[maloosh]

"Use your experimental results to calculate the molar heat of reaction for the reaction of calcium with water in Kj/mol of calcium used. Using this value, and the value of the heat of reaction for the equation H2 + 1/2 O2 --> H20

Calculate the molar heat of formation of calcium hydroxide   Ca (s) + O2 + H2 ---> Ca(OH)2 (s) + H2(g) "

the part I don't understand is the last part "Calculate the molar heat of formation". Isn't that the Sum of products- Sum of reactants equation? Or are we using Hess's law?

If it was indeed sum of p- sum of r, how would you do it using two different chemical equations as shown above? I've only been taught to do it with one chemical equation.

[mjc123]

It's a Hess's law problem. Given the 2 heats of reaction mentioned, you can work out the heat of formation using a Hess's law cycle. (Be careful about stoichiometry.) Your quoted equation doesn't balance. Write Ca + O₂ + 2H₂  Ca(OH)₂ + H₂. The reason for the extra mole of hydrogen will become clear when you set up the Hess's law cycle.
In general, the heat of a reaction can be expressed as Σ ΔH_form(products) - Σ ΔH_form(reactants). But in this case, as all the reactants are pure elements, their heat of formation is zero, so the expression reduces to ΔH_form(Ca(OH)₂) = ΔH_form(Ca(OH)₂).