of intermolecular force. They form in molecules with electronegative atoms such as oxygen, nitrogen, chlorine, and fluorine. For example, a molecule with fluorine can develop a temporary dipole because fluorine pulls electron density toward itself. That leaves the attached carbon with less electron density. The partially negative fluorine is then attracted to the partially positive carbon of a neighboring molecule. London dispersion forces are the weakest type and are present in all compounds. They are especially important in nonpolar molecules such as methane, ethane, pentane, and octane. Larger molecules with more surface area have stronger London dispersion forces, so octane has stronger attractions than methane. Because hydrogen bonding is strongest, compounds with hydrogen bonds usually have higher melting points than nonpolar compounds that rely only on London dispersion forces.

Impurities can lower the melting point of a sample. Literature values assume a pure compound, but lab samples are often mixtures. An impure sample usually melts over a broader range and at a lower temperature than the pure substance. This effect is called melting point depression. In a pure compound, the solid is uniform and ordered, so it needs more heat to melt. In a mixture, impurities disrupt that order and make the solid less stable. The more impurities present, the lower the melting point and the wider the melting range.

How Impurities Change Melting Points

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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

Kimya laboratuvar temelleri için, hem eğitmenlere hem de öğrencilere yönelik, kapsamlı ve müfredat odaklı vidolar.

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Learn how melting point helps identify compounds, how intermolecular forces affect it, and why impurities cause melting point depression in lab samples.

How Impurities Change Melting Points | JoVE K12

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.C

Georgia Standards of Excellence

Physical Science

SPS2.

Physical Properties

Erime Noktaları öğretimine bu 2 dakikalık eğitmen rehberiyle hazırlanın. JoVE Lab Manual üzerindeki Chemistry laboratuvarı için temel hazırlık adımlarını kapsar.

Chemistry içinde Erime Noktaları Eğitmen Hazırlığı - 6.5K+ Görüntülenme | JoVE Lab Manual

8 dakikalık bu öğrenci protokol videosunda Erime Noktaları prosedürlerini adım adım takip edin. JoVE Lab Manual üzerindeki Chemistry koleksiyonunun bir parçasıdır.

Chemistry içinde Erime Noktaları Öğrenci Protokolü - 47K+ Görüntülenme | JoVE Lab Manual

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Atomic Mass

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Molar Mass

The identity of a substance is defined not only by the types of atoms or ions it contains but by the quantity of each type of atom or ion. For example, ...

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

1. Use the periodic table as a model to predict the structure and properties of atoms and elements.

9-12.C.1.1

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

Elements: Chemical Symbols and Isotopes

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

Ions and Ionic Charges

In ordinary chemical reactions, the nucleus &mdash; which contains the protons and neutrons of each atom and thus identifies the element &mdash; remains ...

a. Assess the merits and limitations of historic and modern atomic models pertaining to the presence, position, mass, and charge of subatomic particles.

9-12.C.1.1.a

b. Develop and use models of an element's subatomic particles to compare and contrast its atoms, ions, and isotopes.

9-12.C.1.1.b

The Wave Nature of Light

The nature of light has been a subject of inquiry since antiquity. In the seventeenth century, Isaac Newton performed experiments with lenses and prisms ...

The Electromagnetic Spectrum

The electromagnetic spectrum consists of all the types of electromagnetic radiation arranged according to their frequency and wavelength. Each of the ...

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

Atomic Orbitals

An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution ...

The Energies of Atomic Orbitals

In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are ...

The Aufbau Principle and Hund's Rule

To determine the electron configuration for any particular atom, we can build the structures in the order of atomic numbers. Beginning with hydrogen, and ...

electron-configuration-of-multielectron-atoms

Electron Configuration of Multielectron Atoms

The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

c. Analyze and interpret data to identify or describe an element based on its number of protons, its relative abundance of isotopes, its organization and placement of electrons, and its light emission spectrum. Examples: average atomic mass, electron configuration, orbital notation, noble-gas notation, photoemission spectroscopy

9-12.C.1.1.c

comparing-intermolecular-forces-melting-point-boiling-point

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, ...

surface-tension-capillary-action-and-viscosity

Surface Tension, Capillary Action, and Viscosity

Surface Tension
The various IMFs between identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded ...

Chemical Reactions in Aqueous Solutions

Chemical substances interact in many different ways. Certain chemical reactions exhibit common patterns of reactivity. Due to the vast number of chemical ...

d. Ask questions to determine the relationship between an element's physical and chemical properties and its position on the periodic table. Examples: density, melting point, chemical hardness; reactivity, chemical stability, oxidation state

9-12.C.1.1.d

e. Construct explanations of how periodic trends can be used to predict the properties of elements. Examples: atomic radii, ionization energy, electronegativity

9-12.C.1.1.e

Reaction Stoichiometry

A balanced chemical equation provides a great deal of information in a very succinct format. Chemical formulas provide the identities of the reactants and ...

Limiting Reactant

The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. However, in ...

Reaction Yield

The theoretical yield of a reaction is the amount of product estimated to form based on the stoichiometry of the balanced chemical equation. The ...

Stoichiometry, Product Yield, and Limiting Reactants

Chemical equations represent how a chemical reaction proceeds from reactants to products through physical or chemical change using chemical formulas.
...

4. Use stoichiometric ratios to support the claim that atoms, and therefore mass, are conserved during chemical reactions.

9-12.C.1.4

How Impurities Change Melting Points

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