-571.7 kJ. Let's assume the formation of water, H2O, from hydrogen gas, H2, and oxygen gas, O2. To find enthalpy:
Write the formation reaction: 2H2 + O2 → 2H2O.
Note that the enthalpy of H2 and O2 in their elemental state is zero.
Calculate the enthalpy as:
∑ΔHproducts − ΔHreactants = 2 × -285.83 kJ − ( 2 × 0 kJ + 0 kJ) = -571.7 kJ.
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