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

Le point de fusion d’une substance est la température à laquelle cette substance commence à passer de la phase solide à la phase liquide. À cette température, les phases liquide et solide sont en équilibre. Avec une chaleur supplémentaire, la substance fondra complètement. Mais qu’est-ce qui détermine le point de fusion d’une substance ? Pensons aux solides et aux liquides. Les molécules d’un solide se maintiennent les unes les autres dans une structure rigide et ordonnée appelée réseau, tandis que les molécules d’un liquide ont des interactions plus faibles et se déplacent.
Le chauffage d’un solide transfère de l’énergie aux molécules. Avec suffisamment d’énergie, les molécules surmontent les forces qui les maintiennent dans le réseau et commencent à se déplacer. En d’autres termes, si nous chauffons suffisamment un solide, il fond en un liquide. Ainsi, le point de fusion dépend de l’énergie nécessaire pour surmonter les forces entre les molécules, ou les forces intermoléculaires, qui les maintiennent dans le réseau. Plus les forces intermoléculaires sont fortes, plus l’énergie nécessaire est importante, donc plus le point de fusion est élevé.
De nombreuses forces intermoléculaires dépendent de la force avec laquelle les atomes de la molécule attirent les électrons – ou de leur électronégativité. L’azote, l’oxygène, le fluor et le chlore sont fortement électronégatifs, tandis que le carbone, l’hydrogène et le soufre ne sont que modérément électronégatifs. Les liaisons entre atomes ayant des électronégativités significativement différentes sont polaires. Par exemple, une liaison carbone-oxygène typique est polaire, mais une liaison carbone-hydrogène typique ne l’est pas.
Les électrons d’une molécule passent plus de temps autour de ses atomes les plus électronégatifs, ce qui lui donne une légère charge négative de ce côté et une légère charge positive de l’autre côté. C’est ce qu’on appelle un dipôle. Si le dipôle n’est pas annulé par un dipôle égal et opposé dans la même molécule, la molécule a un dipôle permanent et est polaire.
Parlons maintenant de trois forces intermoléculaires importantes : la liaison hydrogène, les interactions dipôle-dipôle et les forces de dispersion de Londres. La liaison hydrogène se produit entre un atome qui retire des électrons avec un doublet non liant d’électrons et un hydrogène lié à un atome plus électronégatif. Les liaisons hydrogène sont parmi les forces intermoléculaires les plus puissantes.
Des interactions dipôle-dipôle se produisent entre les molécules polaires. Dans une interaction dipôle-dipôle attractive, le côté négatif d’un dipôle s’aligne avec le côté positif d’un autre dipôle. Les interactions dipôle-dipôle sont généralement plus faibles que les liaisons hydrogène.
Les forces de dispersion de London proviennent de changements brefs et aléatoires dans la distribution des électrons d’une molécule, qui provoquent des changements correspondants dans les molécules voisines. Ces décalages aléatoires se produisent dans chaque molécule, c’est donc l’une des rares interactions disponibles pour les molécules non polaires. Les forces de dispersion de Londres sont parmi les forces intermoléculaires les plus faibles.
Plus tôt, nous avons prédit que des forces intermoléculaires plus fortes correspondaient à des points de fusion plus élevés. Nous pouvons le voir en action avec l’hexadécane, le 2-hexadécanone et l’acide hexadécanoïque. À mesure que la force des interactions intermoléculaires disponibles pour chaque molécule augmente, le point de fusion augmente également.
Les forces intermoléculaires ne sont pas le seul facteur qui détermine le point de fusion d’une substance. Sa pureté affecte de manière significative ses points de fusion et de congélation dans un effet appelé « dépression du point de congélation ». Cet effet signifie qu’une solution a un point de congélation plus bas que le solvant pur. C’est pourquoi les rues sont saupoudrées de sel lorsqu’il fait très froid. Si de l’eau s’accumule dans la rue, le sel se dissout rapidement pour former une solution dont le point de congélation est beaucoup plus bas que l’eau pure.
Dans un solide, les impuretés sont incorporées dans la structure du treillis. Ces zones ont souvent des interactions intermoléculaires plus faibles, ce qui rend certaines parties de la structure plus faciles à perturber. Ainsi, par rapport à un solide pur, la fusion commence à une température plus basse et se produit sur une plage de température plus large.
Dans cet atelier, vous mesurerez les points de fusion de deux composés organiques connus, puis analyserez un mélange pour explorer comment les impuretés affectent la plage du point de fusion.

points de fusion

<h4>Points de fusion en chimie organique</h4>
Le point de fusion d’un composé est la température à laquelle la phase solide passe à la phase liquide à une pression standard de 1 atmosphère. Le point de fusion d’un composé est une propriété physique, comme la solubilité, la densité, la couleur et l’électronégativité qui peut être utilisée pour identifier un composé. Déterminer la température exacte à laquelle un composé commence à fondre est une tâche difficile. Pour cette raison, le point de fusion des composés est indiqué sous forme de plage. La limite inférieure de la plage de point de fusion est la température à laquelle les premières gouttes de liquide sont observées. La limite supérieure de la plage est la température à laquelle toute la phase solide est passée à la phase liquide. Il existe dans la littérature des guides de référence avec des valeurs acceptées, qui sont utilisés pour identifier les composés.
<h4>L’effet des forces intermoléculaires sur les points de fusion</h4>
L’un des principaux facteurs qui ont un impact sur le point de fusion du composé est le type de forces intermoléculaires qui existent à l’intérieur du composé. Les forces intermoléculaires sont soit attractives, soit répulsives entre les molécules d’un composé. Dans la phase solide, les molécules d’un composé formeront une structure en réseau organisée car les molécules sont regroupées à proximité les unes des autres. Il existe trois grands types de forces intermoléculaires :
<ol>
	<li style="font-size :17px">Liaison hydrogène - La liaison hydrogène est un type de force intermoléculaire qui se produit en raison des forces d’attraction entre un oxygène électronégatif et un atome d’hydrogène. Par conséquent, pour que ce type de force intermoléculaire soit présent, le composé doit contenir de l’oxygène et de l’hydrogène. Par conséquent, les composés qui contiennent des groupes hydroxyles, tels que les alcools, forment facilement des liaisons hydrogène. Au sein du groupe hydroxyle, un dipôle se forme à mesure que l’oxygène plus électronégatif attire la densité électronique vers lui, ce qui donne à l’oxygène une charge négative partielle. Cela laisse également à l’hydrogène une charge positive partielle. Les oxygènes électronégatifs à proximité sont attirés par la charge positive partielle, formant une liaison hydrogène. Des trois types de forces intermoléculaires, la liaison hydrogène est la plus forte.</li>
	<li style="font-size :17px">Interactions dipôle-dipôle - Le deuxième type de force intermoléculaire le plus fort, les interactions dipôle-dipôle se forment dans les molécules qui contiennent des atomes électronégatifs tels que l’oxygène, l’azote et l’un des halogénures tels que le chlore et le fluor. Par exemple, une molécule d’hydrocarbure contenant du fluor va former des interactions dipôle-dipôle. Comment? L’atome de fluor électronégatif attirera la densité électronique vers lui, ce qui lui donnera une charge négative partielle. L’atome de connexion, le carbone, perd une partie de cette densité électronique et acquiert ainsi une charge négative partielle. Cela forme un dipôle temporaire au niveau de la liaison fluor-carbone. Lorsque les charges opposées s’attirent, le fluor partiellement négatif est attiré par le carbone partiellement positif d’une autre molécule voisine, formant une interaction dipôle-dipôle.</li>
	<li style="font-size :17px">Forces de dispersion de Londres - Ce type d’interaction est une forme des forces de van der Waals et est présent dans tous les composés. Les forces de dispersion de Londres sont le type le plus faible de forces intermoléculaires. Comme les interactions dipôle-dipôle, il y a une redistribution de la densité électronique autour de la molécule, provoquant la formation de charges temporaires. Contrairement aux interactions dipôle-dipôle, les dipôles formés à Londres sont très faibles et minimes. Par exemple, les composés non polaires comme le méthane, l’éthane, le pentane et l’octane interagissent via les forces de dispersion de Londres. La surface et la longueur de la molécule déterminent l’intensité des forces d’attraction, de sorte que les composés ayant une plus grande surface ont des forces de dispersion de London plus importantes que les composés plus petits. Par conséquent, l’octane aurait des forces de dispersion de London plus fortes que le méthane.</li>
</ol>
Chaque type de force intermoléculaire a une force d’attraction différente. Par conséquent, les composés contenant des liaisons hydrogène nécessitent plus d’énergie pour briser l’attraction entre les molécules qu’un composé non polaire qui n’a que des forces de dispersion de Londres. Ainsi, la présence de liaisons hydrogène augmente le point de fusion d’un composé.
<h4>L’effet des impuretés sur les points de fusion</h4>
Les valeurs des points de fusion rapportées dans la littérature supposent que vous disposez d’un échantillon pur du composé en question. Souvent en laboratoire ou dans des échantillons inconnus, les échantillons testés ne sont pas des composés purs. Les impuretés font que le point de fusion observé d’un mélange est inférieur à la température de fusion réelle du composé pur. La portée observable est plus grande que celle de la substance pure.
Dans un composé pur, le solide est composé d’une structure uniforme et ordonnée et nécessite une certaine quantité de température pour briser la structure afin que le composé passe en phase liquide. Dans un mélange contenant des impuretés, la phase solide est composée d’une structure désorganisée. Cela nécessite beaucoup moins d’énergie pour passer en phase liquide, ce qui abaisse le point de fusion. Ce phénomène est connu sous le nom de dépression du point de fusion. Plus il y a d’impuretés dans l’échantillon, plus la plage de point de fusion est large et plus la température de fusion est basse.

Points de fusion

Points de fusion en chimie organique
Le point de fusion d’un composé est la température à laquelle la phase solide passe à la phase liquide à un ...

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

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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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