The accepted value for the heat of combustion of methane is
-890 kj/mol.
explain why the experimental data might differ from the theoretical value

Theoretical value is what answer you would get, basically on paper.

Experimental Data will always have other factors that affect the experiment such as not complete control of the temperature or even the pressure can effect the activation barrier needed to reach combustion

The activity series is shown in the attached picture. if the element is below the other, then you can displace the other. following the activity series as a guide, the reactions are as follows: nickel and aluminum chloride3 ni + 2 alcl₃  → 3 nicl₂ + 2 albarium and copper bromideba + cubr  →  no reaction because cu is less oxidizable than bapotassium nitrate and aluminum3 kno₃ + al  → al(no₃)₃ + 3 ksodium phosphate and zincnaso₄ + zn  → znso₄ + na