Mothballs are composed primarily of the hydrocarbon naphthalene (C10H8). When 1.025 gof 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 figures.ΔErxn = kJ/mol

Explanation:

There are two energy flows in this reaction.

q₁ = heat from reaction

q₂ = heat to warm calorimeter .

Per the Law of Conservation of Energy,

q₁ + q₂ = 0

Data:

For the reaction

mass of naphthalene = 1.025 g

For the calorimeter

Ccal = 5.11 kJ·°C⁻¹; Ti = 24.25 °C; T_f = 32.33 °C

Calculations

(a) Moles of naphthalene

(b) ΔT

ΔT = T_f - Ti = 32.33 °C - 24.25 °C = 8.08 °C

(c) ΔE

ΔErxn = -5161 kJ/mol

Explanation:

Step 1: Data given

Mass of naphthalene = 1.025 grams

The temperature rises from 24.25 °C to 32.33 °C

Molar mass of napthalene = 128.17 g/mol

Step 2: Calculate moles

Moles = mass / molar mass

Moles naphthalene = 1.025 grams / 128.17 g/mol

Moles naphthalene = 0.0080 moles

Step 3: Calculate the amount of heat released

heat = (5.11 kJ/°C)(32.33 °C - 24.25 °C) = 41.3 kJ

Since there is heat released, this is negative = -41.3 kJ

Step 4: Calculate  Δ(Hrxn) per mole of naphthalene is:

H = (-41.3 kJ)/(0.0080 moles) = -5161 kJ/mol

ΔErxn = -5161 kJ/mol

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