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]