Investigation 12: Calorimetry
Calorimetry measures heat flow by tracking temperature change, but the calorimeter itself absorbs heat and leaks some away, so you calibrate it first and extrapolate the temperature graph back to the moment of mixing to get the true value.
A calorimeter is never a perfect container. Some of the heat released by a reaction warms the calorimeter's own parts (the water, the stirrer, the cup) instead of staying in the system you care about, and some escapes to the room before you can measure it. Before trusting any real reaction, you calibrate the calorimeter: add a known amount of heat, usually by running current through a resistor or mixing known hot and cold water masses, and measure the temperature rise. Dividing the heat added by that temperature change gives the calorimeter's heat capacity in J/C, a number that tells you how much heat the device itself soaks up per degree. That calibration constant gets added into later calculations so the calorimeter's own appetite for heat doesn't get mistaken for the reaction's.
Once calibrated, you run the real reaction and record temperature against time continuously, not just once at the peak. Heat escapes gradually the whole time, so by the moment the thermometer hits its highest reading, the system has already lost some heat to the surroundings. The fix is graphical: plot temperature versus time, let the curve rise then slowly fall as cooling takes over, and draw a straight line through the cooling portion. Extrapolate that line backward to the exact time mixing occurred. The temperature where that line crosses the mixing moment is the corrected maximum, the value you use in q = mcdeltaT instead of the observed peak.
This investigation ties directly into error analysis elsewhere in the unit: heat loss is a systematic error, not a random one, and extrapolation is the standard correction rather than just averaging repeated trials.
Key terms
- heat capacity of the calorimeter
- The heat in joules needed to raise the calorimeter itself by one degree Celsius, found by calibration before the real experiment.
- extrapolation
- Extending a graphed trend line beyond the measured points to estimate a value, here the temperature at the exact moment of mixing.
- systematic error
- A consistent, repeatable error in one direction, such as heat loss, that calibration or extrapolation can correct.
- q = mcdeltaT
- The equation relating heat transferred (q) to mass (m), specific heat (c), and temperature change (deltaT).
Watch it explained
- How it worksHow do you find the heat capacity of a calorimeter before trusting it with a real reaction?
- Worked exampleGraph temperature against time in a calorimetry run and extrapolate back to the mixing moment.
- Common mistakeWhy isn't the peak temperature reading on the thermometer the right value to use?
- Going furtherWhat happens to the measured enthalpy when heat leaks out through the calorimeter walls?
Check yourself
End of topic.