A quantity of 1.435 g of naphthalene (C10H8), a pungent-smelling substance used in moth repellents, was burned in a constant-volume bomb calorimeter. Consequently, the temperature of the water rose from 20.28°C to 25.95°C. If the heat capacity of the bomb plus water was 10.17 kJ/°C, calculate the

Question

A quantity of 1.435 g of naphthalene ([latex]C_{10}H_{8}[/latex]), a pungent-smelling substance used in moth repellents, was burned in a constant-volume bomb calorimeter. Consequently, the temperature of the water rose from 20.28°C to 25.95°C. If the heat capacity of the bomb plus water was 10.17 kJ/°C, calculate the the heat of combustion of naphthalene on a molar basis; that is, find the molar heat of combustion.

Strategy Knowing the heat capacity and the temperature rise, how do we calculate the heat absorbed by the calorimeter? What is the heat generated by the combustion of 1.435 g of naphthalene? What is the conversion factor between grams and moles of naphthalene?

Accepted Answer

The heat absorbed by the bomb and water is equal to the product of the heat capacity and the temperature change. From Equation (6.16), assuming no heat is lost to the surroundings, we write

[latex]q_{cal} = C_{cal}lΔt[/latex]

= (10.17 kJ/°C)(25.95°C − 20.28°C)

= 57.66 kJ

Because [latex]q_{sys} = q_{cal} + q_{rnx} = 0, q_{cal} = −q_{rnx}[/latex]. The heat change of the reaction is −57.66 kJ. This is the heat released by the combustion of 1.435 g of [latex]C_{10}H_{8}[/latex]; therefore, we can write the conversion factor as

[latex]\frac{-57.66 kJ}{1.435 g C_{10}H_{8}}[/latex]

The molar mass of naphthalene is 128.2 g, so the heat of combustion of 1 mole of naphthalene is

molar heat of combustion=[latex]\frac{-57.66 kJ}{1.435 \cancel{g C_{10}H_{8}} } imes \frac{128.2 \cancel{g C_{10}H_{8}} }{1 mol C_{10}H_{8}} [/latex]

[latex]=-5.151 imes 10^{3} kJ/mol[/latex]

Check Knowing that the combustion reaction is exothermic and that the molar mass of naphthalene is much greater than 1.4 g, is the answer reasonable? Under the reaction conditions, can the heat change (−57.66 kJ) be equated to the enthalpy change of the reaction?

Question 6.6: A quantity of 1.435 g of naphthalene (C10H8), a pungent-smel...

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