The volume is halved
(Explanation): this is boyle's law PV=PV. In order to keep this equation true, if one variable (pressure in this case) is doubled, the other variable must be the reciprocal of that (in this case 1/2, which is reciprocal of double).
Evaporating and condensing
All the changes to a system cancel out.
Changing the temperature will change Keq
Increasing temperature (by heating up a reaction mixture of reactants and products at equilibrium) will also increase the value of Keq (i.e. more products formed) if products are formed endothermically.
Keq = A * e ^[-H/R.T]