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What is meant by positive and negative deviation from Raoult’s law and how is the sign Δmix H related to positive and negative deviation from Raoult’s law?

 According to Raoult’s law, the partial vapour pressure of each volatile component in any solution is directly proportional to its mole fraction. The solutions which obey Raoult’s law over the entire range of concentration are known as ideal solutions. The solutions that do not obey Raoult’s law (non-ideal solutions) have vapour pressures either higher or lower than that predicted by Raoult’s law. If the vapour pressure is higher, then the solution is said to exhibit positive deviation, and if it is lower, then the solution is said to exhibit negative deviation from Raoult’s law.

Vapour pressure of a two-component solution showing positive deviation from Raoult’s law

Vapour pressure of a two-component solution showing negative deviation from Raoult’s law

In the case of an ideal solution, the enthalpy of the mixing of the pure components for forming the solution is zero.

ΔsolH = 0

In the case of solutions showing positive deviations, absorption of heat takes place.

∴ΔsolH = Positive

In the case of solutions showing negative deviations, evolution of heat takes place.

∴ΔsolH = Negative

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 you are asking the question and u urself are answering it.~~!!! ?????

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why to ask and answer it urself???but ur answer gave some information..thnk you.

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Solutions showing positive deviations from Raoult's law: Let Take a  binary solution with have components A and B. If the force of attraction between molecular of and A and B in the solution are weaker than that of between  A — A and B — B, then the tendency of escaping of molecules A—B from the solution becomes more than that of pure liquids. The total pressure of the solution will be greater than the corresponding vapour pressure of ideal solution of the same component  A and B. This type of solution shows positive deviation from Raoult's law. The positive deviation  of solution have been shown in figure. Some energy loose when we mix both solutions. This reaction is endothermic reaction. For Endothermic reaction delta H is always positive       Solutions showing negative deviations from Raoult's law : Let Take a  binary solution with have components A and B. If the force of attraction between molecular of and A and B in the solution are stronger  than that of between A — A and B — B, then the tendency of escaping of molecules A—B from the solution becomes less than that of pure liquids. The total pressure of the solution will be lower than the corresponding vapour pressure of ideal solution of the same component  A and B. This type of solution shows negative deviation from Raoult's law. The negative deviation  of solution have been shown in figure. Some energy released when we mix both solutions. This reaction is exothermic reaction. For Exothermic reaction delta H is always negative 
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i don't know the answer.
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Negative Deviation from Raoult’s law

 
The negative deviation occurs when the new interactions are stronger than old interactions. It means the new interactions are more stable and have lower energy so that more heat is librated when they are formed. While the old interactions are less stable and weak so less energy is needed to break them. In this overall reaction the energy released in the formation of new interactions is greater than energy required to break old interactions. So the temperature rises on mixing such solvents. 

 
​solvent + solute ⟶ ​solution​

 
ΔHreaction = ‘-ve’ ⟶ ƩHreactants > ƩHproducts ⟶ New interactions are stable than old interactions

Excess heat is librated on formation of new interactions = exothermic reaction.

 
In this case the new interactions are so strong that they can hold molecules back from going into vapor phase. That’s why the observed vapor pressure of such solution is found to be lower than expected which shows negative deviation from Raoult’s law. If you plot a graph between vapor pressure and mole fraction you will find a curve instead of straight line. Mixtures those show large negative deviation from Raoult’s law produces vapor pressure even lower than the pure constituents at certain composition.

 

Positive Deviation from Raoult’s law

The positive deviation occurs when new interactions are weaker than old interactions. In this case, more energy is required to break stable old interactions and less energy is librated on the formation of new weaker interactions. In overall reaction, energy required to break old interactions is higher than the energy librated in formation of new ones. That’s why temperature falls on mixing the constituents.

 
ΔHreaction = ‘+ve’ ⟶ ƩHreactants < ƩHproducts ⟶ Extra energy needs to be absorbed by the molecules to break stronger old interactions  = endothermic reaction.

 
Mixture of ethanol and water shows positive deviation from Raoult’s law. In addition to the van der Waal’s forces hydrogen bonding is also present in both the solvents. In water molecule two hydrogen atoms are available for hydrogen bonding while in ethanol molecule one hydrogen atom is available. The negative charge on oxygen atom is more concentrated in water molecules as compared to ethanol molecules where oxygen atom is bonded to alkyl group which withdraws some electron density from oxygen atom and makes it less negative. So when you compare the strength of hydrogen bonding in water and ethanol you will find that it is stronger in water as it has more possibilities for hydrogen bond formation. When ethanol is mixed with water, energy required to break old hydrogen bond of water is much higher than energy released in the formation of new hydrogen bonds between water and ethanol. That’s why temperature falls on mixing ethanol with water.

 
In such mixtures where new interactions are weaker than older interactions, molecules can easily escape into vapor phase that’s why the observed vapor pressure is higher than that of expected. Mixtures that show large positive deviation from maximum boiling azeotropes. If you observe the phase diagram of ethanol- water, you will find that at composition of 95% ethanol by volume, it forms azeotropic mixture which boils at lower temperature than both water and ethanol.

 
 
These are the cases where mixtures exhibit abnormal behavior which we can predict to some extent by molecular interactions. Almost every mixture shows some deviation from Raoult’s law. In the coming post we will focus our discussion on the solutions of non volatile solute and solvent. And discuss their properties in detail. 
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According to Raoult’s law, the partial vapour pressure of each volatile component in any solution is directly proportional to its mole fraction. The solutions which obey Raoult’s law over the entire range of concentration are known as ideal solutions. The solutions that do not obey Raoult’s law (non-ideal solutions) have vapour pressures either higher or lower than that predicted by Raoult’s law. If the vapour pressure is higher, then the solution is said to exhibit positive deviation, and if it is lower, then the solution is said to exhibit negative deviation from Raoult’s law. Vapour pressure of a two-component solution showing positive deviation from Raoult’s law Vapour pressure of a two-component solution showing negative deviation from Raoult’s law In the case of an ideal solution, the enthalpy of the mixing of the pure components for forming the solution is zero. ΔsolH = 0 In the case of solutions showing positive deviations, absorption of heat takes place. ∴ΔsolH = Positive In the case of solutions showing negative deviations, evolution of heat takes place. ∴ΔsolH = Negative
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