Cecily Zamora
Raoult's law states that the vapour pressure of a solvent above a solution is equal to the pure solvent's vapour pressure at the same temperature, scaled by the solvent mole fraction in the solution. However, many solutions exhibit deviations from this law due to various factors. For instance, hydrogen bonding between molecules can cause a positive divergence, increasing the attraction between the molecules and resulting in a higher vapour pressure than predicted. Conversely, negative deviations occur when the interactions between solute and solvent molecules are stronger than those within the solute and solvent individually, leading to lower vapour pressure. The type of deviation depends on the relative strengths of intermolecular attractions, with non-ideal solutions frequently exhibiting positive deviations. Additionally, the dissociation or association of solutes and the lack of chemical equivalency between components can also cause deviations from Raoult's law.
| Characteristics | Values |
|---|---|
| Hydrogen bonding between molecules | Positive deviation |
| Intermolecular attractions between solute and solvent particles | Positive deviation |
| Solute-solvent intermolecular attractions are weaker than solute-solute and solvent-solvent attractions | Positive deviation |
| Interactions between solute and solvent molecules are stronger than solute-solute or solvent-solvent interactions | Negative deviation |
| Solutions of ethanol and water | Positive deviation |
| Solutions of phenol and aniline | Negative deviation |
| Solutions of chloroform and acetone | Negative deviation |
| Solutions of benzene and methanol | Positive deviation |
| Solutions of chloroform and ethanol | Positive deviation |
| Solutions of HCl and water | Negative deviation |
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What You'll Learn
Hydrogen bonding between molecules
Raoult's law states that the vapour pressure of a solvent above a solution is equal to the pure solvent's vapour pressure at the same temperature, scaled by the solvent mole fraction in the solution. However, many solutions deviate from Raoult's law due to the lack of chemical equivalency between different chemical components. These deviations can be positive or negative.
The system of chloroform (CHCl3) and acetone (CH3COCH3) exhibits a negative deviation from Raoult's law, indicating an attractive interaction between the two components that can be described as a hydrogen bond. The system HCl–water has a large enough negative deviation to form a minimum in the vapour pressure curve known as a (negative) azeotrope, corresponding to a mixture that evaporates without changing composition. When these two components are mixed, the reaction is exothermic as ion-dipole intermolecular forces of attraction are formed between the resulting ions (H3O+ and Cl–) and the polar water molecules.
The hydrogen bonding between molecules in a solution can also cause a positive deviation from Raoult's law. The hydrogen bonds increase the intermolecular forces in the solution, resulting in a higher vapour pressure than expected. This positive deviation indicates that the vapour pressure above the solution is higher than that of the pure solvent.
Positive deviations from Raoult's law occur when the vapour pressure above the solution is higher than expected. This happens when the solute-solvent intermolecular attractions are weaker than those between solute-solute molecules and solvent-solvent molecules. The vapour pressures of such solutions are higher than those of pure components.
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Strong solute-solvent interactions
Raoult's law applies only to ideal solutions, and in such solutions, the interactions between solute and solvent molecules are expected to be similar to the interactions between solute molecules and solvent molecules. However, strong solute-solvent interactions can cause deviations from Raoult's law.
When the interactions between solute and solvent molecules are stronger than the interactions between solute molecules or solvent molecules, the solution exhibits negative deviations from Raoult's law. This means that the vapour pressure of such solutions is lower than that of the pure components. For instance, the system of chloroform (CHCl3) and acetone (CH3COCH3) shows a negative deviation from Raoult's law, indicating an attractive interaction between the two components.
The strong solute-solvent interactions reduce the number of solvent molecules on the surface with sufficient energy to escape to the vapour phase. This results in a lower vapour pressure for the solvent. Additionally, if the solute is volatile, the number of solute particles in the vapour phase will be lower compared to pure solute liquid due to the presence of solvent particles occupying the space between solute particles.
The hydrogen bonding between molecules is an example of strong solute-solvent interactions that can cause a positive divergence from Raoult's law. The attraction between the molecules in the solution is increased by hydrogen bonds, resulting in a vapour pressure that exceeds the calculated value.
The mixing enthalpy and mixing volume of the pure components are both zero in ideal solutions, meaning no heat is absorbed or released when they are mixed. However, non-ideal solutions, which deviate from Raoult's law, exhibit interactions between solutes and solvents that differ from interactions between solutes and solutes or solvents and solvents.
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Dissociation of solute
Raoult's law, a principle of physical chemistry, was established in 1887 by French chemist François-Marie Raoult. It states that the partial vapour pressure of each component of an ideal mixture of liquids is equal to the vapour pressure of the pure component (liquid or solid) multiplied by its mole fraction in the mixture.
Ideal solutions are those that adhere to Raoult's law throughout the concentration range. However, most solutions are non-ideal and deviate from the law. This is due to the lack of chemical equivalency between different chemical components.
One factor that causes deviation from Raoult's law is the dissociation of the solute. When a solute present in a solution dissociates into its constituent ions, the resultant ions interact strongly with the solvent, causing a deviation from Raoult's law. For example, when potassium chloride (KCl) dissolves in water, it dissociates into K+ and Cl- ions, which form strong ion-dipole interactions with water molecules. This results in a deviation from ideal behaviour, as the interactions between the solute and solvent molecules differ from those between solute-solute and solvent-solvent molecules.
The degree of dissociation of the solute can be determined by calculating the relative amount of free water per mole of solute added and the actual number of entities formed in solution per mole of solute. However, this calculation may be beyond the scope of introductory chemistry courses. Instead, a simple exercise can be performed to demonstrate the concept. For example, consider an aqueous solution of aluminium sulfate (Al2(SO4)3) at a concentration of 1 molal (342.1 g of solute/kg of solvent). By assuming complete dissociation of the solute and ignoring the contribution of water molecules bound to the solute, the van't Hoff factor can be used to determine the degree of dissociation.
In summary, the dissociation of the solute can cause deviations from Raoult's law by altering the interactions between solute and solvent molecules and affecting the vapour pressure of the solution. To accurately describe the behaviour of these solutions, it is important to consider the degree of dissociation and the resulting interactions between ions and solvent molecules.
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Association of solute
Raoult's law, a principle of physical chemistry, states that the partial vapour pressure of each component of an ideal mixture of liquids is equal to the vapour pressure of the pure component (liquid or solid) multiplied by its mole fraction in the mixture. In other words, the vapour pressure of a solvent above a solution is equal to the vapour pressure of the pure solvent at the same temperature, scaled by the mole fraction of the solvent present.
The association of solute molecules can cause a deviation from ideal behaviour. This is because, in an ideal solution, the solvent-solute interaction is the same as the solvent-solvent or solute-solute interaction. This implies that both the solute and the solvent take the same amount of energy to escape to the vapour phase as when they are in their pure states.
However, in reality, the decrease in vapour pressure will be greater than that calculated by Raoult's Law for extremely dilute solutions. This is because the additional solute particles will fill the gaps between the solvent particles and take up space. The solute-solvent intermolecular attractions can be weaker than those between solute-solute molecules and solvent-solvent molecules, leading to positive deviations. In such cases, the vapour pressures of solutions are higher than those of pure components. Conversely, negative deviations occur when the interactions between solvents and solute molecules are stronger than solute-solute or solvent-solvent interactions, leading to lower vapour pressures than those of pure components.
The attraction between the molecules in the solution is increased by hydrogen bonds, which cause a positive divergence in Raoult's law. The solution's vapour pressure exceeds the calculation in this case. On the other hand, when the attraction between the molecules is lessened, a negative deviation occurs, resulting in a lower vapour pressure than expected.
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Non-ideal solutions
Raoult's law, proposed by French chemist François-Marie Raoult in 1887, is a relation of physical chemistry with implications in thermodynamics. It states that the partial pressure of each component of an ideal mixture of liquids is equal to the vapour pressure of the pure component (liquid or solid) multiplied by its mole fraction in the mixture.
Ideal solutions are those that adhere to Raoult's law throughout the whole concentration range. There are two important qualities of ideal solutions: the mixing enthalpy and the mixing volume, both of which are zero. This means that no heat is absorbed or released when the components are mixed together, and the volume of the solution is equal to the sum of the volumes of the two components.
However, most solutions are non-ideal and deviate from Raoult's law. This deviation can be positive or negative. A positive deviation indicates that the vapour pressure above the solution is higher than expected, while a negative deviation indicates that the vapour pressure is lower than expected. The following factors cause non-ideal solutions to deviate from Raoult's law:
- Solute-solvent interactions: In an ideal solution, the interactions between solute molecules (A-A), solvent molecules (B-B), and solute and solvent molecules (A-B) are expected to be similar. If these interactions are dissimilar, there will be a deviation from ideal behaviour.
- Dissociation of solute: When a solute present in a solution dissociates into its constituent ions, the resultant ions interact strongly with the solvent and cause a deviation from Raoult's law. For example, a solution of potassium chloride in water deviates from ideal behaviour because the solute dissociates into K^+ and Cl^- ions, which form strong ion-dipole interactions with water molecules.
- Association of solute: The association of solute molecules can also cause a deviation from ideal behaviour.
- Hydrogen bonding: Hydrogen bonding between molecules causes a positive divergence from Raoult's law by increasing the attraction between the molecules in the solution, resulting in a higher vapour pressure than calculated.
- Molecular size and structure: An ideal solution is formed by mixing two elements of the same molecular size and structure. Deviations from this ideal scenario can result in non-ideal solutions.
- Intermolecular forces: Raoult's law assumes that the intermolecular forces between unlike molecules are equal to those between similar molecules. If the forces between unlike molecules are stronger or weaker than expected, the solution will deviate from ideal behaviour.
Examples of non-ideal solutions that exhibit negative deviations from Raoult's law include solutions of phenol + aniline and chloroform + acetone. Solutions of ethanol + acetone and carbon disulphide + acetone show positive deviations.
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Frequently asked questions
Raoult's Law states that the vapour pressure of a solvent above a solution is equal to the pure solvent's vapour pressure at the same temperature, scaled by the solvent mole fraction in the solution.
Positive deviation occurs when the intermolecular attractive forces between solute and solvent particles are weaker than those between solute-solute and solvent-solvent molecules. This results in a higher vapour pressure than expected.
Negative deviation occurs when the interactions between solute and solvent molecules are stronger than the interactions within solutes and solvents themselves. This results in a lower vapour pressure than expected.
Solutions of ethanol and acetone, or carbon disulphide and acetone, exhibit positive deviation from Raoult's Law.
Solutions of chloroform and acetone, or HCl and water, exhibit negative deviation from Raoult's Law.
