Use the standard enthalpies of formation to determine ΔH∘rxn for the following chemical equation: 2 H2 (g) + O2 (g) →2 H2O (l)

Answer 1

Be careful to:

-not get this mixed up with bond enthalpies

-pay attention to negatives

-use the coefficients

-watch out that the Hf value for the same chemical formula is unique for each state of matter (different values for liquid water and for ice and for steam).

ΔHrxn=∑ΔHf(products)-∑ΔHf(reactants)

ΔHrxn=(2[H2O(l)])-(2[H2(g)]+O2(g))

ΔHf H2O(l) = -285.8 kJ/mol

ΔHf H2(g) = 0 kJ/mol

ΔHf O2(g) = 0 kJ/mol

ΔHrxn=(2[-285.8])-(0)

ΔHrxn= -571 kJ/mol