Ideal Solution

What is an Ideal Solution?

Ideal solution, 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 itself 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 vapour 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 = ΧsolventP0solvent

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 vapour particles, hence, exert partial pressure contributes to the total pressure above the solution.

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Further, Raoult’s law states that at equilibrium,

PA = P°A xA ,PB = P°B xB

Where PA is the partial pressure of A

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 + 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.

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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.

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Now, this new pressure 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 A = XAB

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.

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Combining the two graphs, we get the resultant one as below.

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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, the raoult's law 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.

FAQ (Frequently Asked Questions)

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 ideal solution provided below.

1. It Should Obey Raoult’s Law, Which is Defined as,

PA= XAA and PB= XBB

2. ΔHmix = 0, i.e. during mixing, no heat should be either absorbed or evolved

3. 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.