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Energy Lecture 4 Hesss Law & Review

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Calculating Enthalpy Change A theoretical way to determine H for a chemical reaction is provided by Hesss law, which states that if two or more thermochemical equations can be added to produce a final equation for a reaction, then the enthalpy change for the final reaction equals the sum of the enthalpy changes for the individual reactions.

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Hesss Law There is an amount of heat associated with every chemical reaction.

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Hesss Law Often you know the heat for parts of the reaction, and you must add them together to find the heat for the total reaction.

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Applying Hesss Law Use 2 thermochemical reactions to determineH for the oxidation of ethanol (C 2 H 5 OH) to form acetaldehyde (C 2 H 4 O) and water. Here is the overall reaction: Here are the two component reactions: ethanol + oxygen gas acetaldehyde + water

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Applying Hesss Law For the overall reaction, Acetaldehyde should be on the right side of the equation, so reverse equation a. Note that you must change the sign of H. The desired equation has two moles of ethanol, so double equation b and its H.

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Applying Hesss Law Add these two equations, and cancel any terms common to both sides of the combined equation. Note that H is negative, which means the reaction is exothermic. (releasing energy) H = -349kJ 21

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Basic Assessment Questions Practice Hesss Law! Use reactions a and b to determine H for this single-displacement reaction. Cl 2 (g) + 2HBr(g) 2 HCl (g) + Br 2 (g) a. H 2 (g) + Cl 2 (g) 2HCl (g) H= -185 b. H 2 (g) + Br 2 (g) 2HBr (g) H= -73

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Practice Hesss Law Keep equation a as written because HCl is on the right in the total reaction: a. H 2 (g) + Cl 2 (g) 2HCl (g) H= -185kJ Flip equation b because HBr needs to be on the left in the overall equation b. H 2 (g) + Br 2 (g) 2HBr (g) H= -73kJ b. 2HBr (g) H 2 (g) + Br 2 (g) H=73kJ

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Now add the two equations together a. H 2 (g) + Cl 2 (g) 2HCl (g) H= -185 kJ b. 2HBr (g) H 2 (g) + Br 2 (g) H= 73kJ Cl 2 (g) + 2HBr(g) 2 HCl (g) + Br 2 (g) = -112kJ H = -112

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Answer H rxn = –112 kJ

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Practice using q=cmT A 15.6-g sample of ethanol absorbs 868 J as it is heated. If the initial temperature of the ethanol was 21.5°C, what is the final temperature of the ethanol? Hint: solve for T then add 21.5!

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A 15.6-g sample of ethanol absorbs 868 J as it is heated. If the initial temperature of the ethanol was 21.5°C, what is the final temperature of the ethanol? T = q. c m = 868 J (2.44J/gºC ) (15.6g) T= 22.8 ºC Remember this is not your final answer. You are looking for the final temp… so add the initial temp to this number 22.8ºC ºC = 44.3ºC = final temp

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Practice q=mcT again! If 335 g water at 65.5°C loses 9750 J of heat, what is the final temperature of the water?

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Answer 58.5°C

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Practice using q=mol x H How much heat is evolved when 24.9 g of propanol (C 3 H 7 OH) is burned?H comb = –2010 kJ/mol Molar mass of C 3 H 7 OH is 60.1g/mol

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Answer 833 kJ

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