Name: hydrophobic effects in solvation
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 5 s
Channel: JoVE

K12 Standards

Tennessee Academic Standards

High School

Chemistry

motion-and-stability-forces-and-interactions

Motion and Stability: Forces and Interactions

CHEM1.PS2.3

Entropy and Solvation

Die Entropie, S, spiegelt die Wahrscheinlichkeit wider, einen bestimmten Zustand in einem System zu erreichen. In einem isolierten System tritt spontan ein Prozess auf, der eine Zunahme der Gesamtentropie verursacht.
Die Änderung der Entropie, die sich aus der Lösungsbildung ergibt, die als Entropie des Mischens oder ΔS-Mischung bezeichnet wird, ist unabhängig von intermolekularen Wechselwirkungen.
Während der Lösungsbildung werden der gelöste Stoff und das Lösungsmittel vermischt, und der gelöste Stoff dispergiert sich in dem Lösungsmittel. Die Lösungsmittelmoleküle, die direkt mit den gelösten Molekülen interagieren, werden zusammenfassend als Lösungsmittelhülle oder Lösungsmittelkäfig bezeichnet.
Aufgrund der Bildung von Lösungsmittelhüllen hat das Lösungsmittel weniger energetisch äquivalente Konfigurationen als der gelöste Stoff, und der gelöste Stoff gewinnt Entropie auf Kosten der Energie aus dem Lösungsmittel.
Wenn ein Kohlenwasserstoff in Wasser gelöst wird, ordnen sich die Wassermoleküle an der Wasser-Kohlenwasserstoff-Grenzfläche neu an, um die Anzahl der Wasserstoffbrückenbindungen, die sie miteinander eingehen, zu maximieren. Ein Teil des Lösungsmittelwassers wird in das Wasser der Lösungsmittelhüllen umgewandelt, was zu einem Lösungsmittelkäfig um jedes Kohlenwasserstoffmolekül führt.
Das Wasser in der Lösungsmittelhülle ist im Vergleich zum Lösungsmittelwasser geordneter angeordnet und hat eine geringere Bewegungsfreiheit. Dadurch wird die Entropie des Wassers in der Lösungsmittelhülle im Vergleich zu der des Lösungsmittelwassers verringert. Die Auflösung geht also mit einer Verringerung der Entropie einher.
Wenn die Kohlenwasserstoffmoleküle verklumpen, wird das Solvatationswasser mit niedriger Entropie freigesetzt, um zu Lösungsmittelwasser mit höherer Entropie zu werden, wodurch die Entropie erhöht wird.
Diese entropiegetriebene Trennung von Kohlenwasserstoff- und Wassermolekülen wird als hydrophober Effekt bezeichnet. Die Verklumpung von Kohlenwasserstoffen wird aufgrund der damit verbundenen Zunahme der Entropie begünstigt, was zu getrennten Kohlenwasserstoff- und Wasserschichten führt.

entropie und solvatation

Der Vorgang, eine gelöste Substanz mit Lösungsmittel zu umgeben, wird Solvatation genannt. Dabei geht es darum, die gelöste Substanz gleichmäßig im Lösungsmittel zu verteilen. Die Faustregel zur Bestimmung eines Lösungsmittels für eine bestimmte Verbindung lautet: Gleiches löst Gleiches. Ein gutes Lösungsmittel hat ähnliche molekulare Eigenschaften wie die zu lösende Verbindung. So lösen beispielsweise polare Lösungen polare Substanzen und unpolare Lösungsmittel lösen unpolare Substanzen. Ein polares Lösungsmittel ist ein Lösungsmittel mit einer hohen Dielektrizitätskonstante (ϵ ≥ 15); Ein unpolares Lösungsmittel ist eines mit einer niedrigen Dielektrizitätskonstante. Die Dielektrizitätskonstante wird durch das elektrostatische Gesetz definiert, das die Wechselwirkungsenergie E zwischen zwei Ionen mit den jeweiligen Ladungen q1 und q2 angibt, die durch einen Abstand r voneinander getrennt sind. Ein polares Lösungsmittel trennt oder schirmt Ionen effektiv voneinander ab. Daher ist die Tendenz entgegengesetzt geladener Ionen zur Assoziation in einem polaren Lösungsmittel geringer als in einem unpolaren Lösungsmittel.

Im Fall von Kohlenwasserstoff und Wasser ist eines polar (Wasser) und das andere unpolar (Kohlenwasserstoff). Beim Einbringen von Kohlenwasserstoffmolekülen in Wasser bilden die Wassermoleküle entlang der Kohlenwasserstoff-Wasser-Grenzfläche eine schalenartige Anordnung, die so genannte Lösungsmittelschale, um jedes Kohlenwasserstoffmolekül. Das Wasser innerhalb dieser schalenartigen Anordnungen ist geordneter und weist im Vergleich zum Wasser im Lösungsmittel eine geringere Entropie auf. Da jedes System in der Natur versucht, einen Zustand maximaler Entropie zu erreichen, versucht das System, die Wechselwirkungen zwischen Kohlenwasserstoff und Wasser zu minimieren, was zur Bildung getrennter Kohlenwasserstoff- und Wasserschichten führt. Diese entropiebedingte Trennung zwischen Kohlenwasserstoff und Wasser wird als hydrophober Effekt bezeichnet.

Da die Entropie der treibende Faktor für die Unlöslichkeit von Kohlenwasserstoffen in Wasser ist, beeinflusst auch die Temperatur des Systems den Prozess, beispielsweise in Gashydraten oder Clathraten, einem der größten Erdgasvorkommen. Gashydrate sind kristalline feste Formen von Wasser und Gas. Sie entstehen, wenn Methan und Wasser unter hohem Druck und niedrigen Temperaturen gefrieren. Die Kohlenwasserstoffmoleküle sind in stabilen Käfigen aus Eis eingeschlossen, das in seiner Kristallstruktur relativ große offene Räume aufweist. Die Kohlenwasserstoffmoleküle passen in diese Löcher, was es ermöglicht, die maximale Größe der Kohlenwasserstoffmoleküle vorherzusagen, die Clathrate bilden können.

Entropie und Solvatation

Der Prozess, bei dem ein gelöster Stoff mit Lösungsmittel umgegeben wird, wird als Solvatation bezeichnet. Dabei geht es darum, den gelösten Stoff gleichmäßig im Lösungsmittel zu verteilen. Die Faustregel für ...

Education

Lehre

JoVE Core

Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Thermodynamics and Chemical Kinetics

Thermodynamik und chemische Kinetik

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KERNBEGRIFFE UND KONZEPTE

hydrophobic effects in solvation

Solvation is the process of surrounding a solute with solvent. It spreads the solute evenly through the solvent. A useful rule is that like dissolves like. A good solvent has molecular features similar to the substance being dissolved.

Polar solvents dissolve polar solutes, and apolar solvents dissolve apolar solutes. A polar solvent has a high dielectric constant, with ε greater than or equal to 15. An apolar solvent has a low dielectric constant. The dielectric constant is defined by the electrostatic law, which describes the interaction energy between two ions with charges q1 and q2 separated by a distance r.

A polar solvent can separate or shield ions from one another. Because of this, oppositely charged ions are less likely to associate in a polar solvent than in an apolar solvent. Water and hydrocarbons show this difference clearly. Water is polar, while hydrocarbons are apolar.

When hydrocarbon molecules are added to water, the water at the interface forms a shell-like arrangement around each molecule. This shell is called the solvent shell. The water inside these shells is more ordered and has lower entropy than the surrounding water. Since natural systems tend to move toward maximum entropy, the system reduces contact between hydrocarbon and water and forms separate layers.

This entropy-driven separation is called the hydrophobic effect. Because entropy drives the insolubility of hydrocarbons in water, temperature also affects the process. This is seen in gas hydrates, also called clathrates, which are among the largest reserves of natural gas.

Gas hydrates are crystalline solids made of water and gas. They form when methane and water freeze under high pressure and low temperature. The hydrocarbon molecules are trapped in stable cages of ice. Ice has relatively large open spaces in its crystal structure, so hydrocarbon molecules can fit inside these holes. That makes it possible to predict the maximum size of hydrocarbon molecules that can form clathrates.

Hydrophobic Effects in Solvation

chemical reactions

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entropy and solvation

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gibbs free energy and thermodynamic favorability

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rate law and reaction order

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bond dissociation energy and activation energy

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energy diagrams, transition states, and intermediates

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key terms and concepts

Learn how solvation depends on polarity, dielectric constant, and entropy, and why hydrocarbons separate from water through the hydrophobic effect.

Hydrophobic Effects in Solvation | JoVE K12

organic chemistry

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Draw, identify, and contrast graphical representations of chemical bonds (ionic, covalent, and metallic) based on chemical formulas. Construct and communicate explanations to show that atoms combine by transferring or sharing electrons. EXPLANATION: This standard resumes discussions of interactions between atoms from 7.PS1. In seventh grade, students observed patterns within the periodic table that related the physical and chemical properties of substances to the location of the constituent elements on the periodic table. Revisiting general trends is likely to be necessary however a focus should be placed on the transfer of electrons underlying differences in the differences in properties. It is beneficial to show that electrostatic interactions occur even between two non-conductors. Demonstrations can be carried out by using static charge to hold a balloon against a wall or showing that oppositely charged pieces of invisible tape are attracted not only to each other but also to a neutral conductors and non-conductors such as foil strips or paper.

CHEM1.PS2.1

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Understand that intermolecular forces created by unequal distribution of charges result in the varying degrees of attraction between molecules. Compare and contrast the intermolecular forces (hydrogen bonding, dipole-dipole bonding and London dispersion forces) within different types of simple substances (only those following the octet rule) and predict and explain their effect on chemical and physical properties of those substances using models or graphical representations. EXPLANATION: This standard resumes where students left off in 7.PS1. Students' first exposure to all three phases of matter was in third grade, by fifth grade students were investigating phase changes in matter. These discussions have not included a mechanism to explain why states of matter are dependent on temperature and pressure. Students should consider this standard in conjunction with CHEM1.PS2.1 to explore differences in intermolecular attractions within molecular and ionic compounds and the behavior of electrons leading to these differences.

CHEM1.PS2.2

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Construct a model to explain the process by which solutes dissolve in solutes, and develop an argument to describe how intermolecular forces affect the solubility of different chemical compounds. EXPLANATION: A similar task and standard exist in the fifth grade (5.PS1.3). At that time, students performed an investigation (which is again mentioned in CHEM1.PS2.4) into factors affecting solubility. However, student background knowledge was not sufficient to explore any explanation of their results. Explorations into intermolecular attractions in the previous standard (CHEM1.PS2.2) have introduced students to the existence of forces between molecules in a substance. Students can now apply these attractions to form explanations for how solute-solute, solute-solvent, and solvent-solvent attractions all interact as substances dissolve. (Solubility constants and saturation of solutions are beyond the scope of this standard.)

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Conduct an investigation to determine how temperature, surface, and stirring affect the rate of solubility. Construct an argument to explain the relationship observed in experimental data using collision theory. EXPLANATION: This standard is nearly identical to a fifth grade standard which explores factors affecting the solubility of a compound. Though collision theory is more closely related to rates of reaction, in this case the application of collision theory explores how the same factors underlying collision theory for how reactions also facilitate solvation processes.

CHEM1.PS2.4

Hydrophobic Effects in Solvation

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