Calorimetry & Bomb Calorimeter | Thermodynamics Class 1

Calorimetry

Measurement of Heat Transfer | Thermodynamics

1. Introduction

Calorimetry is the experimental technique used to measure the energy changes (heat transfer) associated with chemical or physical processes. The device used is called a Calorimeter.

2. Heat Capacity Concepts

The heat absorbed or released ($q$) is related to temperature change ($\Delta T$).

A. Heat Capacity ($C$)

Heat required to raise the temperature of the entire body/system by 1°C (or 1 K). It is an Extensive Property.

$$ q = C \Delta T $$

B. Specific Heat Capacity ($c$ or $s$)

Heat required to raise the temperature of 1 gram of substance by 1°C. It is an Intensive Property.

$$ q = m \cdot c \cdot \Delta T $$

C. Molar Heat Capacity ($C_m$)

Heat required to raise the temperature of 1 mole of substance by 1°C.

$$ q = n \cdot C_m \cdot \Delta T $$

3. Bomb Calorimeter ($\Delta U$ Measurement)

This device is used for measuring the Heat of Combustion at Constant Volume ($q_v$).

Principle: The reaction takes place in a rigid steel container (the "bomb") so volume remains constant ($\Delta V = 0$). Therefore, the heat measured ($q_v$) is equal to the change in Internal Energy ($\Delta U$).

Construction & Formula

• A steel vessel immersed in a water bath with a thermometer and stirrer.
• The sample is burnt in excess oxygen.
• Total heat released ($q$) warms the water and the calorimeter.

$$ q = \Delta U = - \frac{C_{cal} \cdot \Delta T}{n} $$

Where:

• $C_{cal}$ = Heat capacity of the entire calorimeter system (Water + Vessel).
• $\Delta T$ = Rise in temperature.
• $n$ = Moles of substance burnt.
• (Negative sign indicates exothermic heat release).

4. Coffee Cup Calorimeter ($\Delta H$ Measurement)

Used for reactions occurring at Constant Pressure (atmospheric pressure), such as neutralization or dissolution.

• Since pressure is constant, the heat measured ($q_p$) is equal to the change in Enthalpy ($\Delta H$).
• Typically uses polystyrene (styrofoam) cups which are good insulators.

5. Relationship between $\Delta H$ and $\Delta U$

If we measure $\Delta U$ from a Bomb Calorimeter, we can calculate $\Delta H$ using:

$$ \Delta H = \Delta U + \Delta n_g RT $$

Where $\Delta n_g$ is the change in the number of gaseous moles in the balanced chemical equation.

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