1.Using your boiling point composition diagram and Dalton's and Raoult's Laws, explain
where the boiling point range of the mixture will be during the distillation relative to the
individual boiling points of the components.
Answers
Answer:
Ideal mixtures
An ideal mixture is one which obeys Raoult's Law, but I want to look at the characteristics of an ideal mixture before actually stating Raoult's Law. The page will flow better if I do it this way around.
Examples of ideal mixtures
There is actually no such thing as an ideal mixture! However, some liquid mixtures get fairly close to being ideal. These are mixtures of two very closely similar substances.
Commonly quoted examples include:
hexane and heptane
benzene and methylbenzene
propan-1-ol and propan-2-ol
Ideal mixtures and intermolecular forces
In a pure liquid, some of the more energetic molecules have enough energy to overcome the intermolecular attractions and escape from the surface to form a vapour.
The smaller the intermolecular forces, the more molecules will be able to escape at any particular temperature.If you have a second liquid, the same thing is true. At any particular temperature a certain proportion of the molecules will have enough energy to leave the surface.
In an ideal mixture of these two liquids, the tendency of the two different sorts of molecules to escape is unchanged.
You might think that the diagram shows only half as many of each molecule escaping - but the proportion of each escaping is still the same. The diagram is for a 50/50 mixture of the two liquids. That means that there are only half as many of each sort of molecule on the surface as in the pure liquids. If the proportion of each escaping stays the same, obviously only half as many will escape in any given time.
If the red molecules still have the same tendency to escape as before, that must mean that the intermolecular forces between two red molecules must be exactly the same as the intermolecular forces between a red and a blue molecule.
If the forces were any different, the tendency to escape would change.
Exactly the same thing is true of the forces between two blue molecules and the forces between a blue and a red. They must also be the same otherwise the blue ones would have a different tendency to escape than before.
If you follow the logic of this through, the intermolecular attractions between two red molecules, two blue molecules or a red and a blue molecule must all be exactly the same if the mixture is to be ideal.
This is why mixtures like hexane and heptane get close to ideal behaviour. They are similarly sized molecules and so have similarly sized van der Waals attractions between them. However, they obviously aren't identical - and so although they get close to being ideal, they aren't actually ideal.
For the purposes of this topic, getting close to ideal is good enough!
Ideal mixtures and enthalpy change of mixing
When you make any mixture of liquids, you have to break the existing intermolecular attractions (which needs energy), and then remake new ones (which releases energy).
If all these attractions are the same, there won't be any heat either evolved or absorbed.
That means that an ideal mixture of two liquids will have zero enthalpy change of mixing. If the temperature rises or falls when you mix the two liquids, then the mixture isn't ideal.
Raoult's Law
You may have come cross a slightly simplified version of Raoult's Law if you have studied the effect of a non-volatile solute like salt on the vapour pressure of solvents like water. The definition below is the one to use if you are talking about mixtures of two volatile liquids.
Explanation:
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