The Fire had spotted the Impact a 1-0 lead, but Sherjill MacDonald notched the equalizer in the 34th minute on a header off a cross from Jalil Anibaba.
Then in the second half, Chicago grabbed the lead for good when Alvaro Fernandez sent a pass into the penalty area and found Alex, who beat Montreal goalkeeper Troy Perkins with a shot to the upper corner.
Fernandez later added an insurance tally for the Fire (15-8-5), who are looking to secure a Western Conference playoff spot.
Marco Di Vaio scored for the Impact (12-15-3).
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