2 Calculating Enthalpy Change A theoretical way to determine ∆H for a chemical reaction is provided by Hess’s 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.
3 Hess’s LawThere is an amount of heat associated with every chemical reaction.
4 Hess’s LawOften you know the heat for parts of the reaction, and you must add them together to find the heat for the total reaction.
5 Applying Hess’s LawUse 2 thermochemical reactions to determine ∆H for the oxidation of ethanol (C2H5OH) to form acetaldehyde (C2H4O) and water.Here is the overall reaction:ethanol + oxygen gas acetaldehyde + waterHere are the two component reactions:
6 Applying Hess’s LawFor 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.
7 Applying Hess’s LawAdd these two equations, and cancel any terms common to both sides of the combined equation.12∆ H = -349kJNote that ∆H is negative, which means the reaction is exothermic. (releasing energy)
8 Basic Assessment Questions Practice Hess’s Law!Use reactions a and b to determine ∆H for this single-displacement reaction.Cl2(g) + 2HBr(g) HCl (g) + Br2(g)a. H2(g) + Cl2(g) HCl (g) ∆H= -185b. H2(g) + Br2(g) HBr (g) ∆H= -73
9 Practice Hess’s Law a. H2(g) + Cl2(g) 2HCl (g) ∆H= -185kJ Keep equation “a” as written because HCl is on the right in the total reaction:a. H2(g) + Cl2(g) HCl (g) ∆H= -185kJFlip equation “b” because HBr needs to be on the left in the overall equationb. H2(g) + Br2(g) HBr (g) ∆H= -73kJb HBr (g) H2(g) + Br2(g) ∆H=73kJ
10 a. H2(g) + Cl2(g) 2HCl (g) ∆H= -185 kJ Now add the two equations togethera. H2(g) + Cl2(g) HCl (g) ∆H= -185 kJb HBr (g) H2(g) + Br2(g) ∆H= 73kJCl2(g) + 2HBr(g) HCl (g) + Br2(g)∆H = -112= -112kJ
12 Practice using q=cm∆TA 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!
13 A 15. 6-g sample of ethanol absorbs 868 J as it is heated 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?868 J(2.44J/gºC ) (15.6g)∆T =q .c m=Remember this is not your final answer.You are looking for the final temp… so add the initial temp to this number∆T= 22.8ºC22.8ºC ºC = 44.3ºC = final temp
14 Practice q=mc∆T 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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