Chemistry help!!!

Teacher gave me a couple of questions but I don't understand this one:
2. There are 2 isotopes of Cl. 35Cl and 37Cl. What would you observe when they react?
Could someone help me please

35Cl and 37Cl have moer particles in the nucleus, so they a heavier relative atomic mass. Because of this fact, the reactivity of those Isotopes are a bit weaker in comparison to ordinary chlorine atoms which are react with each other. That is to say those Chlorine atoms may react a bit slowlier than ordinary Chlorine atoms. Or to be a bit clearer: the speed of reaction is a bit slowlier.

You are talking kinetic isotope effect, but they don't teach this at A' level. With chlorine isotopes it will be very small. There was a paper in Nature about 5 years ago which described an experiment measuring kinetic isotope effects for other atoms than hydrogen, however this probably hasn't filtered down to your teachers yet. What they teach at A' Level is that isotopes behave chemically exactly the same.