Songlines Posted January 24, 2017 Report Share Posted January 24, 2017 (edited) I was trying to solve a Hess's Law problem, but I can't seem to "get rid of" one of the compounds. The compound is water. There is no H2O in the final equation (the equation I'm trying to determine the enthalpy for) but I really can't figure out a way to cancel it out. Is water negligible in this case? Here is the question if anyone is interested: Calculate the standard enthalpy change when one mole of methane is formed from its elements in their standard states. The standard enthalpies of combustion, ΔHθ c , of carbon, hydrogen and methane are −393, −286 and −890 kJmol-1 respectively. C (s ) + 2H2 (g) → CH4 ΔH1 = ? C (s) + O2 (g) → CO2(g) ΔH2 =−393 kJmol-1 H2 (g) + 1/2 O2 (g) → H2O (l) ΔH3 = −286 kJmol-1 CH4 (g) + 2O2 (g) → CO2 (g) + H2O (l) ΔH4 = −890 kJmol-1 Edited January 24, 2017 by Songlines Reply Link to post Share on other sites More sharing options...
kw0573 Posted January 24, 2017 Report Share Posted January 24, 2017 Check stoichiometry for reaction 4. After correct stoic, add reaction 2 to twice reaction 3, then subtract by reaction 4. 1 Reply Link to post Share on other sites More sharing options...
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