Question

1. 3.000 grams of Ca is burned in a bomb calorimeter. The water’s temperature rose from 20.0 degrees centigrade to 21.79 degrees centigrade. The heat capacity of the calorimeter is 26.60 kJ/C. What is the enthalpy change for this reaction as written. The thermochemical equation is:

2Ca (s) + O2 (g) —> 2CaO (s)

2. 1.30 grams of C7H6O2 is combusted in a bomb calorimeter. The water’s temperature rose from 20.00 degrees centigrade to 21.58 degrees centigrade. The heat capacity of the calorimeter is 10.90 kJ/C. What is the enthalpy change for this reaction. The thermochemical reaction is:

2C7H6O2 + 15O2 —> 14CO2 + 6H2O

3. 0.560 grams of C6H6 is burned in a bomb calorimeter. The water rises in temperature from 22.0 degrees centigrade to 29.90 degrees centigrade. The heat capacity of the calorimeter (bomb plus water) is 2.96 kJ/C. What is the enthalpy change for the combustion of one mole of C6H6? The thermochemical equation is:

C6H6 (l) + 15/2O2 —> 6CO2 (g) + 3H2O (l)

Answer 1

1):

Moles of Ca = Mass of Ca/ atomic mass of Ca = 3.000 g/(40.078 g/mol) = 0.07485 mol

Heat of calorimeter (q_cal) = heat capacity of calorimeter x temperature change

                                        = (26.60 kJ/^oC) x (21.79 ^oC - 20.0 ^oC)

                                        = 47.61 kJ

Enthalpy change of the reaction = - q_cal /mol Ca = - 47.61 kJ/0.07485 mol = - 636 kJ/mol