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

Il punto di fusione di una sostanza è la temperatura alla quale tale sostanza inizia a passare dalla fase solida alla fase liquida. A questa temperatura, le fasi liquida e solida sono in equilibrio. Con ulteriore calore, la sostanza si scioglierà completamente. Ma cosa determina il punto di fusione di una sostanza? Pensiamo ai solidi e ai liquidi. Le molecole di un solido si tengono l'una con l'altra in una struttura rigida e ordinata chiamata reticolo, mentre le molecole di un liquido hanno interazioni più deboli e si muovono.
Il riscaldamento di un solido trasferisce energia alle molecole. Con abbastanza energia, le molecole superano le forze che le tengono nel reticolo e iniziano a muoversi. In altre parole, se riscaldiamo abbastanza un solido, si scioglie in un liquido. Quindi, il punto di fusione dipende dall'energia necessaria per superare le forze tra le molecole, o le forze intermolecolari, che le trattengono nel reticolo. Più forti sono le forze intermolecolari, maggiore è l'energia richiesta, quindi più alto è il punto di fusione.
Molte forze intermolecolari dipendono dalla forza con cui gli atomi nella molecola attraggono gli elettroni o dalla loro elettronegatività. L'azoto, l'ossigeno, il fluoro e il cloro sono altamente elettronegativi, mentre il carbonio, l'idrogeno e lo zolfo sono solo moderatamente elettronegativi. I legami tra atomi con elettronegatività significativamente diverse sono polari. Ad esempio, un tipico legame carbonio-ossigeno è polare, ma un tipico legame carbonio-idrogeno non lo è.
Gli elettroni di una molecola trascorrono più tempo intorno ai suoi atomi più elettronegativi, dandogli una leggera carica negativa su quel lato e una leggera carica positiva sull'altro lato. Questo è chiamato dipolo. Se il dipolo non è annullato da un dipolo uguale e uno contrario nella stessa molecola, la molecola ha un dipolo permanente ed è polare.
Ora, discutiamo tre importanti forze intermolecolari: legame idrogeno, interazioni dipolo-dipolo e forze di dispersione di London. Il legame idrogeno si verifica tra un atomo che si ritira di elettroni con una coppia solitaria di elettroni e un idrogeno legato a un atomo più elettronegativo. I legami idrogeno sono tra le forze intermolecolari più forti.
Le interazioni dipolo-dipolo si verificano tra molecole polari. In un'interazione dipolo-dipolo attrattivo, il lato negativo di un dipolo si allinea con il lato positivo di un altro dipolo. Le interazioni dipolo-dipolo sono generalmente più deboli dei legami idrogeno.
Le forze di dispersione di Londra derivano da brevi cambiamenti casuali nella distribuzione elettronica di una molecola, che causano cambiamenti corrispondenti nelle molecole vicine. Questi spostamenti casuali avvengono in ogni molecola, quindi questa è una delle poche interazioni disponibili per le molecole non polari. Le forze di dispersione di Londra sono tra le forze intermolecolari più deboli.
In precedenza, avevamo previsto che forze intermolecolari più forti corrispondevano a punti di fusione più elevati. Possiamo vederlo in azione con l'esadecano, il 2-esadecanone e l'acido esadecanoico. All'aumentare della forza delle interazioni intermolecolari disponibili per ciascuna molecola, aumenta anche il punto di fusione.
Le forze intermolecolari non sono l'unico fattore che determina il punto di fusione di una sostanza. La sua purezza influisce in modo significativo sui suoi punti di fusione e congelamento in un effetto chiamato "depressione del punto di congelamento". Questo effetto significa che una soluzione ha un punto di congelamento più basso rispetto al solvente puro. Ecco perché le strade vengono cosparse di sale quando fa molto freddo. Se dell'acqua si raccoglie per strada, il sale si dissolve rapidamente per creare una soluzione con un punto di congelamento molto più basso rispetto all'acqua pura.
In un solido, le impurità sono incorporate nella struttura reticolare. Queste aree hanno spesso interazioni intermolecolari più deboli, rendendo parti della struttura più facili da distruggere. Quindi, rispetto a un solido puro, la fusione inizia a una temperatura più bassa e avviene in un intervallo di temperature più ampio.
In questo laboratorio, misurerai i punti di fusione di due composti organici noti e quindi analizzerai una miscela per esplorare come le impurità influenzano l'intervallo del punto di fusione.

punti di fusione

<h4>Punti di fusione in chimica organica</h4>
Il punto di fusione di un composto è la temperatura alla quale la fase solida passa alla fase liquida a una pressione standard di 1 atmosfera. Il punto di fusione di un composto è una proprietà fisica, come la solubilità, la densità, il colore e l'elettronegatività che può essere utilizzata per identificare un composto. Determinare la temperatura esatta alla quale un composto inizia a sciogliersi è un compito impegnativo; Per questo motivo, il punto di fusione dei composti è riportato come intervallo. Il limite inferiore dell'intervallo del punto di fusione è la temperatura alla quale si osservano le prime gocce di liquido. Il limite superiore dell'intervallo è la temperatura alla quale tutta la fase solida è passata alla fase liquida. In letteratura esistono guide di riferimento con valori accettati, che vengono utilizzate per identificare i composti.
<h4>L'effetto delle forze intermolecolari sui punti di fusione</h4>
Uno dei principali fattori che influisce sul punto di fusione del composto è il tipo di forze intermolecolari che esistono all'interno del composto. Le forze intermolecolari sono attrattive o repulsive tra le molecole di un composto. Nella fase solida, le molecole di un composto formeranno una struttura reticolare organizzata poiché le molecole sono impacchettate vicine tra loro. Esistono tre tipi principali di forze intermolecolari:
<ol>
	<li style="font-size:17px">Legame idrogeno - Il legame idrogeno è un tipo di forza intermolecolare che si verifica a causa delle forze di attrazione tra un ossigeno elettronegativo e un atomo di idrogeno. Pertanto, affinché questo tipo di forza intermolecolare sia presente, il composto deve contenere ossigeno e idrogeno. Pertanto, i composti che contengono gruppi ossidrilici, come gli alcoli, formano facilmente legami idrogeno. All'interno del gruppo ossidrilico, si forma un dipolo quando l'ossigeno più elettronegativo attira la densità elettronica verso di esso, facendo sì che l'ossigeno abbia una carica negativa parziale. Questo lascia anche l'idrogeno con una carica positiva parziale. Gli ossigeni elettronegativi vicini sono attratti dalla carica parziale positiva, formando un legame idrogeno. Dei tre tipi di forze intermolecolari, il legame idrogeno è il più forte.</li>
	<li style="font-size:17px">Interazioni dipolo-dipolo - Il secondo tipo più forte di forza intermolecolare, le interazioni dipolo-dipolo si formano in molecole che contengono atomi elettronegativi come ossigeno, azoto e qualsiasi alogenuri come cloro e fluoro. Ad esempio, una molecola di idrocarburo contenente fluoro formerà interazioni dipolo-dipolo. Come? L'atomo di fluoro elettronegativo attirerà la densità elettronica verso di sé, facendogli avere una carica negativa parziale. L'atomo di connessione, il carbonio, perde parte di quella densità elettronica e quindi guadagna una carica negativa parziale. Questo forma un dipolo temporaneo in corrispondenza del legame fluoro-carbonio. Quando le cariche opposte si attraggono, il fluoro parzialmente negativo è attratto dal carbonio parzialmente positivo di un'altra molecola vicina, formando un'interazione dipolo-dipolo.</li>
	<li style="font-size:17px">Forze di dispersione di Londra - Questo tipo di interazione è una forma di forze di van der Waals ed è presente in tutti i composti. Le forze di dispersione di London sono il tipo più debole di forze intermolecolari. Come le interazioni dipolo-dipolo, c'è una ridistribuzione della densità elettronica attorno alla molecola, causando la formazione di cariche temporanee. A differenza delle interazioni dipolo-dipolo, i dipoli formati nelle forze di dispersione di Londra sono molto deboli e minimi. Ad esempio, i composti non polari come il metano, l'etano, il pentano e l'ottano interagiscono attraverso le forze di dispersione di London. L'area superficiale e la lunghezza della molecola determinano la forza delle forze attrattive, in modo tale che i composti con più area superficiale abbiano forze di dispersione di London maggiori rispetto ai composti più piccoli. Pertanto, l'ottano avrebbe forze di dispersione di Londra più forti del metano.</li>
</ol>
Ogni tipo di forza intermolecolare ha una diversa forza di attrazione. Pertanto, i composti contenenti legami idrogeno richiedono più energia per rompere l'attrazione tra le molecole rispetto a un composto non polare che ha solo forze di dispersione di Londra. Pertanto, la presenza di legami idrogeno aumenta il punto di fusione di un composto.
<h4>L'effetto delle impurità sui punti di fusione</h4>
I valori riportati in letteratura dei punti di fusione presuppongono che si disponga di un campione puro del composto in questione. Spesso in laboratorio o in campioni sconosciuti, i campioni da testare non sono composti puri. Le impurità fanno sì che il punto di fusione osservato di una miscela sia inferiore alla temperatura di fusione effettiva del composto puro. L'intervallo osservabile è maggiore di quello della sostanza pura.
In un composto puro, il solido è composto da una struttura uniforme e ordinata e richiede una certa quantità di temperatura per rompere la struttura affinché il composto passi alla fase liquida. In una miscela contenente impurità, la fase solida è composta da una struttura disorganizzata. Ciò richiede molta meno energia per passare alla fase liquida, abbassando così il punto di fusione. Questo fenomeno è noto come depressione del punto di fusione. Maggiore è il numero di impurità nel campione, più ampio è l'intervallo del punto di fusione e minore è la temperatura di fusione.

Punti di fusione

Punti di fusione in chimica organica
Il punto di fusione di un composto è la temperatura alla quale la fase solida passa alla fase liquida ad 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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Video completi e incentrati sul curriculum sia per gli istruttori che per gli studenti nei corsi di laboratorio di Chimica introduttiva.

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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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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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