Imagine you are performing a calorimetry experiment using a constant-pressure ca

Question

Imagine you are performing a calorimetry experiment using a constant-pressure calorimeter. First, you calibrate the calorimeter using the reaction of KOH with HCl (Delta H^o rxn = -56.2 kJ/mol). Then, you use the calorimeter to investigate the dissolution of solid ammonium nitrate (NH4NO3). The questions below involve your data. Assume the density and specific heat capacity of your solutions is the same as liquid water.

A quantity of 85.0 mL of 0.600 M KOH is mixed with 85.0 mL of 0.600 M HCl in a coffee-cup calorimeter. The initial temperature of both solutions is 17.36C, and the final temperature of the mixed solution is 19.02C. What is the heat capacity of the calorimeter? Provide fully explained calculations.

A 3.52 g sample of NH4NO3 is then dissolved in 80.0 mL of water in the same calorimeter, and temperature of the solution changes from 17.54C to 16.64C. What is the enthalpy of solution of ammonium nitrate, in kJ/mol? Provide fully explained calculations.

How does the value calculated in part (b) relate to the use of ammonium nitrate in instant cold packs (e.g., in first aid kits)? Explain briefly.

Explanation / Answer

Reaction:

KOH + HCl ---> KCl + H2O

Moles of each reactant (KOH or HCl) = (85.0 mL*0.600 mmol/mL) = 51.0 mmol

Mass of solution = (85.0+85.0) mL * 1.00 g/mL = 170.0 g

Heat absorbed by solution and calorimeter,

q = (170.0 g * 4.18 J/g oC + Ccal) (19.02-17.36) oC = (710.6 + Ccal) (1.66)

Heat released by reaction = (56.2 kJ/mol)(51.0 mmol) = 2866.2 J

Equate heat absorbed to heat released:

(710.6 + Ccal) (1.66) = 2866.2

Ccal = 1016 J/oC = 1.02 kJ/oC

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