determine the amount of thermal energy

Question

When 1.14 g of octane (molar mass = 114 g/mol) reacts with excess oxygen in a constant volume calorimeter, the temperature of the calorimeter increases by 10.0°C. The heat capacity of the calorimeter is 6.97 kJ/°C. Determine the amount of heat or thermal energy associated with the reaction.

Answer 1

First, write a correctly balanced equation for the reaction taking place:

2C₈H₁₈ + 25O₂ ==> 16CO₂ + 18H₂O

q = C_cal∆T

q = heat = ?

Ccal = heat capacity of the calorimeter = 6.97 kJ/ºC

∆T = change in temperature =10.0ºC

q = 6.98 kJ/ºC x 10.0ºC = 69.8 kJ = the heat or thermal energy associated with this reaction, using 1.14 g octane. However, if you want to know the molar heat of combustion, which is the heat associated with combustion of 1 mole of octane, then...

69.8 kJ/1.14 g x 114 g/mole = 6980 kJ/mole