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

Температура плавления вещества — это температура, при которой это вещество начинает переходить из твердой фазы в жидкую фазу. При этой температуре жидкая и твердая фазы находятся в равновесии. При дополнительном нагреве вещество полностью расплавится. Но от чего зависит температура плавления вещества? Давайте подумаем о твердых и жидких телах. Молекулы твердого тела удерживают друг друга в жесткой упорядоченной структуре, называемой решеткой, в то время как молекулы жидкости имеют более слабые взаимодействия и движутся.
Нагревание твердого тела передает энергию молекулам. Обладая достаточным количеством энергии, молекулы преодолевают силы, удерживающие их в решетке, и начинают вращаться. Другими словами, если мы достаточно нагреем твердое тело, оно превратится в жидкость. Таким образом, температура плавления зависит от энергии, необходимой для преодоления сил между молекулами, или межмолекулярных сил, удерживающих их в решетке. Чем сильнее межмолекулярные силы, тем больше энергии требуется, поэтому тем выше температура плавления.
Многие межмолекулярные силы зависят от того, насколько сильно атомы в молекуле притягивают электроны, или от их электроотрицательности. Азот, кислород, фтор и хлор являются высокоэлектроотрицательными, в то время как углерод, водород и сера являются умеренно электроотрицательными. Связи между атомами со значительно различающимися электроотрицательностями являются полярными. Например, типичная углерод-кислородная связь является полярной, а типичная углерод-водородная связь — нет.
Электроны молекулы проводят больше времени вокруг ее наиболее электроотрицательных атомов, придавая ей небольшой отрицательный заряд с одной стороны и небольшой положительный заряд с другой. Это называется диполем. Если диполь не нейтрализуется равным и противоположным диполем в той же молекуле, то молекула имеет постоянный диполь и является полярной.
Теперь давайте обсудим три важные межмолекулярные силы: водородные связи, диполь-дипольные взаимодействия и дисперсионные силы Лондона. Водородная связь происходит между атомом, захватывающим электроны с единственной парой электронов, и водородом, связанным с более электроотрицательным атомом. Водородные связи являются одними из самых сильных межмолекулярных сил.
Между полярными молекулами происходят диполь-дипольные взаимодействия. При притягивающем диполь-дипольном взаимодействии отрицательная сторона одного диполя выравнивается с положительной стороной другого диполя. Диполь-дипольные взаимодействия обычно слабее, чем водородные связи.
Дисперсионные силы Лондона возникают в результате коротких, случайных сдвигов в распределении электронов молекулы, которые вызывают соответствующие сдвиги в соседних молекулах. Эти случайные сдвиги происходят в каждой молекуле, так что это одно из немногих взаимодействий, доступных для неполярных молекул. Дисперсионные силы Лондона являются одними из самых слабых межмолекулярных сил.
Ранее мы предсказывали, что более сильные межмолекулярные силы соответствуют более высоким температурам плавления. Мы можем наблюдать это в действии с гексадеканом, 2-гексадеканоном и гексадекановой кислотой. По мере увеличения силы межмолекулярных взаимодействий, доступных каждой молекуле, увеличивается и температура плавления.
Межмолекулярные силы — не единственный фактор, определяющий температуру плавления вещества. Его чистота значительно влияет на температуру плавления и замерзания, что называется «депрессией точки замерзания». Этот эффект означает, что раствор имеет более низкую температуру замерзания, чем чистый растворитель. Именно поэтому улицы посыпают солью, когда становится очень холодно. Если на улице собирается какая-либо вода, соль быстро растворяется, образуя раствор с гораздо более низкой температурой замерзания, чем чистая вода.
В твердом веществе примеси встраиваются в решетчатую структуру. Эти области часто имеют более слабые межмолекулярные взаимодействия, что делает части структуры более подверженными разрушению. Таким образом, по сравнению с чистым твердым телом, плавление начинается при более низкой температуре и происходит в более широком диапазоне температур.
В этой лаборатории вы измерите температуру плавления двух известных органических соединений, а затем проанализируете смесь, чтобы выяснить, как примеси влияют на диапазон температур плавления.

Температуры плавления

<h4>Температуры плавления в органической химии</h4>
Температура плавления соединения — это температура, при которой твердая фаза переходит в жидкую фазу при стандартном давлении в 1 атмосферу. Температура плавления соединения — это физическое свойство, такое как растворимость, плотность, цвет и электроотрицательность, которое может быть использовано для идентификации соединения. Определение точной температуры, при которой соединение начинает плавиться, является сложной задачей; Из-за этого температура плавления соединений сообщается в виде диапазона. Нижняя граница диапазона температур плавления – это температура, при которой наблюдаются первые капли жидкости. Верхняя граница диапазона — это температура, при которой вся твердая фаза переходит в жидкую фазу. В литературе существуют справочники с принятыми значениями, которые используются для идентификации соединений.
<h4>Влияние межмолекулярных сил на температуру плавления</h4>
Одним из основных факторов, влияющих на температуру плавления соединения, является тип межмолекулярных сил, которые существуют внутри соединения. Межмолекулярные силы могут быть либо притягивающими, либо отталкивающими между молекулами соединения. В твердой фазе молекулы соединения образуют организованную решетчатую структуру, поскольку молекулы упакованы близко друг к другу. Существует три основных типа межмолекулярных сил:
<ol>
	<li style="font-size:17px">Водородная связь - это тип межмолекулярной силы, которая возникает из-за сил притяжения между электроотрицательным кислородом и атомом водорода. Следовательно, чтобы этот тип межмолекулярной силы присутствовал, соединение должно содержать кислород и водород. Поэтому соединения, содержащие гидроксильные группы, такие как спирты, легко образуют водородные связи. Внутри гидроксильной группы диполь образуется, когда более электроотрицательный кислород притягивает к себе электронную плотность, в результате чего кислород имеет частичный отрицательный заряд. Это также оставляет водород с частичным положительным зарядом. Близлежащие электроотрицательные кислороды притягиваются к частичному положительному заряду, образуя водородную связь. Из трех типов межмолекулярных сил водородная связь является самой сильной.</li>
	<li style="font-size:17px">Диполь-дипольные взаимодействия – второй по силе тип межмолекулярных взаимодействий, диполь-дипольные взаимодействия формируются в молекулах, содержащих электроотрицательные атомы, такие как кислород, азот и любой из галогенидов, таких как хлор и фтор. Например, молекула углеводорода, содержащая фтор, будет образовывать диполь-дипольные взаимодействия. Как? Электроотрицательный атом фтора будет притягивать к себе электронную плотность, в результате чего у него будет частичный отрицательный заряд. Связующий атом, углерод, теряет часть этой электронной плотности и, таким образом, получает частичный отрицательный заряд. Это образует временный диполь на фтор-углеродной связи. По мере притяжения противоположных зарядов частично отрицательный фтор притягивается к частично положительному углероду другой соседней молекулы, образуя диполь-дипольное взаимодействие.</li>
	<li style="font-size:17px">Лондонские дисперсионные силы – этот тип взаимодействия является формой сил Ван-дер-Ваальса и присутствует во всех соединениях. Дисперсионные силы Лондона являются самым слабым типом межмолекулярных сил. Как и при диполь-дипольных взаимодействиях, происходит перераспределение электронной плотности вокруг молекулы, что приводит к образованию временных зарядов. В отличие от диполь-дипольных взаимодействий, диполи, образующиеся при дисперсионных силах Лондона, очень слабы и минимальны. Например, неполярные соединения, такие как метан, этан, пентан и октановое число, взаимодействуют через дисперсионные силы Лондона. Площадь поверхности и длина молекулы определяют силу сил притяжения, так что соединения с большей площадью поверхности имеют большие дисперсионные силы Лондона, чем меньшие соединения. Следовательно, октановое число будет иметь более сильные дисперсионные силы Лондона, чем метан.</li>
</ol>
Каждый тип межмолекулярной силы имеет разную силу притяжения. Таким образом, соединениям, содержащим водородные связи, требуется больше энергии для преодоления притяжения между молекулами, чем неполярному соединению, которое обладает только дисперсионными силами Лондона. Таким образом, наличие водородных связей повышает температуру плавления соединения.
<h4>Влияние примесей на температуру плавления</h4>
Опубликованные в литературе значения температур плавления предполагают, что у вас есть чистый образец рассматриваемого соединения. Часто в лаборатории или на неизвестных образцах испытуемые образцы не являются чистыми соединениями. Примеси приводят к тому, что наблюдаемая температура плавления смеси оказывается ниже, чем фактическая температура плавления чистого соединения. Наблюдаемый диапазон больше, чем у чистого вещества.
В чистом соединении твердое вещество состоит из однородной и упорядоченной структуры и требует определенного количества температуры для разрушения структуры для перехода соединения в жидкую фазу. В смеси, содержащей примеси, твердая фаза состоит из дезорганизованной структуры. Для этого требуется гораздо меньше энергии для перехода в жидкую фазу, что снижает температуру плавления. Это явление известно как депрессия температуры плавления. Чем больше примесей в образце, тем шире диапазон температур плавления и тем ниже температура плавления.

Температуры плавления в органической химии
Температура плавления соединения – это температура, при которой твердая фаза переходит в жидкую фазу при ...

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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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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. <br...

boiling points

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...

recrystallization

Purifying Crystals by Solubility

Purifying crystals by solubility is a key organic chemistry technique. Recrystallization uses differences in solubility to separate a desired solid product from starting materials, solvent, and impurities. It is one of the three standard purification methods, along with distillation and extraction. Solubility is the maximum amount of a substance that can dissolve in a fixed volume of solvent at a given temperature. Different solutes dissolve in different solvents, and their behavior can vary...

extraction

Separating Mixtures with Solvent Extraction

Solvent extraction is a common way to separate and isolate a target compound in organic chemistry. In this process, a solute moves from one phase to another so it can be separated from unreacted starting materials or impurities. Extraction is also useful when a product is dissolved in a reaction solvent that is hard to remove by evaporation, such as a solvent with a high boiling point. There are three main types of extraction. In solid-liquid extraction, the solute moves from a solid phase...

simple distillation

Separating Liquids by Boiling Point

Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

thin-layer chromatography

TLC Separation by Polarity

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column. The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a...

column chromatography

Separating Mixtures by Polarity

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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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

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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

formula-mass-and-mole-concepts-of-compounds

Formula Mass and Mole Concepts of Compounds

Formula Mass of Covalent Compounds

Chemical formulas represent the elemental makeup of substances. For covalent compounds, the formula represents the ...

a. Quantitatively apply the concepts of the mole and Avogadro's number to conceptualize and calculate percent composition and empirical or molecular formulas of common compounds.

9-12.C.1.4.a

How Impurities Change Melting Points

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