Name: raoult's law in real solutions
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 24 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.6.1

Ideal Solutions

In a solution, there are three major attractive intermolecular forces: attractions between the solvent molecules, attractions between the solute molecules, and attractions between the solute and the solvent molecules.
If the strength of each of the three types of interactions are similar in magnitude, the solution is called an ideal solution.
An ideal solution obeys Raoult&#8217;s law at all concentrations.&#160;
For an ideal solution with two volatile components, such as toluene and benzene, the partial vapor pressure of each component will be given by Raoult&#8217;s law as a product of the vapor pressure of the pure component and its mole fraction.
In a given solution, the mole fraction for toluene is 0.4 and the mole fraction for benzene is 0.6. As the vapor pressures of the pure toluene and pure benzene are 22 and 75 torr, respectively, the partial pressures of toluene and benzene in this solution will be 8.8 and 45 torr, respectively.
The total vapor pressure is the sum of the partial pressure of each component and is equal to 54 torr.
For such an ideal solution, a plot of vapor pressure against mole fraction yields a straight line.
When the intermolecular forces within a solution are not unif

ideal solutions

According to Raoult&#8217;s law, the partial vapor pressure of a solvent in a solution is equal or identical to the vapor pressure of the pure solvent multiplied by its mole fraction in the solution. However, Raoult&#39;s Law is only valid for ideal solutions. For a solution to be ideal, the solvent-solute interaction must be just as strong as a solvent-solvent or solute-solute interaction. This suggests that both the solute and the solvent would use the same amount of energy to escape to the vapor phase as when they are in their pure states. This is only possible when the different components of the solution are chemically similar, as in the case of benzene and toluene or hexane and heptane.
Since many solutions do not have uniform attractive forces, the vapor pressure of these solutions deviates away from the pressure predicted by Raoult&#8217;s law. For instance, when ethanol is dissolved in water, there are strong attractions between the water molecules and the ethanol molecules. These attractive forces tend to slow down the loss of water molecules from the surface of the solution. However, if the solution is sufficiently dilute, the surface will have more water molecules. Some of these surface water molecules may not be surrounded by any ethanol molecules and can still escape to the vapor phase at the same rate as they would in pure water. Such dilute solutions are said to approach ideal behavior.
For non-ideal solutions, deviation from Raoult&#8217;s law can either be negative or positive. The negative deviation takes place when the vapor pressure is lower than that expected due to Raoult&#8217;s law. A solution of water and hydrochloric acid exhibits negative deviation because the hydrogen bonds between water and hydrochloric acid prevent the surface water molecules from vaporizing as readily.
Alternatively, positive deviation occurs when the attraction between the molecules of each component, either solute-solute or solvent-solvent, is greater than the attraction between the solvent and the solute. In such solutions, both components can easily escape into the vapor phase. An example of a positive deviation is a solution of benzene and methanol as the intermolecular forces between the benzene and methanol are weaker than found in pure methanol.

According to Raoult&#8217;s law, the partial vapor pressure of a solvent in a solution is equal or identical to the vapor pressure of the pure solvent ...

Education

JoVE Core

Solutions and Colloids

KEY TERMS AND CONCEPTS

raoult's law in real solutions

Raoult's law describes how vapor pressure changes in a solution. For a solvent, the partial vapor pressure equals the vapor pressure of the pure solvent multiplied by its mole fraction in the solution. This relationship works best for ideal solutions.

An ideal solution is one in which solvent-solute attractions are just as strong as solvent-solvent and solute-solute attractions. In that case, both components need about the same amount of energy to escape into the vapor phase as they do in their pure states. This usually happens when the substances are chemically similar, such as benzene and toluene or hexane and heptane.

Many solutions are not ideal because their intermolecular forces are not uniform. As a result, their vapor pressures shift away from the value predicted by Raoult's law. Ethanol dissolved in water is one example, because strong water-ethanol attractions slow the loss of water molecules from the surface. If the solution is very dilute, however, the surface has more water molecules. Some of those water molecules may not be surrounded by ethanol and can still escape like they do in pure water. Such dilute solutions may approach ideal behavior.

Non-ideal solutions can show either negative or positive deviation from Raoult's law. Negative deviation means the vapor pressure is lower than expected. A water and hydrochloric acid solution shows this behavior because hydrogen bonds between the two substances keep surface water molecules from vaporizing as easily. Positive deviation happens when solute-solute or solvent-solvent attractions are stronger than solvent-solute attractions. In that case, both components can escape into the vapor phase more easily. A benzene and methanol solution is an example because benzene-methanol interactions are weaker than the forces in pure methanol.

Raoult's Law in Real Solutions

solution formation

Why Some Substances Dissolve Better

Some substances dissolve easily in one solvent but not in another. A quick way to predict solubility is the idea that "like dissolves like." Polar substances, such as salt and sugar, dissolve well in polar solvents like water. Non-polar substances are more soluble in non-polar solvents such as carbon tetrachloride. This selective solubility depends on intermolecular forces, which are the attractions within and between molecules. Stronger forces between solute and solvent molecules lead to...

intermolecular forces in solutions

How Solutions Form Without Energy Change

A solution can form as a spontaneous process. That means it happens under the right conditions without energy from an outside source. When the attraction between solute and solvent particles is about the same as the attraction in the separate substances, the solution forms with no energy change. A solution like this is called an ideal solution. Mixtures of ideal gases are one example, and helium and argon behave close to that model. These gases do not have significant intermolecular...

enthalpy of solution

Dissolution and Heat Changes

Dissolution and heat changes help explain why some solutions form on their own. Two factors can favor the spontaneous formation of a solution. One is a decrease in internal energy, which happens in an exothermic change. The other is increased dispersal of matter, which means the solute spreads evenly through the solvent. When a substance dissolves, the internal energy often changes as heat is absorbed or released. This does not happen in every case, but it is common. A solution forms when...

aqueous solutions and heats of hydration

How Water Dissolves Ionic Compounds

Water dissolves ionic compounds because its polar molecules attract ions. This attraction between an ion and a molecule with a dipole is called an ion-dipole attraction. It helps explain why many ionic solids can separate in water and form aqueous solutions. When an ionic compound dissolves, its ions separate and spread out evenly through the solution. Water molecules surround the ions and solvate them, which lowers the strong electrostatic forces between the ions. This separation is a...

solution equilibrium and saturation

Saturation Limits in Solutions

Saturation limits in solutions describe how much solute can dissolve in a solvent under given conditions. As sugar is added to water and stirred, it dissolves at first. More sugar can be added until the solution reaches its natural limit. That limit depends mainly on the relative strengths of three attractions: solute-solute, solute-solvent, and solvent-solvent forces. When no more sugar dissolves, even after long stirring, undissolved sugar stays behind. The concentration of sugar in the...

physical properties affecting solubility

Gas Solubility in Liquids

Gas solubility in liquids depends on the attractive forces between the gas and the liquid. When a gas dissolves, there is no strong solute-solute attraction to break apart first. Gas particles are already far apart, so their internal interactions are negligible. That makes solute-solvent interactions the main energetic factor that controls how much gas can dissolve. Examples show how these interactions matter. Oxygen is about three times more soluble in water than helium because oxygen has...

expressing solution concentration

Measuring Solution Strength

Solution strength can be described with terms like dilute and concentrated, and it can also be measured with several scientific units. A solute is the part of a solution present at a lower amount than the solvent. Different units are useful in different situations, especially in chemistry. Molarity, written as M, is a common unit for concentration. It tells how many moles of solute are in one liter of solution. Because solution volume changes with temperature, molarity can change too when the...

vapor pressure lowering

Raoult's Law and Lower Vapor Pressure

Raoult’s law explains why a solution with a nonvolatile solute has a lower vapor pressure than the pure liquid. Vapor pressure is the pressure from a liquid’s gas phase when vaporization and condensation happen at equal rates. When a nonvolatile substance dissolves in a volatile liquid, the liquid’s equilibrium vapor pressure drops. One reason for this drop is at the surface of the solution. Solvent molecules must be at the surface to vaporize, and added solute molecules reduce the surface...

Raoult's law describes how vapor pressure changes in a solution. For a solvent, the partial vapor pressure equals the vapor pressure of the pure solvent multiplied by its mole fraction in the solution. This relationship works best for ideal solutions. An ideal solution is one in which solvent-solute attractions are just as strong as solvent-solvent and solute-solute attractions. In that case, both components need about the same amount of energy to escape into the vapor phase as they do in...

freezing point depression and boiling point elevation

freezing-point-depression-and-boiling-point-elevation

Colligative Effects in Solutions

Colligative effects in solutions include boiling point elevation and freezing point depression. These properties depend on how many solute particles are present, not on the solute’s chemical identity. Because of that, a solution can behave differently from the pure solvent even when only a nonvolatile solute is added. Boiling point elevation happens because the vapor pressure of a solution is lower than the vapor pressure of the pure solvent. The boiling point is the temperature at which vapor...

osmosis and osmotic pressure of solutions

osmosis-and-osmotic-pressure-of-solutions

Semipermeable Membranes and Reverse Osmosis

Semipermeable membranes let some molecules pass while blocking others. They are found in biological cell membranes and in dialysis tubing used to remove metabolic wastes from blood. These examples show how selective permeation depends on factors such as size, shape, polarity, and charge. Osmosis is the net movement of solvent through a semipermeable membrane. In a U-shaped setup, pure solvent and a solution are separated by a membrane that allows only solvent molecules to pass. Solvent...

electrolytes: van't hoff factor

Electrolyte Ions and the van't Hoff Factor

Electrolyte ions affect colligative properties because these properties depend on the number of dissolved particles, not their identity. The concentration terms used for freezing point depression, boiling point elevation, and osmotic pressure include every solute species present in the solution. Nonelectrolytes dissolve without dissociation, so each molecule that enters solution counts as one particle. Electrolytes behave differently because they can form ions in water. If sodium chloride were...

colloids

Colloids and Everyday Mixtures

Colloids are mixtures with particles between the size of a solution and a suspension. They stay dispersed instead of settling out, even though their particles are larger than most simple molecules. This makes colloids different from cloudy suspensions like mud and water, flour and water, or tempera paint. It also makes them different from solutions, where dissolved molecules or ions are invisible and do not settle. Colloidal systems can be made in two main ways. In dispersion methods, larger...

key terms and concepts

Learn how Raoult's law predicts vapor pressure in ideal solutions, and how real mixtures can show negative or positive deviation from the rule.

Raoult's Law in Real Solutions | JoVE K12

chemistry

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Tennessee Academic Standards

High School

Matter and Its Interactions

CHEM1.PS1.15

Virginia  Standards of Learning

HIgh School

CH.5

CH.5.b

CH.5.e

new-york-state-science-learning-standards

New York State Science Learning Standards

Physical Science

HS. Structure and Properties of Matter

HS-PS1-10.

Scientific Laws and Theories

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Ionization Energy

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PS.Chm.2.2

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Raoult's Law in Real Solutions

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