Paper 1 questions help

[Scienuk]

Hello,

Can you explain how to answer the following questions? the answer for 23 is D and the answer for 22 is A.

Edited June 5, 2016 by Scienuk

[Emmi]

23:

We can't conclude that A is liquid and B is vapor because we're not given any other information than this graph. We could be at conditions where both are in the liquid phase but A has a greater vapor pressure, or both are in the vapor phase with A having a greater vapor pressure. So this rules out A and B. C is incorrect because vapor pressure and boiling point are inversely related. When a species has a higher vapor pressure, its pressure is closer to atmospheric pressure. This lowers the boiling point because it has to overcome less force in order to boil. This only leaves D, which is correct because a molecule with stronger intermolecular forces requires more energy to disrupt. If the forces between molecules are strong, they're less likely to escape and form a vapor - hence their vapor pressure is lower.

22: The definition of Kc is concentration of products over concentration of reactants assuming ideal behavior (there are ways to calculate this with non-ideal behavior but this is way beyond the level of IB). Since Kc is concentration of products over concentration of reactants, and Kc is much greater than 1, then we know either the concentration of products is large, or the concentration of reactants is small. This means the reaction almost goes to completion, so A is correct. B is incorrect because Kc is large, so we know the reaction proceeded to near completion. C is incorrect because Kc doesn't measure how fast a reaction occurs, it measures how far a reaction proceeds. We cannot conclude anything about the reaction rates just knowing Kc, so D is incorrect.

Does this make sense conceptually?