Chemistry Question

This is a question i came across. The correct answer is B as I have marked.
I don’t understand the explanation provide.

  1. Why is the mixing of X & Y exothermic?
  2. Why is a vapor pressure of Z less than X or Y at the same temperature?


This is my reasoning feel free to correct me:

  1. mixing a solute and solvent is exothermic, removing a solute from a solvent is endothermic
    source: Intermolecular Forces in Mixtures And Solutions - Chemistry LibreTexts
    In the answer provided they reasoned with the equation:
    deltaH = sum (deltaH bonds broken) - sum (deltaH bonds formed)
    so deltaH is negative (exothermic) because X-Y IMF has more energy but this feels like a flawed or incomplete answer to me

  2. when IMF increase it will be harder for vapor to escape from the liquid and contribute to the pressure exerted by the vapor on the container wall
    Also according to the formula of vapor pressure:
    Ptot = x solvent * Psolvent
    x solvent = 1/1 before mixing and 1/2 after mixing (with n(x)=1 and n(y)=1 in my example)
    so the Ptot must decrease after mixing


Thank you! I get it now :slight_smile: