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# Sample problem answers - Enthalpy Sample Problems 1 Given...

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Enthalpy Sample Problems: 1. Given the equation 3 O 2(g) 2 O 3(g) Δ H = +285.4 kJ, calculate Δ H for the following reaction. 3/2 O 2(g) O 3(g) . Answer Since 3/2 O 2(g) O 3(g) is ½ of 3 O 2(g) 2 O 3(g) the enthalpy of the reaction will be ½ as well: ½ (+285.4kJ) = +142.7kJ 2. Given the equation: 2Ag 2 S (s) + 2H 2 O (l) 4Ag (s) + 2H 2 S (g) + O 2(g) Δ H = +595.5kJ, calculate Δ H for the following reaction. Ag (s) + ½ H 2 S (g) + ¼ O 2(g) ½ Ag 2 S (s) + ½ H 2 O (l) Answer In this problem, the reaction equation has been reversed and divided by four. The new enthalpy value will be the opposite sign and ¼ its original value: -1/4(+595.5kJ) = -148.9kJ 3. Express the following information as a chemical equation. At 25 o C and at a constant pressure, dinitrogen trioxide gas decomposes to nitrogen monoxide and nitrogen dioxide gases with the absorption of 0.533kJ of heat for every gram of dinitrogen trioxide that decomposes.

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Answer +0.533kJ/g N 2 O 3 0.533kJ/g (76.01g/mol) = +40.5kJ/mol N 2 O 3(g) NO (g) + NO 2(g) Δ H = +40.5kJ 4. What is the enthalpy change when 12.8g H 2(g) reacts with excess Cl 2(g) to form HCl (g) ? H 2(g) + Cl 2(g) 2HCl (g) Δ H = -184.6kJ Answer 12.8g H 2 / 2.016g/mol = 6.35mol H 2 6.35mol H 2 (-184.6kJ/mol H 2 ) = -1.17x10 3 kJ 5. What volume of CH 4(g) , measured at 25 o C and 745Torr, must be burned in excess oxygen to release 1.00x10 6 kJ of heat to the surroundings?
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Sample problem answers - Enthalpy Sample Problems 1 Given...

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