Question

Calculate the enthalpy of the reaction 4B(s)+3O2(g)→2B2O3(s)

Answer 1

I think you want to look up the enthalpy of formation for B2O3 here. Then sum the enthalpy of formations of the products and subtract the sum of the enthalpy of formations of the reactants. Both of the reactants are stable elemental forms, so both are 0.

Answer 2

4B(s)+3O2(g)--> 2B2O3(s)
4(0)kj 3(0)kj 2(-1273.5)kj

-2547kj+4(0)+3(0)
=-2547kj
or = -2547000 Joules