Physical Chemistry : Calorimetry

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #21 : Thermochemistry And Thermodynamics

The heat capacity of a bomb calorimeter assembly is \(\displaystyle 3.90\ \frac{\textup{kJ}}{^\circ\textup{C}}\) . What is the heat of combustion of \(\displaystyle 1.1\textup{ g}\) of sucrose in a bomb calorimeter that results in the temperature rising from \(\displaystyle 22.1\ ^\circ \textup{C}\) to \(\displaystyle 27.8\ ^\circ\textup{C}\)?

Possible Answers:

\(\displaystyle 41.5\ \frac{\textup{kJ}}{\textup{g}}\)

\(\displaystyle -20.2\ \frac{\textup{kJ}}{\textup{g}}\)

\(\displaystyle -15.1\ \frac{\textup{kJ}}{\textup{g}}\)

\(\displaystyle -62.9\ \frac{\textup{kJ}}{\textup{g}}\)

\(\displaystyle 3.0\ \frac{\textup{kJ}}{\textup{g}}\)

Correct answer:

\(\displaystyle -20.2\ \frac{\textup{kJ}}{\textup{g}}\)

Explanation:

A bomb calorimeter is a device used to measure the quantity of heat change for a process. The heat of a reaction which is denoted as \(\displaystyle q\), is the negative of the thermal energy gained by the calorimeter:

\(\displaystyle q_{rxn}=-q_{calorimeter}\)

The heat capacity of a calorimeter is:

\(\displaystyle q_{calorimeter}=heat \ capacity \ of\ calorimeter\times \bigtriangleup T\)

Plugging the values given into the equation gives:

\(\displaystyle q_{calorimeter}=3.90\frac{kJ}{^{o}C}\times (27.8-22.1)^{o}C=22.3kJ\)

Using the relation provided earlier:

\(\displaystyle q_{rxn}=-q_{calorimeter}=-22.3kJ\)

Because we are dealing with 1.1 grams of sucrose, the heat of combustion of sucrose is:

\(\displaystyle q_{rxn}=\frac{-22.3kJ}{1.1g}=-20.2\frac{kJ}{g}\)

Example Question #21 : Thermochemistry And Thermodynamics

The heat capacity of a bomb calorimeter assembly is \(\displaystyle 2.60\ \frac{\textup{kJ}}{^\circ \textup{C}}\) . What is the heat of combustion of \(\displaystyle 1.65 \textup{ g}\) of caffeine in a bomb calorimeter that results in the temperature rising from \(\displaystyle 23.3\ ^\circ \textup{C}\) to \(\displaystyle 31.8\ ^\circ\textup{C}\)?

Possible Answers:

\(\displaystyle -1.48\ \frac{\textup{kJ}}{\textup{g}}\)

\(\displaystyle 50.0\ \frac{\textup{kJ}}{\textup{g}}\)

\(\displaystyle 13.0\ \frac{\textup{kJ}}{\textup{g}}\)

\(\displaystyle -13.39\ \frac{\textup{kJ}}{\textup{g}}\)

\(\displaystyle 3.01\ \frac{\textup{kJ}}{\textup{g}}\)

Correct answer:

\(\displaystyle -13.39\ \frac{\textup{kJ}}{\textup{g}}\)

Explanation:

A bomb calorimeter is a device used to measure the quantity of heat change for a process. The heat of a reaction which is denoted as q, is the negative of the thermal energy gained by the calorimeter:

\(\displaystyle q_{rxn}=-q_{calorimeter}\)

The heat capacity of a calorimeter is:

\(\displaystyle q_{calorimeter}=heat \ capacity \ of\ calorimeter\times \bigtriangleup T\)

Plugging the values given into the equation gives:

\(\displaystyle q_{calorimeter}=2.60\ \frac{kJ}{^{o}C}\times (31.8-23.3)^{o}C=22.1kJ\)

Using the relation provided earlier:

\(\displaystyle q_{rxn}=-q_{calorimeter}=-22.1kJ\)

Because we are dealing with 1.65 grams of sucrose, the heat of combustion of sucrose is:

\(\displaystyle q_{rxn}=\frac{-22.1kJ}{1.65g}=-13.39\frac{kJ}{g}\)

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