Question

Mothballs are composed primarily of the hydrocarbon naphthalene (C10H8). When 1.025 g of naphthalene is burned in a bomb calorimeter, the temperature rises from 24.25 ∘C to 32.33 ∘C.Find ΔErxn for the combustion of naphthalene. The heat capacity of the calorimeter, determined in a separate experiment, is 5.11kJ/∘C. Express the change in energy in kilojoules per mole to three significant figure

Answer 1

Given the heat capacity of calorimeter is 5.11 kJ/oC

The change in temperature = 32.33 - 24.25 = 8.08 oC

So The total heat prroduced is = 5.11 kJ/oC x 8.08 oC

                                            = 41.28 kJ

The balanced equation is : C₁₀H₈ + 12O₂ ----> 10CO₂ + 4H₂O

Molar mass of naphthalene,C₁₀H₈ = (10x12)+(8x1) = 128 g/mol

1.025 g of naphthalene upon reaction produces 41.28 kJ

128 g of naphthalene upon reaction produces M kJ

M = ( 128x41.28) / 1.025

    = 5.16x10³ kJ

Therefore ΔE_rxn = -5.16x10³ kJ (the negative sign indicates the reaction is exothermic)