Physical Chemistry Thermodynamics: The enthalpy of combustion of benzoic acid (C6H5COOH) is commonly used as the standard...

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Chemistry

Physical Chemistry Thermodynamics:

The enthalpy of combustion of benzoic acid (C6H5COOH) iscommonly used as the standard for calibrating constant-volume bombcalorimeters; its value has been accurately determined to be—3226.7 kJ mol-1, (a) When 0.9862 g of benzoic acid was oxidized,the temperature rose from 21.84°C to 25.67°C. What is the heatcapacity of the calorimeter? (b) In a separateexperiment, 0.4654 g of glucose (C6H1206) was oxidized in the samecalorimeter, and the temperature rose from 21.22°C to 22.28°C.Calculate the enthalpy of combustion of glucose, the value of deltar U for the combustion, and the molar enthalpy offormation of glucose.

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4.1 Ratings (831 Votes)

a)

number of moles of benzoic acid = mass/ molar mass

                                                   = 0.9862 / 122.12

                                               =8.076*10^-3 mol

Heat released = 3226.7 KJ/mol*8.076*10^-3 mol

              = 26.06 KJ

Q = C*(Tf-Ti)

26.06 KJ = C* (25.67 – 21.84) oC

C = 6.804 KJ/oC

Answer: 6.804 KJ/oC

b)

Q = C*(Tf-Ti)

   = 6.804*(22.28 – 21.22)

   =7.212 KJ

number of moles of glucose= mass/molar mass

                           = 0.4654 / 180

                                         = 2.586*10^-3 mol

delta H = -Q/number of mol

     = - 7.212 KJ /(2.586*10^-3)mol

     = -2789.3 KJ/mol

Answer: -2789.3 KJ/mol


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