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

Der Schmelzpunkt eines Stoffes ist die Temperatur, bei der dieser Stoff beginnt, von der festen Phase in die flüssige Phase zu wechseln. Bei dieser Temperatur befinden sich die flüssige und die feste Phase im Gleichgewicht. Mit zusätzlicher Hitze schmilzt die Substanz vollständig. Doch was bestimmt den Schmelzpunkt einer Substanz? Denken wir an Feststoffe und Flüssigkeiten. Die Moleküle eines Festkörpers halten sich gegenseitig in einer starren, geordneten Struktur, die als Gitter bezeichnet wird, während die Moleküle einer Flüssigkeit schwächere Wechselwirkungen aufweisen und sich bewegen.
Durch das Erhitzen eines Feststoffs wird Energie auf die Moleküle übertragen. Mit genügend Energie überwinden die Moleküle die Kräfte, die sie im Gitter halten, und beginnen sich zu bewegen. Mit anderen Worten, wenn wir einen Feststoff genug erhitzen, schmilzt er zu einer Flüssigkeit. Der Schmelzpunkt hängt also von der Energie ab, die benötigt wird, um die Kräfte zwischen den Molekülen oder die intermolekularen Kräfte zu überwinden, die sie im Gitter halten. Je stärker die intermolekularen Kräfte sind, desto mehr Energie wird benötigt, also desto höher ist der Schmelzpunkt.
Viele intermolekulare Kräfte hängen davon ab, wie stark die Atome im Molekül Elektronen anziehen – oder von ihrer Elektronegativität. Stickstoff, Sauerstoff, Fluor und Chlor sind stark elektronegativ, während Kohlenstoff, Wasserstoff und Schwefel nur mäßig elektronegativ sind. Bindungen zwischen Atomen mit deutlich unterschiedlichen Elektronegativitäten sind polar. Zum Beispiel ist eine typische Kohlenstoff-Sauerstoff-Bindung polar, eine typische Kohlenstoff-Wasserstoff-Bindung jedoch nicht.
Die Elektronen eines Moleküls verbringen mehr Zeit um seine elektronegativsten Atome, wodurch es auf dieser Seite eine leichte negative Ladung und auf der anderen Seite eine leichte positive Ladung erhält. Dies wird als Dipol bezeichnet. Wenn der Dipol nicht durch einen gleichen und entgegengesetzten Dipol im selben Molekül aufgehoben wird, hat das Molekül einen permanenten Dipol und ist polar.
Lassen Sie uns nun drei wichtige intermolekulare Kräfte diskutieren: Wasserstoffbrückenbindungen, Dipol-Dipol-Wechselwirkungen und Londoner Dispersionskräfte. Wasserstoffbrückenbindungen treten zwischen einem elektronenziehenden Atom mit einem einsamen Elektronenpaar und einem Wasserstoff auf, der an ein elektronegatives Atom gebunden ist. Wasserstoffbrückenbindungen gehören zu den stärksten intermolekularen Kräften.
Dipol-Dipol-Wechselwirkungen treten zwischen polaren Molekülen auf. Bei einer attraktiven Dipol-Dipol-Wechselwirkung richtet sich die negative Seite eines Dipols mit der positiven Seite eines anderen Dipols aus. Dipol-Dipol-Wechselwirkungen sind im Allgemeinen schwächer als Wasserstoffbrückenbindungen.
Londoner Dispersionskräfte entstehen durch kurze, zufällige Verschiebungen in der Elektronenverteilung eines Moleküls, die entsprechende Verschiebungen in benachbarten Molekülen verursachen. Diese zufälligen Verschiebungen finden in jedem Molekül statt, so dass dies eine der wenigen Wechselwirkungen ist, die für unpolare Moleküle verfügbar sind. Die Londoner Dispersionskräfte gehören zu den schwächsten intermolekularen Kräften.
Früher haben wir vorhergesagt, dass stärkere intermolekulare Kräfte mit höheren Schmelzpunkten einhergehen. Wir können dies in Aktion mit Hexadecan, 2-Hexadecanon und Hexadecansäure sehen. Mit zunehmender Stärke der intermolekularen Wechselwirkungen, die jedem Molekül zur Verfügung stehen, steigt auch der Schmelzpunkt.
Intermolekulare Kräfte sind nicht der einzige Faktor, der den Schmelzpunkt einer Substanz bestimmt. Seine Reinheit beeinflusst seine Schmelz- und Gefrierpunkte erheblich, was als "Gefrierpunkt-Depression" bezeichnet wird. Dieser Effekt bedeutet, dass eine Lösung einen niedrigeren Gefrierpunkt hat als das reine Lösungsmittel. Deshalb werden die Straßen mit Salz bestreut, wenn es sehr kalt wird. Wenn sich Wasser auf der Straße ansammelt, löst sich das Salz schnell auf und bildet eine Lösung mit einem viel niedrigeren Gefrierpunkt als reines Wasser.
Bei einem Festkörper werden Verunreinigungen in die Gitterstruktur eingearbeitet. Diese Bereiche weisen oft schwächere intermolekulare Wechselwirkungen auf, wodurch Teile der Struktur leichter zu stören sind. Im Vergleich zu einem reinen Feststoff beginnt das Schmelzen also bei einer niedrigeren Temperatur und erfolgt über einen breiteren Temperaturbereich.
In diesem Labor messen Sie die Schmelzpunkte von zwei bekannten organischen Verbindungen und analysieren dann ein Gemisch, um zu untersuchen, wie sich Verunreinigungen auf den Schmelzpunktbereich auswirken.

schmelzpunkte

<h4>Schmelzpunkte in der organischen Chemie</h4>
Der Schmelzpunkt einer Verbindung ist die Temperatur, bei der die feste Phase bei einem Standarddruck von 1 Atmosphäre in die flüssige Phase übergeht. Der Schmelzpunkt einer Verbindung ist eine physikalische Eigenschaft wie Löslichkeit, Dichte, Farbe und Elektronegativität, die zur Identifizierung einer Verbindung verwendet werden kann. Die genaue Temperatur zu bestimmen, bei der eine Verbindung zu schmelzen beginnt, ist eine schwierige Aufgabe. Aus diesem Grund wird der Schmelzpunkt von Verbindungen als Bereich angegeben. Die untere Grenze des Schmelzpunktbereichs ist die Temperatur, bei der die ersten Tropfen Flüssigkeit beobachtet werden. Die Obergrenze des Bereichs ist die Temperatur, bei der die gesamte feste Phase in die flüssige Phase übergegangen ist. In der Literatur gibt es Referenzleitfäden mit akzeptierten Werten, die zur Identifizierung von Verbindungen verwendet werden.
<h4>Die Wirkung intermolekularer Kräfte auf die Schmelzpunkte</h4>
Ein wichtiger Faktor, der den Schmelzpunkt der Verbindung beeinflusst, ist die Art der intermolekularen Kräfte, die in der Verbindung vorhanden sind. Intermolekulare Kräfte sind zwischen den Molekülen einer Verbindung entweder anziehend oder abstoßend. In der festen Phase bilden die Moleküle einer Verbindung eine organisierte Gitterstruktur, da die Moleküle dicht beieinander liegen. Es gibt drei Haupttypen von intermolekularen Kräften:
<ol>
	<li style="font-size:17px">Wasserstoffbrückenbindung – Wasserstoffbrückenbindung ist eine Art intermolekularer Kraft, die aufgrund der Anziehungskräfte zwischen einem elektronegativen Sauerstoff und einem Wasserstoffatom auftritt. Damit diese Art von intermolekularer Kraft vorhanden ist, muss die Verbindung daher Sauerstoff und Wasserstoff enthalten. Daher bilden Verbindungen, die Hydroxylgruppen enthalten, wie z. B. Alkohole, leicht Wasserstoffbrückenbindungen. Innerhalb der Hydroxylgruppe bildet sich ein Dipol, wenn der elektronegative Sauerstoff die Elektronendichte zu sich zieht, wodurch der Sauerstoff eine teilweise negative Ladung hat. Dadurch bleibt auch der Wasserstoff teilweise positiv geladen. In der Nähe befindliche elektronegative Sauerstoffe werden von der partiellen positiven Ladung angezogen und bilden eine Wasserstoffbrückenbindung. Von den drei Arten intermolekularer Kräfte ist die Wasserstoffbrückenbindung die stärkste.</li>
	<li style="font-size:17px">Dipol-Dipol-Wechselwirkungen – Die zweitstärkste Art der intermolekularen Kraft, Dipol-Dipol-Wechselwirkungen, bilden sich in Molekülen, die elektronegative Atome wie Sauerstoff, Stickstoff und alle Halogenide wie Chlor und Fluor enthalten. Zum Beispiel bildet ein Kohlenwasserstoffmolekül, das Fluor enthält, Dipol-Dipol-Wechselwirkungen. Wie? Das elektronegative Fluoratom zieht die Elektronendichte zu sich heran, wodurch es eine teilweise negative Ladung hat. Das verbindende Atom, Kohlenstoff, verliert einen Teil dieser Elektronendichte und erhält so eine teilweise negative Ladung. Dadurch bildet sich ein temporärer Dipol an der Fluor-Kohlenstoff-Bindung. Wenn sich entgegengesetzte Ladungen anziehen, wird das teilweise negative Fluor vom teilweise positiven Kohlenstoff eines anderen Nachbarmoleküls angezogen, wodurch eine Dipol-Dipol-Wechselwirkung entsteht.</li>
	<li style="font-size:17px">Londoner Dispersionskräfte – Diese Art der Wechselwirkung ist eine Form der Van-der-Waals-Kräfte und ist in allen Verbindungen vorhanden. Die Londoner Dispersionskräfte sind die schwächste Art der intermolekularen Kräfte. Wie bei Dipol-Dipol-Wechselwirkungen kommt es zu einer Umverteilung der Elektronendichte um das Molekül, was zur Bildung temporärer Ladungen führt. Im Gegensatz zu Dipol-Dipol-Wechselwirkungen sind die in London gebildeten Dispersionskräfte sehr schwach und minimal. Zum Beispiel interagieren unpolare Verbindungen wie Methan, Ethan, Pentan und Oktan über die Londoner Dispersionskräfte. Die Oberfläche und Länge des Moleküls bestimmt die Stärke der Anziehungskräfte, so dass Verbindungen mit mehr Oberfläche größere Londoner Dispersionskräfte aufweisen als kleinere Verbindungen. Daher hätte Oktan stärkere Londoner Ausbreitungskräfte als Methan.</li>
</ol>
Jede Art von intermolekularer Kraft hat eine unterschiedliche Anziehungsstärke. Daher benötigen Verbindungen, die Wasserstoffbrückenbindungen enthalten, mehr Energie, um die Anziehungskraft zwischen Molekülen zu brechen, als eine unpolare Verbindung, die nur Londoner Dispersionskräfte hat. Somit erhöht das Vorhandensein von Wasserstoffbrückenbindungen den Schmelzpunkt einer Verbindung.
<h4>Die Wirkung von Verunreinigungen auf die Schmelzpunkte</h4>
In der Literatur wird davon ausgegangen, dass Sie eine reine Probe der betreffenden Verbindung haben. Oft im Labor oder in unbekannten Proben handelt es sich bei den untersuchten Proben nicht um reine Verbindungen. Verunreinigungen führen dazu, dass der beobachtete Schmelzpunkt einer Mischung niedriger ist als die tatsächliche Schmelztemperatur der reinen Verbindung. Der beobachtbare Bereich ist größer als der der reinen Substanz.
In einer reinen Verbindung besteht der Feststoff aus einer gleichmäßigen und geordneten Struktur und benötigt eine bestimmte Temperatur, um die Struktur auseinander zu brechen, damit die Verbindung in die flüssige Phase übergehen kann. In einem Gemisch, das Verunreinigungen enthält, besteht die feste Phase aus einer unorganisierten Struktur. Dadurch wird viel weniger Energie benötigt, um in die flüssige Phase überzugehen, wodurch der Schmelzpunkt gesenkt wird. Dieses Phänomen wird als Schmelzpunktssenkung bezeichnet. Je mehr Verunreinigungen sich in der Probe befinden, desto breiter ist der Schmelzpunktbereich und desto niedriger ist die Schmelztemperatur.

Schmelzpunkte

Schmelzpunkte in der organischen Chemie
Der Schmelzpunkt einer Verbindung ist die Temperatur, bei der die feste Phase bei einem ...

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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...

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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

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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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