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

Temperatura topnienia substancji to temperatura, w której substancja ta zaczyna zmieniać się z fazy stałej w fazę ciekłą. W tej temperaturze faza ciekła i stała są w równowadze. Przy dodatkowym cieple substancja całkowicie się stopi. Ale co decyduje o temperaturze topnienia substancji? Pomyślmy o ciałach stałych i cieczach. Cząsteczki ciała stałego utrzymują się nawzajem w sztywnej, uporządkowanej strukturze zwanej siatką, podczas gdy cząsteczki cieczy mają słabsze oddziaływania i poruszają się.
Ogrzewanie ciała stałego przenosi energię na cząsteczki. Przy wystarczającej ilości energii cząsteczki pokonują siły utrzymujące je w sieci i zaczynają się poruszać. Innymi słowy, jeśli wystarczająco podgrzejemy ciało stałe, topi się ono w ciecz. Tak więc temperatura topnienia zależy od energii potrzebnej do pokonania sił między cząsteczkami lub sił międzycząsteczkowych, utrzymujących je w sieci. Im silniejsze są siły międzycząsteczkowe, tym więcej energii jest potrzebne, a więc tym wyższa jest temperatura topnienia.
Wiele sił międzycząsteczkowych zależy od tego, jak silnie atomy w cząsteczce przyciągają elektrony - lub od ich elektroujemności. Azot, tlen, fluor i chlor są wysoce elektroujemne, podczas gdy węgiel, wodór i siarka są tylko umiarkowanie elektroujemne. Wiązania między atomami o znacznie różniących się elektroujemnościach są polarne. Na przykład typowe wiązanie węgiel-tlen jest polarne, ale typowe wiązanie węgiel-wodór nie jest.
Elektrony cząsteczki spędzają więcej czasu wokół jej najbardziej elektroujemnych atomów, nadając jej lekki ładunek ujemny po tej stronie i lekki ładunek dodatni po drugiej stronie. Nazywa się to dipolem. Jeśli dipol nie jest znoszony przez równy i przeciwny dipol w tej samej cząsteczce, cząsteczka ma stały dipol i jest polarna.
Omówmy teraz trzy ważne siły międzycząsteczkowe: wiązania wodorowe, oddziaływania dipol-dipol i siły dyspersji Londona. Wiązanie wodorowe zachodzi między atomem wycofującym elektrony z samotną parą elektronów a wodorem związanym z bardziej elektroujemnym atomem. Wiązania wodorowe należą do najsilniejszych oddziaływań międzycząsteczkowych.
Oddziaływania dipol-dipol zachodzą między cząsteczkami polarnymi. W przyciągającym oddziaływaniu dipol-dipol ujemna strona jednego dipola wyrównuje się z dodatnią stroną innego dipola. Oddziaływania dipol-dipol są na ogół słabsze niż wiązania wodorowe.
Londyńskie siły dyspersyjne wynikają z krótkich, przypadkowych przesunięć w rozkładzie elektronów cząsteczki, które powodują odpowiednie przesunięcia w pobliskich cząsteczkach. Te losowe przesunięcia zachodzą w każdej cząsteczce, więc jest to jedna z niewielu interakcji dostępnych dla cząsteczek niepolarnych. Londyńskie siły dyspersyjne należą do najsłabszych oddziaływań międzycząsteczkowych.
Wcześniej przewidywaliśmy, że silniejsze siły międzycząsteczkowe odpowiadają wyższym temperaturom topnienia. Możemy to zobaczyć w działaniu w przypadku heksadekanonu, 2-heksadekanonu i kwasu heksadekanowego. Wraz ze wzrostem siły oddziaływań międzycząsteczkowych dostępnych dla każdej cząsteczki wzrasta również temperatura topnienia.
Siły międzycząsteczkowe nie są jedynym czynnikiem określającym temperaturę topnienia substancji. Jego czystość znacząco wpływa na temperaturę topnienia i zamarzania w efekcie zwanym "obniżeniem temperatury zamarzania". Efekt ten oznacza, że roztwór ma niższą temperaturę zamarzania niż czysty rozpuszczalnik. Dlatego ulice są posypywane solą, gdy robi się bardzo zimno. Jeśli na ulicy zbierze się woda, sól szybko się rozpuszcza, tworząc roztwór o znacznie niższej temperaturze zamarzania niż czysta woda.
W ciele stałym zanieczyszczenia są włączane do struktury sieci. Obszary te często mają słabsze oddziaływania międzycząsteczkowe, co sprawia, że części struktury są łatwiejsze do zakłócenia. Tak więc, w porównaniu z czystym ciałem stałym, topnienie rozpoczyna się w niższej temperaturze i zachodzi w szerszym zakresie temperatur.
W tym laboratorium zmierzysz temperaturę topnienia dwóch znanych związków organicznych, a następnie przeanalizujesz mieszaninę, aby zbadać, jak zanieczyszczenia wpływają na zakres temperatur topnienia.

temperatury topnienia

<h4>Temperatury topnienia w chemii organicznej</h4>
Temperatura topnienia związku to temperatura, w której faza stała przechodzi w fazę ciekłą pod standardowym ciśnieniem 1 atmosfery. Temperatura topnienia związku to właściwość fizyczna, taka jak rozpuszczalność, gęstość, kolor i elektroujemność, którą można wykorzystać do identyfikacji związku. Określenie dokładnej temperatury, w której związek zaczyna się topić, jest trudnym zadaniem; Z tego powodu temperatura topnienia związków jest podawana jako zakres. Dolną granicą zakresu temperatur topnienia jest temperatura, w której obserwuje się pierwsze krople cieczy. Górną granicą zakresu jest temperatura, w której cała faza stała przeszła w fazę ciekłą. W literaturze istnieją przewodniki referencyjne z przyjętymi wartościami, które są używane do identyfikacji związków.
<h4>Wpływ sił międzycząsteczkowych na temperatury topnienia</h4>
Jednym z głównych czynników wpływających na temperaturę topnienia związku jest rodzaj sił międzycząsteczkowych, które istnieją w związku. Siły międzycząsteczkowe są przyciągające lub odpychające między cząsteczkami związku. W fazie stałej cząsteczki związku utworzą zorganizowaną strukturę sieciową, ponieważ cząsteczki są upakowane blisko siebie. Istnieją trzy główne typy sił międzycząsteczkowych:
<ol>
	<li style="font-size:17px">Wiązanie wodorowe - Wiązanie wodorowe to rodzaj siły międzycząsteczkowej, która powstaje w wyniku sił przyciągania między elektroujemnym tlenem a atomem wodoru. Dlatego, aby ten rodzaj siły międzycząsteczkowej był obecny, związek musi zawierać tlen i wodór. Dlatego związki zawierające grupy hydroksylowe, takie jak alkohole, łatwo tworzą wiązania wodorowe. W grupie hydroksylowej dipol powstaje, gdy bardziej elektroujemny tlen przyciąga do siebie gęstość elektronów, dzięki czemu tlen ma częściowy ładunek ujemny. Pozostawia to również wodór z częściowym ładunkiem dodatnim. Pobliskie tleny elektroujemne są przyciągane do częściowego ładunku dodatniego, tworząc wiązanie wodorowe. Spośród trzech rodzajów sił międzycząsteczkowych wiązania wodorowe są najsilniejsze.</li>
	<li style="font-size:17px">Interakcje dipol-dipol - Drugi najsilniejszy rodzaj siły międzycząsteczkowej, oddziaływania dipol-dipol powstają w cząsteczkach zawierających atomy elektroujemne, takie jak tlen, azot i wszelkie halogenki, takie jak chlor i fluor. Na przykład cząsteczka węglowodoru zawierająca fluor utworzy oddziaływania dipol-dipol. Jak? Elektroujemny atom fluoru przyciągnie do siebie gęstość elektronów, dzięki czemu będzie miał częściowy ładunek ujemny. Atom łączący, węgiel, traci część tej gęstości elektronów, a tym samym zyskuje częściowy ładunek ujemny. W ten sposób powstaje tymczasowy dipol przy wiązaniu fluor-węgiel. Gdy przyciągają się przeciwne ładunki, częściowo ujemny fluor jest przyciągany do częściowo dodatniego węgla innej sąsiedniej cząsteczki, tworząc oddziaływanie dipol-dipol.</li>
	<li style="font-size:17px">Londyńskie siły dyspersyjne - Ten rodzaj interakcji jest formą sił van der Waalsa i występuje we wszystkich związkach. Londyńskie siły dyspersyjne są najsłabszym rodzajem oddziaływań międzycząsteczkowych. Podobnie jak w przypadku oddziaływań dipol-dipol, zachodzi redystrybucja gęstości elektronów wokół cząsteczki, powodująca powstawanie tymczasowych ładunków. W przeciwieństwie do oddziaływań dipol-dipol, siły dyspersji powstające w Londynie są bardzo słabe i minimalne. Na przykład związki niepolarne, takie jak metan, etan, pentan i oktan, oddziałują poprzez siły dyspersji londyńskiej. Pole powierzchni i długość cząsteczki określa siłę sił przyciągania, tak że związki o większej powierzchni mają większe siły dyspersji londyńskiej niż mniejsze związki. W związku z tym oktan miałby silniejsze londyńskie siły dyspersyjne niż metan.</li>
</ol>
Każdy rodzaj siły międzycząsteczkowej ma inną siłę przyciągania. Dlatego związki zawierające wiązania wodorowe wymagają więcej energii, aby przerwać przyciąganie między cząsteczkami niż związek niepolarny, który ma tylko siły dyspersji Londona. Zatem obecność wiązań wodorowych zwiększa temperaturę topnienia związku.
<h4>Wpływ zanieczyszczeń na temperatury topnienia</h4>
Podane w literaturze wartości temperatur topnienia zakładają, że masz czystą próbkę danego związku. Często w laboratorium lub w nieznanych próbkach badane próbki nie są czystymi związkami. Zanieczyszczenia powodują, że obserwowana temperatura topnienia mieszaniny jest niższa niż rzeczywista temperatura topnienia czystego związku. Obserwowalny zakres jest większy niż w przypadku czystej substancji.
W czystym związku ciało stałe składa się z jednolitej i uporządkowanej struktury i wymaga określonej temperatury do rozbicia struktury, aby związek mógł przejść w fazę ciekłą. W mieszaninie zawierającej zanieczyszczenia faza stała składa się ze zdezorganizowanej struktury. Wymaga to znacznie mniej energii do przejścia w fazę ciekłą, co obniża temperaturę topnienia. Zjawisko to jest znane jako obniżenie temperatury topnienia. Im więcej zanieczyszczeń w próbce, tym szerszy zakres temperatur topnienia i niższa temperatura topnienia.

Temperatury topnienia

Temperatury topnienia w chemii organicznej
Temperatura topnienia związku to temperatura, w której faza stała przechodzi w fazę ciekłą w ...

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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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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

How Impurities Change Melting Points

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