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

O ponto de fusão de uma substância é a temperatura na qual essa substância começa a mudar da fase sólida para a fase líquida. Nessa temperatura, as fases líquida e sólida estão em equilíbrio. Com calor adicional, a substância derreterá completamente. Mas o que determina o ponto de fusão de uma substância? Vamos pensar em sólidos e líquidos. As moléculas de um sólido se mantêm em uma estrutura rígida e ordenada chamada rede, enquanto as moléculas de um líquido têm interações mais fracas e se movem.
O aquecimento de um sólido transfere energia para as moléculas. Com energia suficiente, as moléculas superam as forças que as mantêm na rede e começam a se mover. Em outras palavras, se aquecermos um sólido o suficiente, ele derrete em um líquido. Portanto, o ponto de fusão depende da energia necessária para superar as forças entre as moléculas, ou as forças intermoleculares, mantendo-as na rede. Quanto mais fortes forem as forças intermoleculares, mais energia é necessária, portanto, maior será o ponto de fusão.
Muitas forças intermoleculares dependem de quão fortemente os átomos da molécula atraem elétrons - ou de sua eletronegatividade. Nitrogênio, oxigênio, flúor e cloro são altamente eletronegativos, enquanto carbono, hidrogênio e enxofre são apenas moderadamente eletronegativos. As ligações entre átomos com eletronegatividades significativamente diferentes são polares. Por exemplo, uma ligação carbono-oxigênio típica é polar, mas uma ligação carbono-hidrogênio típica não é.
Os elétrons de uma molécula passam mais tempo em torno de seus átomos mais eletronegativos, dando-lhe uma leve carga negativa desse lado e uma leve carga positiva do outro lado. Isso é chamado de dipolo. Se o dipolo não for cancelado por um dipolo igual e oposto na mesma molécula, a molécula tem um dipolo permanente e é polar.
Agora, vamos discutir três forças intermoleculares importantes: ligações de hidrogênio, interações dipolo-dipolo e forças de dispersão de London. A ligação de hidrogênio ocorre entre um átomo que retira elétrons com um par de elétrons solitário e um hidrogênio ligado a um átomo mais eletronegativo. As ligações de hidrogênio estão entre as forças intermoleculares mais fortes.
As interações dipolo-dipolo ocorrem entre moléculas polares. Em uma interação dipolo-dipolo atraente, o lado negativo de um dipolo se alinha com o lado positivo de outro dipolo. As interações dipolo-dipolo são geralmente mais fracas do que as ligações de hidrogênio.
As forças de dispersão de London vêm de mudanças breves e aleatórias na distribuição de elétrons de uma molécula, que causam mudanças correspondentes em moléculas próximas. Essas mudanças aleatórias acontecem em todas as moléculas, então esta é uma das poucas interações disponíveis para moléculas apolares. As forças de dispersão de Londres estão entre as forças intermoleculares mais fracas.
Anteriormente, previmos que forças intermoleculares mais fortes correspondiam a pontos de fusão mais altos. Podemos ver isso em ação com hexadecano, 2-hexadecanona e ácido hexadecanóico. À medida que a força das interações intermoleculares disponíveis para cada molécula aumenta, o mesmo acontece com o ponto de fusão.
As forças intermoleculares não são o único fator que determina o ponto de fusão de uma substância. Sua pureza afeta significativamente seus pontos de fusão e congelamento em um efeito chamado 'depressão do ponto de congelamento'. Este efeito significa que uma solução tem um ponto de congelamento mais baixo do que o solvente puro. É por isso que as ruas são polvilhadas com sal quando fica muito frio. Se alguma água se acumular na rua, o sal se dissolve rapidamente para formar uma solução com um ponto de congelamento muito mais baixo do que a água pura.
Em um sólido, as impurezas são incorporadas à estrutura da rede. Essas áreas geralmente têm interações intermoleculares mais fracas, tornando partes da estrutura mais fáceis de interromper. Assim, em comparação com um sólido puro, a fusão começa em uma temperatura mais baixa e ocorre em uma faixa de temperatura mais ampla.
Neste laboratório, você medirá os pontos de fusão de dois compostos orgânicos conhecidos e, em seguida, analisará uma mistura para explorar como as impurezas afetam a faixa do ponto de fusão.

pontos de fusão

<h4>Pontos de fusão em química orgânica</h4>
O ponto de fusão de um composto é a temperatura na qual a fase sólida faz a transição para a fase líquida a uma pressão padrão de 1 atmosfera. O ponto de fusão de um composto é uma propriedade física, como solubilidade, densidade, cor e eletronegatividade, que pode ser usada para identificar um composto. Determinar a temperatura exata na qual um composto começa a derreter é uma tarefa desafiadora; Por causa disso, o ponto de fusão dos compostos é relatado como um intervalo. O limite inferior da faixa do ponto de fusão é a temperatura na qual as primeiras gotas de líquido são observadas. O limite superior da faixa é a temperatura na qual toda a fase sólida fez a transição para a fase líquida. Existem guias de referência com valores aceitos na literatura, que são usados para identificar compostos.
<h4>O efeito das forças intermoleculares nos pontos de fusão</h4>
Um fator importante que afeta o ponto de fusão do composto é o tipo de forças intermoleculares que existem dentro do composto. As forças intermoleculares são atrativas ou repulsivas entre as moléculas de um composto. Na fase sólida, as moléculas de um composto formarão uma estrutura de rede organizada à medida que as moléculas são compactadas juntas. Existem três tipos principais de forças intermoleculares:
<ol>
	<li style="font-size:17px">Ligação de hidrogênio – A ligação de hidrogênio é um tipo de força intermolecular que ocorre devido às forças de atração entre um oxigênio eletronegativo e um átomo de hidrogênio. Portanto, para que esse tipo de força intermolecular esteja presente, o composto deve conter oxigênio e hidrogênio. Portanto, compostos que contêm grupos hidroxila, como álcoois, formam prontamente ligações de hidrogênio. Dentro do grupo hidroxila, um dipolo se forma à medida que o oxigênio mais eletronegativo puxa a densidade eletrônica em sua direção, fazendo com que o oxigênio tenha uma carga parcial negativa. Isso também deixa o hidrogênio com uma carga positiva parcial. Os oxigênios eletronegativos próximos são atraídos para a carga positiva parcial, formando uma ligação de hidrogênio. Dos três tipos de forças intermoleculares, a ligação de hidrogênio é a mais forte.</li>
	<li style="font-size:17px">Interações dipolo-dipolo – O segundo tipo mais forte de força intermolecular, as interações dipolo-dipolo se formam em moléculas que contêm átomos eletronegativos, como oxigênio, nitrogênio e qualquer um dos halogenetos, como cloro e flúor. Por exemplo, uma molécula de hidrocarboneto contendo flúor formará interações dipolo-dipolo. Como? O átomo de flúor eletronegativo puxará a densidade eletrônica em sua direção, fazendo com que ele tenha uma carga parcial negativa. O átomo de conexão, o carbono, perde parte dessa densidade eletrônica e, portanto, ganha uma carga negativa parcial. Isso forma um dipolo temporário na ligação flúor-carbono. À medida que cargas opostas se atraem, o flúor parcialmente negativo é atraído pelo carbono parcialmente positivo de outra molécula vizinha, formando uma interação dipolo-dipolo.</li>
	<li style="font-size:17px">Forças de dispersão de Londres – Este tipo de interação é uma forma de forças de van der Waals e está presente em todos os compostos. As forças de dispersão de Londres são o tipo mais fraco de forças intermoleculares. Como as interações dipolo-dipolo, há uma redistribuição da densidade eletrônica ao redor da molécula, causando a formação de cargas temporárias. Ao contrário das interações dipolo-dipolo, os dipolos formados nas forças de dispersão de Londres são muito fracos e mínimos. Por exemplo, compostos apolares como metano, etano, pentano e octano interagem por meio de forças de dispersão de London. A área de superfície e o comprimento da molécula determinam a força das forças de atração, de modo que os compostos com mais área de superfície têm maiores forças de dispersão de London do que os compostos menores. Portanto, a octanagem teria forças de dispersão de Londres mais fortes do que o metano.</li>
</ol>
Cada tipo de força intermolecular tem uma força de atração diferente. Portanto, compostos contendo ligações de hidrogênio requerem mais energia para quebrar a atração entre moléculas do que um composto apolar que possui apenas forças de dispersão de London. Assim, a presença de ligações de hidrogênio aumenta o ponto de fusão de um composto.
<h4>O efeito das impurezas nos pontos de fusão</h4>
Os valores relatados na literatura dos pontos de fusão pressupõem que você tenha uma amostra pura do composto em questão. Muitas vezes, no laboratório ou em amostras desconhecidas, as amostras testadas não são compostos puros. As impurezas fazem com que o ponto de fusão observado de uma mistura seja menor do que a temperatura real de fusão do composto puro. O alcance observável é maior do que o da substância pura.
Em um composto puro, o sólido é composto de uma estrutura uniforme e ordenada e requer uma certa quantidade de temperatura para quebrar a estrutura para que o composto faça a transição para a fase líquida. Em uma mistura contendo impurezas, a fase sólida é composta por uma estrutura desorganizada. Isso requer muito menos energia para fazer a transição para a fase líquida, diminuindo assim o ponto de fusão. Esse fenômeno é conhecido como depressão do ponto de fusão. Quanto mais impurezas na amostra, mais ampla a faixa do ponto de fusão e menor a temperatura de fusão.

Pontos de fusão

Pontos de fusão em química orgânica
O ponto de fusão de um composto é a temperatura na qual a fase sólida faz a transição para a fase líquida em um ...

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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.

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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