The enthalpy of reaction in this case will be
The enthalpy of reaction is defined as the difference between the sum of the standard enthalpies of formation of the products and the sum of the standard enthalpies of formation of the reactants.
The reaction you're interested in is
You know that you must use the following reactions
Because the standard enthalpy of formation for elements in their most stable form is zero, the enthalpies of reaction given to you for the above three reactions are actually the standard enthalpies of formation of
This means that you can use them in equation (1) to solve for
Since equation (2) gives you an enthalpy value for 2 moles of
Rounded to three sig figs, the answer will be
SIDE NOTE Enthalpies of formation are usually given for the formation of 1 mole of a substance, which would make equation (2) actually look like this
In this case, you'll have to multiply this value by 4 in order to get the correct enthalpy needed for your reaction because, like I've said, you have 4 moles of HCl that are produced.