Question

in a bomb calorimeter, reactions are carried out at

A. constant pressure

B. constant volume

C. 1 atm pressure and 25C

D. 1 atm pressure and 0C

The answer is B, can someone explain why? Shouldn't the pressure stay the same also in a bomb calorimeter? Can someone please explain how a bomb calorimeter works in general too.

Answer 1

Most serious calorimetry carried out in research laboratories involves the determination of heats of combustion ΔHcombustion

All of these components are contained within the double-walled outer part of the calorimeter (Figure 1). After the initial temperature of the water is measured, the heated wire inside the bomb starts the reaction. After combustion the final temperature of the water is measured, and then the change in temperature of the reactants can be calculated. Through the combustion reaction, the temperature rises due to the conversion from chemical energy to thermal energy that occurs through the reaction. To ensure complete combustion, the experiment is carried out in the presence of oxygen above atmospheric pressure. This requires that the combustion be confined to a fixed volume.

Since the process takes place at constant volume, the reaction vessel must be constructed to withstand the high pressure resulting from the combustion process, which amounts to a confined explosion. The vessel is usually called a “bomb”, and the technique is known as bomb calorimetry. The reaction is initiated by discharging a capacitor through a thin wire which ignites the mixture.

Another consequence of the constant-volume condition is that the heat released corresponds to qv , and thus to the internal energy change ΔU rather than to ΔH. The enthalpy change is calculated according to the formula

ΔH=qv+ΔngRT(1)

in which Δn_g  is the change in the number of moles of gases in the reaction.

Hence,answer is B.constant volume and pressure wont remain constant due to high heat generated