Lesson 5 of 12 · 6 min
Calorimetry
NCERT § "Measurement of ΔU and ΔH: Calorimetry"
The gas company says methane gives out about 890 kJ per mole when it burns. Someone had to burn it in a steel can and read a thermometer to know that.
The lesson in notes
In short
Energy changes are measured by calorimetry: the reaction is run in a vessel immersed in a known amount of liquid, and the heat released or absorbed is found from the temperature change and the calorimeter's heat capacity.
A bomb calorimeter is a sealed steel vessel of constant volume, so no pressure-volume work is done and it measures ΔU.
In a bomb calorimeter the sample is burnt in pure oxygen, and the heat given out is found from the rise in temperature of the surrounding water.
Reactions carried out at constant (atmospheric) pressure in a simple calorimeter give q_p, which is ΔH.
To convert a bomb calorimeter ΔU into ΔH, use ΔH = ΔU + Δn_gRT with gaseous species only.
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Constant-volume against constant-pressure calorimeters, solved
The Organic Chemistry Tutor · English · Solved problems · Open on YouTube