A student needs to calibrate a coffee-cup calorimeter. To do so she places 100.0
ID: 723521 • Letter: A
Question
A student needs to calibrate a coffee-cup calorimeter. To do so she places 100.0g of water at 58.60C into the calorimeter which already contains 100.0g of water at 19.30C. The final temperature of the water in the calorimeter is 38.70C. What is the heat capacity of the calorimeter?The calorimeter is then emptied and 345.00ml of water at 21.5000C is then added to it. Then 2.00ml of 6.50M NaOH and 3.00ml of 5.00M acetic acid at the same temperature are added to the calorimeter. The temperature of the solution rises to 26.2180C. The density of the resulting solution is 1.02g/ml. What is ?H (J/mol rxn) for the reaction?
Explanation / Answer
a) First assume that the heat capacity of water be 4.186 J/gm.K, heat capacity (and please not specific heat capacity, don't confuse yourself) of calorimeter be C then heat lost by water at 58.6 degC = heat gained by water at 19.3 degC + heat gained by calorimeter at 19.3 degC then, balance heat flows both sides 100*4.186*(58.6-38.7) = 100*4.186*(38.7-19.3) + C(38.7-19.3) solving, C = 10.788 J/K b) 2ml of 6.5M NaOH and 3ml of 5M acetic acid are added. I am assuming both compounds are fully dissociated. so we have (2/1000)*6.5 = 0.013 moles of NaOH and (3/1000)*5 = 0.015 moles of acetic acid. as 1 mol NaOH reacts with 1 mol acetic acid, NaOH is limiting reactant and only 0.013 moles of both reactants participate in reaction. so heat released by neutralization = heat gained by mixture + heat gained by calorimeter. the volume of solution = 345+2+3 = 350 ml density of solution = 1.02 gm/ml mass of solution = 350*1.02= 357 gm as trace amounts of reactants are added, the sp. heat capacity of the solution can be safely assumed to be constant and equal to 4.186 J/gm.K so, 357*4.186*(26.218-21.5) + 10.788(26.218-21.5) = heat of reaction so heat of reaction = 7101.6 J this amount of heat is released when 0.013 mol of reactants reacted. hence heat of reaction = 7101.6/0.013 = 54621.2 J/mol rxn
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