In terms of players and home advantage which is obvious. If Barcelona got Lewandowski in their lineup then they might have a shot but with their current lineup is just not enough to win. If they can pull a 2-1 lost or keep it draw on the 1st leg then they might have a shot on the 2nd leg though but Real Madrid will definitely pressure them starting from the 1st half whistle
This is a quite difficult condition for Barcelona playing away at Real Madrid's home and they played without some of their key players.
What they expect is possibly for getting a drawn result for this first-leg match. But, we know Real Madrid will not let it be, moreover with playing at home, Real Madrid must be more motivated to win the match whatever happened. they have a bigger chance, expect that real Madrid let it lose the chance.
This El Classico may not influence to the position in La Liga, but whatever, El Classico will be always much interesting, moreover this is semi-final of Copa del Rey