I don't think it's will be very hard for Chelsea to beat Madrid. Chelsea's current position is not very good, the change of the coach will greatly affect the game. Chelsea players must adapt to the new coach's style, and sometimes this is not necessarily going well. Indeed, motivation can beat everything, but Madrid players also has motivation to add their collection of UCL trophies even though they have become king of UCL with the most winners.
I think the chances of FC Chelsea to beat Real Madrid and to advance into the semi final of the Champions League are significantly higher than most people think. First off all Chelsea still has a good squad and even though they have a pretty bad run in the premier league at the moment, the champions league simply is a totally different competition and every player will go all out in such matches.
Secondly i think that Real Madrid is not really that strong. For me clubs like Manchester City or Bayern Munich are way stronger than Real at the moment. So it is not like Chelsea is facing an unbeatable opponent here. Of course Real also was not the favorite to win it last year, so that does not necessarily mean that much.