A bomb calorimeter is a type of constant-volume calorimeter, which works under the First law of Thermodynamics. It states that “The total energy in an isolated system is always conserved” . The bomb calorimeter is used in determining the heat of combustion of a particular reaction, where electrical energy is used to initiate the reaction (combustion). The change in temperature of the water allows for calculating calorie content of the given sample. In order to ensure complete combustion, the experiment is carried out in the presence of oxygen above atmospheric pressure (25 atm). The heat released corresponds toqv, and thus to the internal energy change ΔU rather than to ΔH. The enthalpy change is calculated according to the formula

ΔH=qv+ΔngRT

Δ ng is the change in the number of moles of gases in the reaction.

Enthalpy is the amount of heat content used or released in a system at constant pressure. It is a state function and depends on T, P and U. Enthalpy is usually expressed as change in enthalpy for a process between initial and final states as

∆H = ∆U + ∆PV

If temperature and pressure remain constant through the process and the work is limited to pressure-volume work, then the enthalpy change is given by the equation:

ΔH = ΔU + PΔV

Also at constant pressure the heat flow (q) for the process is equal to the change in enthalpy defined by the equation:

ΔH = qp

Entropy is the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work. All spontaneous processes are irreversible so the entropy of the universe is increasing i.e. more and more energy becomes unavailable for conversion into work.

According to second law of thermodynamics, △ Svap=ΔQ vap

T=ΔH vap

T

Hess's Law of Constant Heat Summation states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes. This law is a manifestation that enthalpy is a state function , which is used to determine the theoretical value of heat of combustion