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💏intro to chemistry review

Constant-Volume Calorimeter

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

A constant-volume calorimeter is a device used in calorimetry to measure the amount of heat released or absorbed during a chemical reaction or physical process. It maintains a constant volume throughout the experiment, allowing for the direct measurement of changes in temperature and the calculation of the associated enthalpy change.

5 Must Know Facts For Your Next Test

  1. Constant-volume calorimeters are commonly used to measure the heat of reaction, heat of combustion, and heat of solution.
  2. The temperature change observed in a constant-volume calorimeter is directly proportional to the enthalpy change of the reaction or process.
  3. Constant-volume calorimeters maintain a fixed volume throughout the experiment, unlike constant-pressure calorimeters which operate at constant pressure.
  4. The design of a constant-volume calorimeter typically includes an insulated chamber, a thermometer, and a stirrer to ensure uniform temperature distribution.
  5. The heat capacity of the calorimeter itself must be taken into account when calculating the enthalpy change of the system.

Review Questions

  • Explain the key principle behind the operation of a constant-volume calorimeter.
    • The key principle behind the operation of a constant-volume calorimeter is that the temperature change observed within the insulated chamber is directly proportional to the enthalpy change of the reaction or process occurring inside. Since the volume is maintained at a constant value, any change in temperature can be directly related to the heat absorbed or released, allowing for the calculation of the enthalpy change using the formula $q = m \cdot c \cdot \Delta T$, where $q$ is the heat, $m$ is the mass of the system, $c$ is the specific heat capacity, and $\Delta T$ is the change in temperature.
  • Describe the advantages of using a constant-volume calorimeter over a constant-pressure calorimeter.
    • The primary advantage of using a constant-volume calorimeter over a constant-pressure calorimeter is that the enthalpy change measured in a constant-volume system is equal to the internal energy change, $\Delta U$, whereas in a constant-pressure system, the enthalpy change, $\Delta H$, includes the work done by or on the system. This makes the constant-volume calorimeter a more direct measurement of the energy changes within the system, without the need to account for the work term. Additionally, constant-volume calorimeters are simpler in design and operation, as they do not require the complex mechanisms to maintain a constant pressure environment.
  • Evaluate the importance of the heat capacity of the calorimeter itself in the calculations of enthalpy change.
    • The heat capacity of the calorimeter itself is a crucial factor in the accurate calculation of the enthalpy change in a constant-volume calorimeter experiment. Since the calorimeter is an integral part of the system, any change in its temperature will contribute to the overall temperature change observed. To account for this, the heat capacity of the calorimeter must be determined and included in the calculation of the enthalpy change using the formula $q = (m \cdot c)_{\text{system}} \cdot \Delta T + (m \cdot c)_{\text{calorimeter}} \cdot \Delta T$. Failure to consider the heat capacity of the calorimeter will result in an inaccurate determination of the enthalpy change of the reaction or process being studied.