Non-ideal solutions

All it shows is that there has been an exothermic event as a result of the mixing process.

Diluting sulphuric acid with water will release heat, as will mixing hydrochloric acid and sodium hydroxide solutions, as two simple examples. In both cases, the result is a revised system with less chemical energy than beforehand. By the First_Law_of_Thermodynamics it has to go somewhere, so it appears as heat coming out of the system. By convention, heat leaving a chemical system is shown as negative.

so, are you saying that it would be -ve deviation, coz exothermic processes show that?

but, what my friend is saying is that because the soln becomes warm, it will expand and ΔV(mix)>0, showing +ve deviation & also, that the total vapour pressure wud inc, again leading to +ve dev.

if i'm right about it being -ve dev, plz help me in justifying my ans more appropriately.

thanx.

If the solution is in a totally closed system with no heat loss to outside, its 0 different.

If system is not totally close and heat is lost to outside, different is -ve.

A+B = C + heat so C got less energy then when it start.

A+B+heat = C so C got more energy then it start.

Pineapple

Elika,

The question you are asking is does the solution show positive or negative deviations from ideality. Therefore, the question you should be asking yourself is what is ideality? For the case of phase behavior, ideality is Raoult's Law (RL). This shows how the compostion varies with changes in temperature or pressure.

deviations from RL are shown by differences in the measured vapor pressure and the one determined by RL. Therefore, positive deviations indicate that the vapor pressure is greater than that calculated from RL. And negative deviation is when the vapor pressure measured is lower than that calculated by RL.

Vapor pressure is strongly dependent on temperature. Therefore increasing the temperature of the system will increase the vapor pressure. From the problem statement, the mixing causes an exothermic reaction. the temperature therefore increases and so does the vapor pressure. So it is a positive deviation from ideality.

I would go with your friend on this one. Also, the sign convention for energy leaving a system is different for every book. I would not use that as an agrument.

But by convention, for an exothermic process, ΔH[mix]<0 and that shows -ve deviation.

Also, ΔV[mix]<0 (because of formn of vapours) that too leads to -ve deviation.

And, if vol of mixture is decreasing, wouldn't it mean an inc in vol of container w.r.t. vapors, which would in fact, dec the vapor pressure, again showing -ve deviation.

Please reply.

Hi Elika,

It appears my argument as well as your friends is completely different to what the answer is. There does exist the concept of deviations from ideality w.r.t. Raoult's Law. This is what possibly confused myself and your friend.

However, after re-reading some thermodynamics, I must agree on the basis of energy and volume changes in the system that the problem is in fact a negative deviation since for the ideal case both changes in enthalpy and volume are equal to zero.

You can also make a similar argument for entropy of the mixture.