[Rejjy13]

Q. Two liquids A & B are mixed and the resulting solution is found to be cooler. What type of deviation from the Raoult's law is this?

Attempted Answer :
I assumed that since the solution is cooler, it must have lost heat. A solution losing heat signifies that stronger molecular interactions are formed than those originally in the individual components.
It means the A----B type interactions are stronger than A----A and B----B type interactions.
As a result, the molecules present in the solution are more closely bound to each other than in the original liquid components themselves.
Hence the molecules cannot vapourise easily. Therefore, the vapour pressure of the solution must be less than that expected as per Raoult Law. That is, a negative deviation is observed.

Is this correct?

My teacher claims that since the solution is cooler, it has gained heat. Therefore the deviation is negative. Kindly explain if this is in fact the correct explanation.

[XGen]

When a reaction becomes cooler, it is because it is taking heat from the surroundings and making the surroundings cooler. For example, if the two liquids were at room temperature and in a glass, they would absorb heat from the glass for the reaction to occur. This makes the glass cooler, and makes it cool to the touch.

Another way to think about it is the evaporation of water. When we sweat, beads of water form on our skin. Now, vaporization is clearly an endothermic process and therefore absorbs heat. We sweat because as the water vaporizes, it absorbs heat from our skin and the air around us, cooling ourselves down.

[Rejjy13]

@XGen

I agree with that part completely. However, the question says that the solution becomes cooler, not (explicitly) the apparatus. Kindly re explain.

Thanks for your help.

[XGen]

I guess the wording is slightly ambiguous, but generally people consider that a solution becomes cooler when the temperature drops. The temperature drops when the reaction is endothermic, even though it is absorbing energy. This is probably because the heat absorbed is used in forming the bonds.

[Rejjy13]

I guess the wording is slightly ambiguous, but generally people consider that a solution becomes cooler when the temperature drops. The temperature drops when the reaction is endothermic, even though it is absorbing energy. This is probably because the heat absorbed is used in forming the bonds.

How is that possible? I thought heat is released in the formation of stronger, newer, better bonds.

[XGen]

That is for exothermic reactions.

Endothermic reactions, if I recall correctly, require energy to proceed because the bond energies of the products are lower than the bond energies of the reactants. It takes more energy to keep the bonds there.

Usually endothermic reactions are less likely to be spontaneous than exothermic reactions, because the reactants are more stable than the products. However, in this case, the dissociation of a solid into many many ions is a large increase in entropy. This allows the reaction to be spontaneous even though it is endothermic.