Raoult’s Law – Ideal Solutions

Introduction

A similar mixture of compounds consists of physical properties that are straightforwardly connected to the element’s quality. The interactions in-between the molecules of the components do not vary from the interactions in-between the molecules of each component are known as the ideal solution. The molecules in the ideal solution apply forces on one another. For all levels of concentration and temperatures, the solutions follow Raoult’s Law. This only works for the ideal solution. In some other cases, it works fairly for solvent in the dilute solution, but as a result, the decrease in vapour pressure is greater than the calculation of Raoult’s Laws.

Ideal Solution

The ideal solution R and S is formed when R-S interactions in the solution are similar to the R-Rand S-S interactions. It should be of an equal type of component that will interact with itself and the other component. Their ideal solution always obeys Raoult’s Law. 

The Characteristics of Ideal Solution

• The mixing of solute and a solvent that consists of analogous molecular size and structure can provide us with an ideal solution. 
• The Ideal solution follows Raoult’s Law.
• The capacity of a solution differs directly with the compositions.
• In mixing the liquid, there is neither an evaluation of heat nor absorption. 
• The vapour pressure of a solution differs directly with composition. 

• For an example:               

• R and R experience intermolecular forces of attraction
• S and S experience intermolecular forces of attraction
• R and S experience intermolecular forces of attraction. 

So here, the equality between the intermolecular forces of attraction can help us to secure an ideal solution. 

• For studying topics like colligative properties and thermodynamics of chemicals, it is very much required as well as important to have the knowledge and understanding about the ideal solution. 

State Raoult’s Law:

Francois Marte Raoult, a French Chemist. In 1986, formulated a connection between a mole of a fraction of volatile liquids and partial pressure. 

The limited vapour pressure of a solvent present in a solution is equivalent to the vapour pressure of a pure solvent which is increased by the mole fraction of the solution, which is what Raoult’s Law infers. 

Raoult’s Law Formula:

PSolution = Χsolvent PSolvent 

Whereas;

• The PSolution  refers to – the exact vapour pressure of a solution
• The Χsolvent refers to – the mole fraction of a solution
• The PSolvent refers to – the vapour pressure of a pure solution

For an example:

An antifreeze solution is prepared from 222.6 g of ethylene glycol (C2H6O2) and 200 g of water. Calculate the morality of the solution. If the density of the solution is 1.072 g mL-1, then what shall be the molarity of the solution?

Ans: Calculation of Molality :

Mass of ethylene glycol = 222.6 (Given)

Molar mass of ethylene glycol [C2H4(OH)2]

= 2 X 12 + 6 x 1 + 2 x 16 = 62

Therefore moles of ethylene glycol

= 222.6g / 62 gmol-1 = 3.59 mol

Mass of water = 200g   (Given)

Therefore molality of the solution is = (moles of ethylene glycol / mass of water) x 1000

= (3.59 / 200) x 1000 = 17.95 m

Calculation of Molarity:

Moles of ethylene glycol = 3.59 mol (already calculated)

Total Mass of solution = 200 + 222.6 = 422.6g

Volume of solution = mass / density volume

= 422.6 / 1.072 = 394.22 ml

now molarity of the solution is = (moles of ethylene glycol / volume of solution) x 1000

= (3.59 / 394.22) x 1000

= 9.11 M

At equilibrium:

PA = P°A xA ,PB = P°B xB

• In which PA refers to the partial pressure of A

• P°A refers to the vapour pressure of pure A at the same temperature.

• xA refers to the mole fraction that is present in the liquid phase. 

Limitations of Raoult’s Law

There are also some limitations of Raoult’s Law and they are as follows;

• It applies only to the very dilute Solutions. 
• It is applicable only for the solution that consists of a nonvolatile solute. 
• A solute that can dissociate and associate to the exact solution, Raoult’s law is not applicable for those.

Some Properties of an Ideal Solution:

Many of the times, an ideal solution contains physical properties which are almost related to the properties of the pure components. Few properties are as follows;

• When an enthalpy of a solution nearly gets close to zero, in such a case, it shows the behaviour of an ideal solution. Therefore, zero is the enthalpy of the solution.

ΔmixH=0 

• Similarly, zero is also the volume of mixing. 

ΔmixV=0

• It can only take place with components of the same size and polarity. 

Examples of Ideal Solutions:

Securing an accurate and well-balanced ideal solution can be rare, but in some of the solutions, we can see a possible behaviour of displaying an ideal solution. For example, 

• Ethyl Iodide (C2H5I) is a colourless explosive compound, and Ethyl Bromide (C2H5Br) is also known as Bromoethane. 
• Bromobenzene (C6H5Br)  is a colourless compound, but the old liquid can appear yellow, and Chlorobenzene (C6H5Cl) is a liquid with an almond-like odour. 
• N-heptane (C7H16) is one of the important components of gasoline, and hexane(C₆H₁₄)  is also an important component of gasoline. 

State Raoult’s Law for a Solution of Volatile Liquids:

The partial vapour pressure of a solvent in a solution (or mixture) is identical to or equal to the vapour pressure of a pure solvent increased by the mole fraction of the solution. 

​PA ∝ XA 

​

The Non-ideal Solution

It is the solution that doesn’t obey Raoult’s Law at all the levels of temperatures and concentrations. It is very much dissimilar from the ideal solution and referred to as Non- ideal solution. 

Limitations

Raoult’s law does not apply to all kinds of solutions but only ideal solutions. An ideal solution has solvent-solute interactions the same as the solvent-solvent or solute-solute interaction. 

Characteristics of Non-Ideal Solution:

• The interaction between the solute- solute and solvent- solvent is dissimilar from the solute-solvent interaction. 

• The heat might be released if the enthalpy of mixing is negative, and if the enthalpy of mixing is positive, that means the heat has been absorbed. 

Examples of Non-ideal solutions:

• Acetone (C3H6O) is a highly volatile and colourless liquid, and Carbon disulfide (CS2) is a colourless and ether-like odour liquid. 
• Acetone (C3H6O) is a highly volatile and colourless liquid, and Benzene (C6H6) is an organic chemical compound found in essential parts of gasoline and crude oil. 
• Methyl Alcohol (CH3OH) is colourless and strong like odour and Water (H₂O) odourless and tasteless liquid at a normal room temperature.

Conclusion 

In this of Raoult’s Law – Ideal solutions, we understood that the main purpose of Raoult’s Law is the ideal solution. The Force of attraction between the solute and solvent is similar to the force of attraction between solvent-solute. It implies that the total vapour pressure over a solution can be related to a mole of the fraction of any one component. Raoult’s Law only works fairly for the ideal solutions. The ideal solution obeys Raoult’s Law, whereas the non-ideal reaction does not obey Raoult’s Law and contains different molecules between the solute and solvent.