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

El punto de fusión de una sustancia es la temperatura a la que esa sustancia comienza a cambiar de la fase sólida a la fase líquida. A esta temperatura, las fases líquida y sólida están en equilibrio. Con calor adicional, la sustancia se derretirá por completo. Pero, ¿qué determina el punto de fusión de una sustancia? Pensemos en sólidos y líquidos. Las moléculas de un sólido se mantienen entre sí en una estructura rígida y ordenada llamada red, mientras que las moléculas de un líquido tienen interacciones más débiles y se mueven.
El calentamiento de un sólido transfiere energía a las moléculas. Con suficiente energía, las moléculas superan las fuerzas que las mantienen en la red y comienzan a moverse. En otras palabras, si calentamos un sólido lo suficiente, se funde en un líquido. Por lo tanto, el punto de fusión depende de la energía que se necesita para superar las fuerzas entre las moléculas, o las fuerzas intermoleculares, que las mantienen en la red. Cuanto más fuertes son las fuerzas intermoleculares, más energía se requiere, por lo que mayor es el punto de fusión.
Muchas fuerzas intermoleculares dependen de la fuerza con la que los átomos de la molécula atraen electrones, o de su electronegatividad. El nitrógeno, el oxígeno, el flúor y el cloro son altamente electronegativos, mientras que el carbono, el hidrógeno y el azufre son solo moderadamente electronegativos. Los enlaces entre átomos con electronegatividades significativamente diferentes son polares. Por ejemplo, un enlace carbono-oxígeno típico es polar, pero un enlace carbono-hidrógeno típico no lo es.
Los electrones de una molécula pasan más tiempo alrededor de sus átomos más electronegativos, lo que le da una ligera carga negativa en ese lado y una ligera carga positiva en el otro lado. A esto se le llama dipolo. Si el dipolo no es cancelado por un dipolo igual y opuesto en la misma molécula, la molécula tiene un dipolo permanente y es polar.
Ahora, analicemos tres fuerzas intermoleculares importantes: enlaces de hidrógeno, interacciones dipolo-dipolo y fuerzas de dispersión de London. El enlace de hidrógeno ocurre entre un átomo que retira electrones con un par solitario de electrones y un hidrógeno unido a un átomo más electronegativo. Los enlaces de hidrógeno se encuentran entre las fuerzas intermoleculares más fuertes.
Las interacciones dipolo-dipolo se producen entre moléculas polares. En una interacción atractiva dipolo-dipolo, el lado negativo de un dipolo se alinea con el lado positivo de otro dipolo. Las interacciones dipolo-dipolo son generalmente más débiles que los enlaces de hidrógeno.
Las fuerzas de dispersión de Londres provienen de cambios breves y aleatorios en la distribución de electrones de una molécula, que causan los cambios correspondientes en las moléculas cercanas. Estos cambios aleatorios ocurren en todas las moléculas, por lo que esta es una de las pocas interacciones disponibles para las moléculas no polares. Las fuerzas de dispersión de London se encuentran entre las fuerzas intermoleculares más débiles.
Anteriormente, predijimos que las fuerzas intermoleculares más fuertes correspondían a puntos de fusión más altos. Podemos ver esto en acción con el hexadecano, la 2-hexadecanona y el ácido hexadecanoico. A medida que aumenta la fuerza de las interacciones intermoleculares disponibles para cada molécula, también lo hace el punto de fusión.
Las fuerzas intermoleculares no son el único factor que determina el punto de fusión de una sustancia. Su pureza afecta significativamente a sus puntos de fusión y congelación en un efecto llamado "depresión del punto de congelación". Este efecto significa que una solución tiene un punto de congelación más bajo que el solvente puro. Es por eso que las calles se rocían con sal cuando hace mucho frío. Si se acumula agua en la calle, la sal se disuelve rápidamente para formar una solución con un punto de congelación mucho más bajo que el agua pura.
En un sólido, las impurezas se incorporan a la estructura de la red. Estas áreas a menudo tienen interacciones intermoleculares más débiles, lo que hace que partes de la estructura sean más fáciles de interrumpir. Por lo tanto, en comparación con un sólido puro, la fusión comienza a una temperatura más baja y se produce en un rango de temperatura más amplio.
En este laboratorio, medirá los puntos de fusión de dos compuestos orgánicos conocidos y luego analizará una mezcla para explorar cómo las impurezas afectan el rango del punto de fusión.

puntos de fusión

<h4>Puntos de fusión en química orgánica</h4>
El punto de fusión de un compuesto es la temperatura a la que la fase sólida pasa a la fase líquida a una presión estándar de 1 atmósfera. El punto de fusión de un compuesto es una propiedad física, como la solubilidad, la densidad, el color y la electronegatividad que se puede utilizar para identificar un compuesto. Determinar la temperatura exacta a la que un compuesto comienza a derretirse es una tarea desafiante; Debido a esto, el punto de fusión de los compuestos se informa como un rango. El límite inferior del rango del punto de fusión es la temperatura a la que se observan las primeras gotas de líquido. El límite superior del rango es la temperatura a la que toda la fase sólida ha pasado a la fase líquida. En la literatura existen guías de referencia con valores aceptados, que se utilizan para identificar los compuestos.
<h4>El efecto de las fuerzas intermoleculares en los puntos de fusión</h4>
Un factor importante que afecta el punto de fusión del compuesto es el tipo de fuerzas intermoleculares que existen dentro del compuesto. Las fuerzas intermoleculares son atractivas o repulsivas entre las moléculas de un compuesto. En la fase sólida, las moléculas de un compuesto formarán una estructura reticular organizada a medida que las moléculas se agrupan juntas. Hay tres tipos principales de fuerzas intermoleculares:
<ol>
	<li style="font-size:17px">Enlace de hidrógeno - El enlace de hidrógeno es un tipo de fuerza intermolecular que ocurre debido a las fuerzas de atracción entre un oxígeno electronegativo y un átomo de hidrógeno. Por lo tanto, para que este tipo de fuerza intermolecular esté presente, el compuesto debe contener oxígeno e hidrógeno. Por lo tanto, los compuestos que contienen grupos hidroxilo, como los alcoholes, forman fácilmente enlaces de hidrógeno. Dentro del grupo hidroxilo, se forma un dipolo a medida que el oxígeno más electronegativo atrae la densidad de electrones hacia él, lo que hace que el oxígeno tenga una carga negativa parcial. Esto también deja al hidrógeno con una carga positiva parcial. Los oxígenos electronegativos cercanos son atraídos por la carga positiva parcial, formando un enlace de hidrógeno. De los tres tipos de fuerzas intermoleculares, los enlaces de hidrógeno son los más fuertes.</li>
	<li style="font-size:17px">Interacciones Dipolo-Dipolo – El segundo tipo más fuerte de fuerza intermolecular, las interacciones dipolo-dipolo se forman en moléculas que contienen átomos electronegativos como oxígeno, nitrógeno y cualquiera de los haluros como el cloro y el flúor. Por ejemplo, una molécula de hidrocarburo que contiene flúor formará interacciones dipolo-dipolo. ¿Cómo? El átomo de flúor electronegativo tirará de la densidad de electrones hacia él, haciendo que tenga una carga negativa parcial. El átomo de conexión, el carbono, pierde parte de esa densidad de electrones y, por lo tanto, obtiene una carga negativa parcial. Esto forma un dipolo temporal en el enlace flúor-carbono. A medida que las cargas opuestas se atraen, el flúor parcialmente negativo es atraído por el carbono parcialmente positivo de otra molécula vecina, formando una interacción dipolo-dipolo.</li>
	<li style="font-size:17px">London dispersion forces – Este tipo de interacción es una forma de las fuerzas de van der Waals y está presente en todos los compuestos. Las fuerzas de dispersión de London son el tipo más débil de fuerzas intermoleculares. Al igual que las interacciones dipolo-dipolo, hay una redistribución de la densidad de electrones alrededor de la molécula, lo que provoca la formación de cargas temporales. A diferencia de las interacciones dipolo-dipolo, los dipolos formados en las fuerzas de dispersión de Londres son muy débiles y mínimas. Por ejemplo, los compuestos no polares como el metano, el etano, el pentano y el octano interactúan a través de las fuerzas de dispersión de Londres. El área superficial y la longitud de la molécula determinan la fuerza de las fuerzas de atracción, de modo que los compuestos con más área de superficie tienen mayores fuerzas de dispersión de London que los compuestos más pequeños. Por lo tanto, el octanaje tendría fuerzas de dispersión de Londres más fuertes que el metano.</li>
</ol>
Cada tipo de fuerza intermolecular tiene una fuerza de atracción diferente. Por lo tanto, los compuestos que contienen enlaces de hidrógeno requieren más energía para romper la atracción entre las moléculas que un compuesto no polar que solo tiene fuerzas de dispersión de London. Por lo tanto, la presencia de enlaces de hidrógeno aumenta el punto de fusión de un compuesto.
<h4>El efecto de las impurezas en los puntos de fusión</h4>
Los valores de los puntos de fusión reportados en la literatura asumen que tiene una muestra pura del compuesto en cuestión. A menudo, en el laboratorio o en muestras desconocidas, las muestras que se analizan no son compuestos puros. Las impurezas hacen que el punto de fusión observado de una mezcla sea más bajo que la temperatura de fusión real del compuesto puro. El rango observable es mayor que el de la sustancia pura.
En un compuesto puro, el sólido está compuesto por una estructura uniforme y ordenada y requiere una cierta cantidad de temperatura para romper la estructura y que el compuesto pase a la fase líquida. En una mezcla que contiene impurezas, la fase sólida está compuesta por una estructura desorganizada. Esto requiere mucha menos energía para pasar a la fase líquida, lo que reduce el punto de fusión. Este fenómeno se conoce como depresión del punto de fusión. Cuantas más impurezas haya en la muestra, más amplio será el rango de punto de fusión y menor será la temperatura de fusión.

Puntos de fusión

Puntos de fusión en química orgánica
El punto de fusión de un compuesto es la temperatura a la que la fase sólida pasa a la fase líquida ...

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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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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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