i)Q is heat of the system while w is the work done on or done by the system.
Q and W both depend on the path so neither of them is a state function.
By 1st law of thermodynamics q+w =U
Here U is internal energy of the system which is a state function.
Here the property of U does not match with Q and W.
It is observed in experiments that whether the reaction takes place in one step or many steps, the change in internal energy for a specified reactant and product will be same and it does not depend upon the path taken.
ii) The process of dissolving can be endothermic (temperature goes down) or exothermic (temperature goes up).
When water dissolves a substance, the water molecules tends to separate the particles by breaking the bond between them.
It takes energy to break the bonds between the molecules or ions of the solute. Energy is released when water molecules bond to the solute molecules or ions.If it takes more energy to separate the particles of the solute than is released when the water molecules bond to the particles, then the temperature goes down (endothermic).
During the dissolution of NH4Cl in water more energy is consumed to separate NH4Cl to NH4+ and Cl- but the energy released after the dissolution is comparatively less hence the reaction is endothermic.
heat + NH4Cl(s) --H2O --> NH4+(aq) + Cl-(aq)
iii) The gaseous form of matter is more disordered than liquid form. That is the entropy of gas is more than liquid. Same with steam and water.At boiling point the steam is the form of gas and water is liquid so the entropy of steam is more than water at boiling point.