Benzene is added to gasoline to increase the octane number. When 0.113 g of benzene, C₆H₆, burns in excess oxygen in a calibrated constant-pressure calorimeter with a heat capacity of 551 J · (°C)^–1, the temperature of the calorimeter rises by 8.60°C. Write the thermochemical equation for the reaction 2 C₆H₆(l) + 15 O₂ (g) → 12 CO₂ (g) + 6 H₂O(l).

ANTICIPATE Because the temperature rises, the reaction is exothermic; therefore you should expect DH to be negative. All combustion reactions are exothermic.

PLAN The heat released by the reaction at constant pressure is calculated from the temperature change multiplied by the heat capacity of the calorimeter. Use the molar mass of one species (benzene) to convert the heat released into the reaction enthalpy corresponding to the thermochemical equation by noting the amount of C₆H₆ that reacts in the experiment (from n = m/M, where m is the mass of benzene and M is its molar mass) and then scaling the heat released to 2 mol C₆H₆. The molar mass of benzene is 78.12 g · mol^–1.

What should you assume? Assume that all the heat released by the reaction is absorbed by the calorimeter, with none lost to the rest of the surroundings.