What is an Ideal Solution?
An ideal solution is a composition where the molecules of separate species are identifiable, however, as opposed to the molecules in an ideal gas, the particles in an ideal solution apply force on each other. When the forces applied across all molecules are the exact same, irrespective of the species, a solution is said to be ideal.
For example, consider two liquids X and Y, and combine them. The solution formed will undergo various intermolecular forces of attraction in it, such as:
X - X intermolecular forces of attraction
Y - Y intermolecular forces of attraction
X - Y intermolecular forces of attraction
For an ideal solution to be formed, the intermolecular forces of attraction between A - A, B - B and A - B must be just about equal.
If we go by the simplest definition of an ideal solution, it is described as a solution that is formed by mixing two elements that are of the same molecular size, structure and that have uniform intermolecular forces. An ideal solution abides by Raoult’s law at almost every range of concentration and temperatures.
Characteristics of an Ideal Solution
Most of the time an ideal solution has physical properties closely related to those of the pure components, some of them are as follows:
The thermodynamics of a solution is zero. If the thermodynamics of the solution is closer to zero, then it is more probable to exhibit ideal behavior. ΔmixH = 0
The mixing volume is zero too. ΔmixV = 0
To get an ideal solution, it can be helpful to mix a solvent and a solute with identical molecular size and structure. The student can even take two substances X and Y and then mix them together. It is observed that there are several intermolecular forces existing between them.
In spite of the fact that getting a balanced and stable ideal solution is an infrequent situation, certain solutions tend to show ideal behavior at times.
The ideal solution is a homogeneous combination of compounds that has physical properties that are linearly related to the quality of the elements. The classic example of this situation is the rule of Raoult, which refers to certain heavily diluted solutions and to a small class of condensed solutions, including those under which the interactions between the solute and solvent molecules are the same as those between the molecules themselves of each substance.
Benzene and toluene solutions, which have very close molecular structures, are ideal: each combination of the two has a volume equivalent to the sum of the concentrations of the respective elements, and the combining phase takes place without heat absorption or evolution. The vapor pressures of the solutions are defined mathematically by the linear structure of the molecular composition.
What is Raoult's Law and Derive it?
Raoult's law implies that the partial vapor pressure of a solvent in a solution (or mixture) is identical to or equal to the vapor pressure of a pure solvent multiplied by the mole fraction of the solution.
Raoult's law equation can be written mathematically as;
Psolution = Χsolvent Posolvent
Where,
Psolution = vapour pressure of the solution
Χsolvent = mole fraction of the solvent
P0solvent = vapour pressure of the pure solvent
As an example to define Raoult’s Law, consider a solution of volatile liquids in a container A and B as listed below. Since A and B both are volatile, in the vapor phase, there would be both particles of A and B.
Both A and B vapor particles, hence, exert partial pressure contributes to the total pressure above the solution.
(Image will be uploaded soon)
Further, Raoult’s law states that at equilibrium,
PA = P°A xA ,PB = P°B xB
Where PA is the partial pressure of A
P°A is the vapour pressure of pure A at the corresponding temperature.
xA is the mole fraction which is in the liquid phase
Similarly,
PB , P°B xB
Hence,
PT = PA + PB (Dalton’s Law) = P°A xA + P°B xB = P°A + xB (P°B - P°A)
What is the Importance of Raoult’s Law?
Imagine we have a locked container loaded with volatile liquid A. For some time, due to evaporation, A vapour particles may begin to form. As time passes, the A vapor particles will be in dynamic equilibrium with the liquid particles (on the surface). The pressure exerted by the vapor particles of A at some given temperature is called the vapor pressure of A at that temperature.
(Image will be uploaded soon)
Vapor pressure is exhibited on both solids and liquids and relies solely on the temperature and the type of liquid.
Now assume that we're attaching another liquid B (solute) to this container. This will result in the B particles occupying the space between the A particles on the surface of the solution.
For any liquid that is given, there are a fraction of molecules on the surface that will have sufficient energy able to escape to the vapour phase.
Because we now have a lower number of A particles on the surface, the amount of A vapor particles in the vapor process would be greater. It would lead to the lower vapour pressure of A.
However, if we believe that B is still volatile, we would have fewer B particles in the vapor phase compared to pure B's liquid.
(Image will be uploaded soon)
Now, this new partial pressure of each (A and B) is given by Raoult’s law and depends completely on the concentration of each component in the liquid phase.
PA α XA, PB α PB = XB = XA P°A = XA P°B
It is evident from Raoult 's law that, as the mole fraction of the component lessens, its partial pressure also lessens during the vapour phase.
The below graphs demonstrate the pressure for mole fractions A and B.
(Image will be uploaded soon)
Combining the two graphs, we get the resultant one as below.
(Image will be uploaded soon)
We have also added the graph for the total vapour pressure of the solution in the above diagram, i.e., PA + PB.
As far as Raoult's law application goes, it is also useful to calculate the molecular mass of an unknown solute.
Properties of an Ideal Solution
Most of the time, Raoult's ideal solution has physical properties that are closely related to the properties of pure components.
Some of its properties are,
The solution enthalpy is zero. If the enthalpy of the solution reaches near to zero, it is more likely to exhibit ideal behavior.
ΔmixH=0
The volume mixing is therefore zero.
ΔmixV=0
Characteristics of Ideal Solution
The ideal solution can be obtained by combining a solute with a solvent composed of a common molecular structure with size. If we take two substances as X and Y, and mixed together, we can see there are quite few intermolecular forces between them.
For instance,
X and X experience the intermolecular forces of attraction.
Y and Y experience the intermolecular forces of attraction.
X and Y experience the intermolecular forces of attraction.
FAQs on Ideal Solution
1. Can you provide any practical examples of ideal solutions?
Ideal solutions are being obtained by mixing two components of the same molecular size, a structure that will have exactly the same intermolecular attraction. For instance, two liquids A and B form and the ideal solution where A-A and B-B molecular attractions are the same, and then A-B molecular attraction is almost similar to A-A and B molecular attraction.
Any of the ideal solutions should have the characteristics of the ideal solution provided below.
It Should Obey Raoult’s Law, Which is Defined as,
PA= XA P°A and PB= XB P°B
ΔHmix = 0, i.e. during mixing, no heat should be either absorbed or evolved
ΔVmix = 0, i.e. no expansion or contraction during mixing.
Some examples of ideal solutions are Ethyl chloride and Ethyl Bromide, n-hexane and n-heptane, Silicon tetrachloride (SiCl4 or Cl4Si).
2. State the differences between ideal and non-ideal solutions.
Ideal Solution- A solution obeying Raoult’s law at all conditions of concentration and temperature is defined as an ideal solution.
The force of attraction between dissimilar molecules will be the same as they were in a pure state.
The volume of solution is described as the sum of the volume of the components,
Vsolution = Vsolute + Vsolvent.
There is no heat absorbed or evolved during solution format H = 0.
In an ideal solution, each component’s activity is equal to its mole fraction under all conditions.
Non-Ideal solution- No need to obey Raoult's law
The force of attraction between dissimilar molecules are not the same as they were in the pure state.
Vsolution ≠ V1 + V2
H ≠ 0
In a non-ideal solution, each component’s activity is not equal to its mole fraction under any condition.
