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CALCULATING HEAT CHANGES Section 11.4

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After reading Section 11.4, you should know: What Hesss Law is How to apply Hesss Law to a chemical equation How to calculate the heat of formation for a given chemical equation

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Adding 2 equations together…you can cancel compounds out IF they are exactly the same on the reactants and products side 2Al + 1 1 / 2 O 2 --> Al 2 O 3 H = -1669.8 kJ Fe 2 O 3 --> 2Fe + 1 1 / 2 O 2 H = 824.2 kJ 2Al + Fe 2 O 3 --> 2Fe + Al 2 O 3 H = -844.8 kJ Hesss Law The heat of 2 reactions added together will equal the heat of a 3rd reaction

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H f = standard heat of formation 0 Change in enthalpy when 1 mol of new compounds form under standard conditions Table of values on page 316 Ex: Calculate the H 0 for 2NO (g) + O 2 --> 2NO 2 H f NO = 90.37 kJ / mol H f O 2 = 0.0 kJ / mol H f NO 2 = 33.85 kJ / mol 0 0 0

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1. Find sum of H f reactants 0 Remember, you have 2 moles of NO 2. Find sum of H f products 3. Find the difference between H f products and H f reactants You should have gotten: -113.04 kJ / mol 0

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After reading Section 11.4, you should know: What Hesss Law is How to apply Hesss Law to a chemical equation How to calculate the heat of formation for a given chemical equation

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