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Hess's law of heat summation, how to calculate unknown ΔHrxn, standard states, formation equation

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Colorado State University
CHEM 111
Dana Johnson

10 December Announcements Check RamCT for updates 3 online assignments All open now, close 12 / 15 at 6 pm Final clicker review Wed, 12 / 15 here 9:10 – 11:10 am Final exam Friday 7 – 9 am Rooms close at 7:30 am, do not be late Review Net reactions Sequential reactions can be additive Gives overall reaction Thermochemical equations Connect a reaction with energy change Can be reversed with a sign change When coefficients are divided, ΔH ’srxne as well Hess’s law of heat summation Can’t experimentally determine ΔH rxnfor every reaction But, can determine ΔH rxnvia Hess’s law: The ΔH rxnof an overall reaction is the sum of the ΔH rxnalues of individual steps Another consequence of the state function property Example reaction Target reaction: CH (4) + 2O (g) 2 CO (g) + 2H2O (l) ΔH 2 rxn= ? CH (4) + 2O (g) 2 CO (g) + 2H2O (g) ΔH 2 rxn= -802 kJ 2H O2(g) → 2H O (l2 ΔH rxn= -88 kJ CH (g) + 2O (g) → CO (g) + 2H O (l) ΔH = -890 kJ 4 2 2 2 rxn How to calculate unknown ΔH rxn 1. Note the coefficients of the target reaction 2. Manipulate equations with known ΔH rxnto match target reaction Reverse and change signs Multiply coefficients and ΔH rxnby the same factor 3. Add reactions and ΔH values to get the target reaction and unknown ΔH rxn rxn More complicated example Calculate ΔH rxnfor the following: 2NOCl (g) → N (g) 2 O (g) + C2 (g) 2 Given the following reactions: ½N (2) + ½ O (g) 2 NO (g) ΔH rxn= 90.3 kJ NO (g) + ½ Cl (g) → NOCl (g) ΔH = -386 kJ
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