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Ch.6 - Thermochemistry
All textbooksTro 4th EditionCh.6 - ThermochemistryProblem 99
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Chapter 6, Problem 99

Evaporating sweat cools the body because evaporation is an endothermic process: H2O(l) → H2O(g) ΔH°rxn = +44.01 kJ. Estimate the mass of water that must evaporate from the skin to cool the body by 0.50°C. Assume a body mass of 95 kg and assume that the specific heat capacity of the body is 4.0 J/g°C.

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Calculate the total heat energy (q) required to cool the body by 0.50°C using the formula q = m * c * ΔT, where m is the mass of the body in grams, c is the specific heat capacity, and ΔT is the change in temperature.
Convert the body mass from kilograms to grams by multiplying by 1000, since 1 kg = 1000 g.
Substitute the values into the formula: m = 95000 g (since 95 kg = 95000 g), c = 4.0 J/g°C, and ΔT = 0.50°C.
Calculate the heat energy (q) in joules required to cool the body by 0.50°C.
Use the enthalpy change of the evaporation process (ΔH°rxn = +44.01 kJ/mol) to find the mass of water that must evaporate. Convert q from joules to kilojoules, then divide by ΔH°rxn to find the number of moles of water that need to evaporate. Finally, multiply by the molar mass of water (18.02 g/mol) to find the mass of water in grams.
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Open Question
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Textbook Question

LP gas burns according to the exothermic reaction: C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g) ΔH°rxn = –2044 kJ What mass of LP gas is necessary to heat 1.5 L of water from room temperature (25.0 °C) to boiling (100.0 °C)? Assume that during heating, 15% of the heat emitted by the LP gas combustion goes to heat the water. The rest is lost as heat to the surroundings.

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Textbook Question

Use standard enthalpies of formation to calculate the standard change in enthalpy for the melting of ice. (The ΔH°f for H2O(s) is –291.8 kJ/mol.)

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Textbook Question

Use standard enthalpies of formation to calculate the standard change in enthalpy for the melting of ice. (The ΔH°f for H2O(s) is –291.8 kJ/mol.) Use this value to calculate the mass of ice required to cool 355 mL of a beverage from room temperature (25.0 °C) to 0.0 °C. Assume that the specific heat capacity and density of the beverage are the same as those of water.

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