The heat of reaction at constant volume and temperature, also known as the heat of formation or enthalpy change, refers to the amount of heat energy released or absorbed during a chemical reaction. It is represented by the symbol ΔH.

ΔH is a measure of the difference in enthalpy between the products and the reactants of a chemical reaction. Enthalpy (H) is a thermodynamic property that represents the heat content or energy of a system.

The heat of reaction at constant volume and temperature can be determined experimentally or calculated using Hess's law or bond enthalpies.

Experimental Determination:
To determine the heat of reaction experimentally, the reaction is carried out in a bomb calorimeter. The bomb calorimeter is a constant volume device that allows for accurate measurement of heat transfer. The reactants are placed in a sealed vessel and ignited, and the heat released or absorbed is measured by the change in temperature of the surrounding water. This provides a direct measurement of the heat of reaction at constant volume and temperature.

Calculation using Hess's Law:
Hess's law states that the enthalpy change of a reaction is independent of the pathway taken and depends only on the initial and final states. This allows us to calculate the heat of reaction at constant volume and temperature by using known enthalpy changes of other reactions.

To calculate ΔH using Hess's law, the reaction of interest is broken down into a series of simpler reactions with known enthalpy changes. The enthalpy changes of these simpler reactions are then algebraically added or subtracted to obtain the overall enthalpy change of the reaction of interest.

Calculation using Bond Enthalpies:
Bond enthalpies are the energy required to break a particular type of bond in a molecule. The heat of reaction at constant volume and temperature can be calculated by determining the difference between the total bond enthalpies of the reactants and the total bond enthalpies of the products. The bond enthalpy values can be obtained from tables.

In conclusion, the heat of reaction at constant volume and temperature is a measure of the heat energy released or absorbed during a chemical reaction. It can be determined experimentally using a bomb calorimeter or calculated using Hess's law or bond enthalpies. These methods allow us to understand the energetics of chemical reactions and provide valuable information for various applications in fields such as chemistry, thermodynamics, and chemical engineering.