Problem: For which of the following reactions is ΔH∘rxn equal to ΔH∘f of the product(s)? You do not need to look up any values to answer this question. Check all that apply.a) Li(s)+12F2(g)→LiF(s)b) Li(s)+12F2(l)→LiF(s)c) H2O2(g)→12O2(g)+H2O(g)d) 2H2(g)+O2(g)→2H2O(g)e) 2Li(s)+F2(g)→2LiF(s)f) H2(g)+12O2(g)→H2O(g)

We are being asked to determine which of the following reactions is ΔH°_rxn equal to ΔH°_f of the product(s)

Recall that enthalpy of reaction is given by the equation

$\boxed{\mathbf{\Delta H}{\mathbf{°}}_{\mathbf{rxn}}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{\Delta H}{\mathbf{°}}_{\mathbf{f}\mathbf{,}\mathbf{ }\mathbf{products}}\mathbf{ }\mathbf{-}\mathbf{ }\mathbf{\Delta H}{\mathbf{°}}_{\mathbf{f}\mathbf{,}\mathbf{ }\mathbf{reactants}}}$

So for ΔH°_rxn = ΔH°_{f, product}, 

• Looking for a formation equation where only 1 mole of product is made
• Look for a reaction in which all reactants are in natural state.
  • ΔH°_f  = reactants = 0 for elements in their natural state, such as diatomics: H₂, N₂, F₂, O₂, I₂, Cl₂, Br₂