Bioengineering Fundamentals-Saterbak Problem 4.5 of Textbook A bomb calorimeter

Question

Bioengineering Fundamentals-Saterbak Problem 4.5 of Textbook

A bomb calorimeter is a device commonly used to measure the internal energy of a substance, especially in combustion reactions. A calorimeter is well insulated and designed to maintain a constant volume. For a calorimeter to work properly, its calorimeter constant C must be known. The calorimeter constant is related to the internal energy change as delta U=CdeltaT. For benzoie acid (C7H6O2), the heat of combustion delta H is -3226.7kJ/mol. A 2.53-g sample of benzoie acid is burned in a bomb calorimeter at 25 degrees Celsius, and the temperature increases by 3.72 degrees. What is the calorimeter constant?

Explanation / Answer

Molecular weight of Benzoic acid =122, moles of Benzoic acid = 2.53/122=0.0207

Heat of combustion = -3226.7 Kj/mol

1 mol of Benzoic acid produces 3226.7 Kj heat

0.0207 mole of Benzoic acid produces 3226.7*0.0207=66.81 Kj =66.81*1000=66810 joules. This is enthalpy change

delH= 66810 joules.

since th system is insultated, this heat of combustion increases the internal energy

for constat Volume process

delH= delU

Hence delU= C*delT

66810= C*3.72

C= 66810/3.72 J/deg.c =17960 J/deg.c

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