2Calculating 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.
3Hess’s LawThere is an amount of heat associated with every chemical reaction.
4Hess’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.
5Applying 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:
6Applying 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.
7Applying 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)
8Basic 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
9Practice 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
12Practice 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!
13A 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
14Practice 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?