AP® Chemistry · Unit 6 · Topic 6.4
Calorimetry
Calorimetry finds reaction heat by measuring the temperature change it causes in surrounding water, since the heat lost by the reaction equals the heat gained by the water, with opposite sign.
A calorimeter works because energy has nowhere else to go. If a reaction happens inside an insulated cup of water, any heat it releases gets absorbed by the water, and any heat it needs gets pulled from the water. The water's temperature change is a direct readout of the reaction's heat, because the system (reaction plus water) is isolated from the rest of the room.
In a coffee-cup calorimeter, you mix two solutions, record the starting temperature, and watch it rise or fall as the reaction proceeds. You calculate q_water = m c (delta T), using the mass of the solution, its specific heat (usually treated as 4.184 J/g C, like water), and the temperature change. That q_water equals q_reaction in size but the opposite sign: the reaction lost exactly what the water gained. Divide q_reaction by the moles of limiting reactant consumed, and you get delta H per mole, the molar heat of reaction.
This connects forward to bond enthalpies and Hess's law, which predict delta H from reaction structure instead of measuring it. Calorimetry is the experimental check: you measure what theory calculates. It also sets up the idea of heat capacity and specific heat used again in heating curves, where the same q = mc(delta T) relationship tracks temperature change within one phase.
Key terms
- Calorimeter
- An insulated container used to measure heat flow by tracking the temperature change of a surrounding substance like water.
- Specific heat capacity
- The energy needed to raise one gram of a substance by one degree Celsius, 4.184 J/g C for water.
- q = mc(delta T)
- The equation relating heat absorbed or released to mass, specific heat, and temperature change.
- Molar heat of reaction (delta H)
- The heat released or absorbed per mole of a specified reactant or product in a reaction.
- Heat capacity of the calorimeter
- Extra heat absorbed by the calorimeter itself rather than the solution, which causes measured delta H to be smaller in magnitude than the true value if ignored.
Watch it explained
- How it worksWalk me through how a coffee cup calorimeter turns a temperature rise into joules of reaction heat.
- Worked exampleFind delta H per mole when mixing 50.0 mL each of 1.0 M HCl and 1.0 M NaOH raises T by 6.7 degrees.
- Common mistakeWhy isn't the heat absorbed by the water the same sign as the heat released by the reaction?
- Going furtherWhat happens to the measured value when the calorimeter itself soaks up some of the heat?
Check yourself
End of topic.