I have a question on a lab report that ask: Explain why the rate constants for your trials should (or should not) differ from each other.

I know that rate constants would not change based on the concentrations, but is would change if the temperature had been raised or lowered. In our experiment trials we never changed the temperature so my initial thought would be that they should have stayed the same, but in the second trial we added a concentration of a substance we did not use in our first trial. I believe that would cause the rate constant to change, but I am not exactly sure of the reasoning.