Which of the following binary solutions of volatile liquids have negative deviation?
#A)# chloroform and acetone
#B)# chlorobenzene and bromobenzene
#C)# benzene and toluene
#D)# hexane and heptane
The mixture is
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Since the two liquids are volatile (i.e. they vaporize easily), we're going to have to find an expression for the total vapor pressure of the ideal binary mixture, DETERMINING THE TOTAL VAPOR PRESSURE We assume the vapor above the solution is ideal so that we can utilize Dalton's law of partial pressures: where In an ideal solution, the solvent always follows Raoult's law, so we can write: where the We assume the solution is also not dilute, since That means the vapor pressure of the mixture of volatile liquids is: But since this is a binary mixture, i.e. two components, we know that the mol fractions add up to The total vapor pressure assuming ideality is therefore: CLASSIFYING THE SOLUTION'S DEVIATION FROM IDEALITY We're not done yet though. The real vapor pressure you've given us is That is indicative of negative deviation, which is depicted by this diagram:
(The dashed diagonal lines and the horizontal line at the top are from Raoult's law, and the solid lines are the real vapor pressures.)
Negative deviation is when the average distance between solvent and solute particles is closer than in an ideal solution. This is when the energy of solute-solvent interaction ( This means that the intermolecular forces formed between solute and solvent are stronger than those that were present previously amongst the solute and solvent within themselves. Recall the most important ones: CONSIDERING THE BINARY MIXTURE OPTIONS We now examine the mixture choices. This is a tricky example, but chloroform has dipole-dipole interactions with itself, and acetone also has dipole-dipole interactions with itself... however, acetone hydrogen-bonds with chloroform!!
The three chlorine atoms are electron-withdrawing groups, which make the hydrogen very electropositive.
This is enough for the carbonyl oxygen on acetone to become a hydrogen-bond acceptor, polarizing the hydrogen atom and forming a hydrogen-bonding interaction. Since hydrogen-bonding is stronger than dipole-dipole interactions on average, this mixture will have negative deviation, which agrees with how we got This mixture has two substances that only have dipole-dipole forces to exchange, and they therefore form essentially ideal solutions.
Both of these primarily have dispersion forces (toluene,
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The binary solutions of volatile liquids that exhibit negative deviation from Raoult's law typically have stronger intermolecular attractions between unlike molecules than within their pure components. Examples include acetone and chloroform or ethanol and water.
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When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.
When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.
When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.
When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.
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