Dortmund will play vs Schalke just after its Champions League game vs Chelsea. Since Dortmund doesn't have a clear advantage in the first leg, then it will take that game seriously.
In Schalke game, Dortmund is playing away vs a team that is trying at every cost to get out of the relegation zone. So far Schalke shrank the gap and might escape once it secure 3 points against Dortmund, even though the odds are in favour of Dortmund obviously.
Maybe in Dortmund's match with Chelsea it will let Chelsea win, but it is possible that when there is an opportunity to score, Dortmund will do it. Meanwhile, Dortmund will take advantage of Schalke because it is a very easy opponent to beat. Dortmund should be able to score more than five goals so that they can win the first position and shift Bayern Munich from the top position of the standings.