Chemistry · Thermodynamics · NEET
When a reaction gives off heat (exothermic), that heat does not vanish. It flows into the water and the metal walls of the calorimeter around it. So the heat the reaction lost is exactly equal to the heat the calorimeter gained. This is just the law of conservation of energy: energy is not created or destroyed, it only moves from the reaction to the surroundings inside the sealed calorimeter.
Sign shows direction of heat flow. The reaction LOSES heat, so its q is negative (energy leaving). The calorimeter GAINS heat, so its q is positive (energy entering). NCERT writes: the heat of the reaction has the same magnitude but opposite sign to the heat gained by the calorimeter. So q(reaction) = - q(calorimeter). The numbers are equal; only the plus/minus differs.
Use q = C x delta-T when you are given the heat capacity C of the whole calorimeter (units kJ/K or J/K). C already includes the water and the vessel together. Use q = m x c x delta-T when you are given the mass m and the specific heat c (like water c = 4.2 J/g degreeC). Both give the heat absorbed; pick the one matching the data in the question.
The reaction happens inside a sealed steel bomb. The heat evolved passes to the water bath and the steel around the bomb. The temperature of that water rises, and you measure delta-T. Because volume is fixed, no work is done, so the heat measured equals the internal energy change, delta-U (this is q at constant volume, q_V).
Multiply the calorimeter's heat capacity by the temperature change: q(calorimeter) = C x delta-T. That is the heat gained. The heat released by the reaction is the same number with a minus sign. Example from NCERT: graphite burnt, temperature rose 298 K to 299 K (delta-T = 1 K), C = 20.7 kJ/K, so heat gained = 20.7 kJ and the reaction released 20.7 kJ (delta-U = -20.7 kJ).
This is the core idea behind measuring delta-U and delta-H. Bomb calorimeter (constant volume) gives delta-U; constant-pressure calorimeter gives delta-H. NEET questions give you C and delta-T, or mass and specific heat, and ask for the heat of reaction. If you flip a sign or use the wrong formula, the answer is wrong. Master this before converting delta-U to delta-H.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes, as long as the calorimeter is well insulated so no heat leaks to the outside surroundings. Then all the reaction's heat goes into the calorimeter and water, so the two amounts are equal with opposite signs.
Two forms: q = C x delta-T (using the total heat capacity C of the calorimeter) or q = m x c x delta-T (using mass m and specific heat c, e.g. water c = 4.2 J/g degreeC). Both give the heat absorbed by the calorimeter.
A bomb calorimeter works at constant volume, so it measures delta-U (internal energy change). To get delta-H you convert using delta-H = delta-U + delta-n(g) x R x T.
The steel bomb is sealed, so the volume cannot change. Since work of expansion needs a volume change (w = -p x delta-V), and delta-V = 0, no pressure-volume work is done. So all the energy shows up as heat, equal to delta-U.
The signs flip. An endothermic reaction absorbs heat, so its q is positive and the calorimeter's temperature falls (delta-T is negative). The reaction gains the heat that the calorimeter loses; magnitudes are still equal.
