Chemistry · Thermodynamics · NEET
It measures ΔU (change in internal energy), NOT ΔH. This is the most common NEET confusion. The bomb is a sealed steel vessel, so its volume stays fixed. At constant volume the heat released equals ΔU, written as qV = ΔU. To get ΔH you must do an extra step: ΔH = ΔU + Δn_g RT. So the calorimeter gives ΔU first, and ΔH is calculated afterward.
Work done by an expanding gas is w = −pΔV. Inside a sealed bomb, the volume is fixed, so ΔV = 0. If ΔV = 0, then w = 0, even if the reaction produces gases. Because no work is done, the First Law ΔU = q + w becomes ΔU = q. This is why the measured heat equals ΔU directly.
The bomb is a strong, sealed steel container. The gases produced by burning cannot escape and the walls do not move, so the volume cannot change. This is called an isochoric (constant-volume) condition. NEET likes this: constant volume forces ΔV = 0, which forces w = 0, which makes qV = ΔU. The whole design exists to trap the heat and pin the volume.
Use ΔH = ΔU + Δn_g RT, where Δn_g = moles of gaseous products − moles of gaseous reactants, R = 8.314 J K⁻¹ mol⁻¹, and T is the temperature in kelvin. Only count gases. If Δn_g = 0, then ΔH = ΔU. Example: NCERT gives cyanamide combustion with ΔU = −742.7 kJ/mol; you plug Δn_g into this formula to get ΔH.
The reaction releases heat into the surrounding water. The temperature of the water and calorimeter rises by ΔT. The heat gained by the calorimeter is q = C_V × ΔT, where C_V is the heat capacity of the calorimeter (given in the question). The heat lost by the reaction equals the heat gained by the calorimeter, but with the opposite sign. So the reaction's ΔU is negative (exothermic) while the water heats up.
A bomb calorimeter is SEALED, so volume is constant → it measures ΔU (qV). A coffee-cup or constant-pressure calorimeter is OPEN to the atmosphere, so pressure is constant → it measures ΔH (qP). Sealed = ΔU, open = ΔH. NEET can test either, so remember which vessel goes with which quantity.
Which amongst the following options is the correct relation between change in enthalpy and change in internal energy?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It measures qV, the heat released at constant volume, which equals ΔU (the change in internal energy of the reaction).
Because the vessel is sealed and rigid, the volume is fixed (ΔV = 0), so no pressure-volume work is done (w = 0). By the First Law ΔU = q + w = qV. Constant pressure would be needed to measure ΔH directly.
A weighed sample of a combustible substance is burned in an excess of pure dioxygen (O₂) supplied inside the sealed steel bomb, so combustion is complete.
When Δn_g = 0, meaning the moles of gaseous products equal the moles of gaseous reactants. Then ΔH = ΔU + 0 = ΔU. This is also true for reactions with only solids and liquids.
The steel bomb sits in a water bath so heat released by the reaction is absorbed by the water. The temperature rise ΔT of the water is measured and used with the calorimeter's heat capacity to find the heat.
