Question

A bomb calorimeter, or a constant volume calorimeter, is a device often used to determine the heat of combustion of fuels and the energy content of foods. In an experiment, a 0.7382 g sample of quinizarin (C14H8O4) is burned completely in a bomb calorimeter. The calorimeter is surrounded by 1.254×103 g of water. During the combustion the temperature increases from 25.17 to 28.32 °C. The heat capacity of water is 4.184 J g-1°C-1. The heat capacity of the calorimeter was determined in a previous experiment to be 868.3 J/°C. Assuming that no energy is lost to the surroundings, calculate the molar heat of combustion of quinizarin based on these data.

C14H8O4(s) + 14O2(g) 4 H2O(l) + 14 CO2(g) + Energy

Molar Heat of Combustion = ____kJ/mol

Answer 1

The heat involved in the reaction is related to the temperature change by the equation below

q_cal= C_cal ΔT

q_cal = (868.3 J / ^oC)×(28.32 ^oC − 25.17 ^oC)

q_cal = 2735 J

Since no heat is lost to the surroundings, the amount of heat absorbed is equal in magnitude to the amount of heat released from the combustion.

q_cal = −q_rxn

q_rxn = −2735 J

The balanced chemical equation for the combustion reaction is given below.

C₁₄H₈O₄(s) + 14O₂(g) --> 4 H₂O(l) + 14 CO₂(g) + Energy

Using the molar mass of quinizarin, we arrive with the molar heat of combustion as shown.

ΔE = - 2735 J / 0.7382 × 1 mol quinizarin / 240 g

ΔE = - 889189.9214 J / mol = 889190 J / mol

ΔE = - 889.190 kJ / mol