You will not measure the enthalpy of the reaction of in the coffee cup calorimeter because this reaction involves gases. Instead, you will use a well-known published value. HCI and MgO
HCI and Mg
H2 and O2

H2 and O2

Explanation:

A coffee cup calorimeter is basically a type of calorimeter that operates at constant pressure. The heat measured in the calorimeter is equal to the change in enthalpy of the reaction.

This type of calorimeter is apt for reactions involving solutions where there is little or no change in volume.

It is not generally applicable to gas phase reactions such as the reaction between H2 and O2.