When 0.96 g of benzoic acid was combusted, C7H6O2(s) + 15/2O2(g) rightarrow 7CO2

Question

When 0.96 g of benzoic acid was combusted, C7H6O2(s) + 15/2O2(g) rightarrow 7CO2(g) + 3H2O(l), using 10 cm of fuse wire for the electrical ignition, the temperature was increased by 2.41 K. The enthalpy of combustion (DeltaHcomb) of benzoic acid (MW = 122.122 g/mol) is 26,430.0 J/g. After the reaction, 2.0 cm of fuse wire was found unburned. For every centimeter of the fuse wire burned 9.62 Joule of energy was released. Calculate the heat capacity (Kc) of the calorimeter. Assume the initial temperature to be 298 K. Gas constant, R = 8.314 J mol-1 K-1.

Explanation / Answer

C7H6O2(s) + 15/2 O2 ----------> 7CO2(g) + 3 H2O (l)

Mass of benzoicacid= 0.96 grams

no of mole of benzoicacid = 0.96/122.122 = 0.0078 moles

Delta H comb= 26,430 j/g = 3227.68446 kj/ mol

Delta n= 1.5

R= 8.314 j mol-1.k-1

T= 298 k

delta E= Delta H- Delta n RT

= 3227.6- 1.5 X8.314X298

= 3227.6- 3.716 = 3223.884 KJ

but energy released by 0.0078 moles of benzoic acid = 3223.88 x 0.0078 = 25.14 kj

Length of the fuse wire initial= 10 cm

Length of the fuse wire after reaction = 2 cm

Length of the fuse wire burned = 10-2 = 8 cm

1cm wire gives 9.62 j energy

8cm wire gives energy= 8x9.62 = 76.96 j = 0.076 kj

Total energy released Delta E = 25.22 KJ

Heat capacity of calorimeter(Z) = - Delta E/Delta T X W/M

Delta T = 2.41 K

w= 0.96 grams

M= molecular weight of benzoic acid = 122.122 g/mol

Z= - 25.22/2.41 x 0.96/122.122 = 0.082270642 KJ . k-1.mol.

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