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Mobile App Chapter 13: Problem 19
Explain the difference between an ideal and a nonideal solution.
Short Answer
Expert verified
Ideal solutions follow Raoult's law, have no change in enthalpy or volume upon mixing, and have similar intermolecular interactions. Nonideal solutions deviate from Raoult's law, often have changes in enthalpy or volume, and have different intermolecular interactions compared to their pure components.
Step by step solution
01
Define an Ideal Solution
An ideal solution is a mixture of substances that obeys Raoult's law throughout its entire range of composition and temperature. This means the partial vapor pressure of each component is directly proportional to the mole fraction of that component in the solution. In an ideal solution, there are no changes in enthalpy or volume when the solution forms, indicating that the molecular interactions between different components are similar to those present in the pure substances.
02
Define a Nonideal Solution
A nonideal solution is a mixture where the components do not obey Raoult's law. The partial vapor pressures of the components are not directly proportional to their mole fractions. Nonideal solutions exhibit changes in enthalpy or volume when they form. This is due to the differences in molecular interactions between the components of the solution compared to the interactions present in their pure forms. Nonideal solutions often experience positive or negative deviations from Raoult's law.
03
Contrast the Behaviors
The key difference between ideal and nonideal solutions lies in their behaviors and the molecular interactions among the solute and solvent. Ideal solutions have component interactions similar to the pure components and follow Raoult's law, while nonideal solutions have different solute-solvent interactions leading to deviations from Raoult's law. This results in observable differences in physical properties such as boiling point, vapor pressure, and mixing enthalpies.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Raoult's Law
Raoult's law is foundational in understanding how solutions behave. It states that the partial vapor pressure of a component in a solution is proportional to the mole fraction of that component in the mixture. Mathematically, it is expressed as:
\[\begin{equation} P_i = P_i^* \times X_i \end{equation}\]
where,
\[\begin{equation} P_i = P_i^* \times X_i \end{equation}\]
where,
- \(P_i\) is the partial vapor pressure of the component i in the solution,
- \(P_i^*\) is the vapor pressure of the pure component i, and
- \(X_i\) is the mole fraction of the component i in the solution.
Solution Composition
The composition of a solution is described by detailing the amounts of each substance present. A common way to quantify this in solutions is by using mole fraction, which is the ratio of the moles of a component to the total moles of all components.
\[\begin{equation} X_B = \frac{{n_B}}{{n_A + n_B}} \end{equation}\]
Understanding the composition is crucial as it directly influences physical properties like boiling point and pressure.
- For a binary solution consisting of solute A and solvent B:
\[\begin{equation} X_B = \frac{{n_B}}{{n_A + n_B}} \end{equation}\]
Understanding the composition is crucial as it directly influences physical properties like boiling point and pressure.
Molecular Interactions
Molecular interactions are the forces between molecules in a substance. These can include van der Waals forces, hydrogen bonds, and ionic interactions. The nature of these interactions dictates whether a solution will be ideal or nonideal. In an ideal solution, the interactions between different components are identical to those in their pure substances. However, nonideal solutions have either stronger or weaker interactions between the components than in their pure forms, which explains the changes in properties like enthalpy and volume upon mixing.
Partial Vapor Pressure
Partial vapor pressure is a measure of the tendency of a substance to vaporize from a mixture. It's proportionate to its presence in the solution, defined by the mole fraction. In an ideal solution, the partial vapor pressure is predicted accurately by Raoult's law. Nonideal solutions, however, show deviations because the interactions among the particles affect the escape tendency of the molecules from the liquid to the gas phase, altering the expected vapor pressure.
Mole Fraction
The mole fraction is a dimensionless number that represents the ratio of the number of moles of a particular component to the total number of moles in the mixture. It is key in characterizing the solution composition and is used in calculations related to Raoult's law. The mole fraction is expressed as:
\[\begin{equation} X_i = \frac{{\text{{number of moles of component i}}}}{{\text{{total number of moles in solution}}}} \end{equation}\]
It's instrumental in predicting properties such as the boiling points and vapor pressures of the components in a solution.
\[\begin{equation} X_i = \frac{{\text{{number of moles of component i}}}}{{\text{{total number of moles in solution}}}} \end{equation}\]
It's instrumental in predicting properties such as the boiling points and vapor pressures of the components in a solution.
Enthalpy Changes in Solutions
The enthalpy change when a solution forms can tell us a lot about the nature of the solution. In ideal solutions, the mixing of components results in no net change in enthalpy—this implies that the energy of solute-solvent interactions is equal to the energy of the solvent-solvent and solute-solute interactions that were broken. Nonideal solutions, on the other hand, may release or absorb heat (exothermic or endothermic respectively) as a result of stronger or weaker new interactions compared to the original ones.
Volumetric Changes in Solutions
When substances dissolve to form a solution, volume changes might occur. In an ideal solution, molecules fit into the spaces between molecules of solvent without causing a change in volume. In nonideal solutions, the interactions between molecules can lead to expansion or contraction upon mixing, which is not accounted for by Raoult’s law. These volumetric changes can often be related to the nature of solute-solvent interactions and can provide insights into solution behaviors.
Solution Properties
The properties of a solution, such as vapor pressure, boiling point, freezing point, and osmotic pressure, depend heavily on the solution composition and the types of molecular interactions that occur. Ideal solutions have properties that can be predicted by simple models like Raoult's law, since they behave like pure substances. Nonideal solutions exhibit properties that deviate from these models, requiring more complex expressions to accurately describe their behavior. These deviations are a stark reminder of the intricate nature of chemical systems and the importance of considering molecular-level interactions in understanding macroscopic properties.
