Calorimetry is the experimental measurement of heat transfer during physical and chemical changes. It matters because heat flow tells us whether a reaction releases energy, absorbs energy, or changes the temperature of its surroundings. In chemistry labs, calorimetry connects observable temperature changes to quantities such as heat of reaction and enthalpy change.
A well-insulated calorimeter helps keep the measured heat exchange as close as possible to the heat produced or absorbed by the process being studied.
The core idea is conservation of energy: heat lost by one part of the system is gained by another. In a coffee-cup calorimeter at constant pressure, the solution absorbs or releases heat and q = mcΔT is often used to calculate that heat. In a bomb calorimeter at constant volume, a reaction occurs inside a sealed metal chamber and the heat warms the surrounding water and calorimeter hardware.
By measuring temperature change and using the proper heat capacity, scientists can calculate reaction energy from laboratory data.
Key Facts
- Heat gained or lost by a substance: q = mcΔT
- q is heat, m is mass, c is specific heat capacity, and ΔT = Tfinal - Tinitial
- For water, c = 4.184 J/(g·°C) is commonly used in calorimetry calculations
- Conservation of energy in an insulated setup: qsystem + qsurroundings = 0
- Coffee-cup calorimeters usually measure heat at constant pressure, so qreaction = ΔH for many solution reactions
- Bomb calorimeters measure heat at constant volume and use q = CcalΔT, where Ccal is the calorimeter heat capacity
Vocabulary
- Calorimetry
- Calorimetry is the measurement of heat transfer during a chemical reaction or physical change.
- Heat
- Heat is energy transferred between objects or systems because of a temperature difference.
- Specific heat capacity
- Specific heat capacity is the amount of heat needed to raise 1 gram of a substance by 1°C.
- Coffee-cup calorimeter
- A coffee-cup calorimeter is an insulated constant-pressure device used to measure heat changes in reactions that occur in solution.
- Bomb calorimeter
- A bomb calorimeter is a sealed constant-volume device used to measure heat released by combustion or other reactions.
Common Mistakes to Avoid
- Using the wrong sign for q, because a temperature increase in the surroundings means the reaction released heat and qreaction is negative.
- Forgetting to convert mass units, because specific heat is often in J/(g·°C), so the mass should be in grams unless units are changed consistently.
- Using the final temperature instead of ΔT, because heat depends on the temperature change, not the absolute final temperature.
- Treating coffee-cup and bomb calorimeters as identical, because coffee-cup calorimeters are usually constant pressure while bomb calorimeters are constant volume and require a calorimeter heat capacity.
Practice Questions
- 1 A 150.0 g sample of water warms from 22.0°C to 28.5°C. Using c = 4.184 J/(g·°C), calculate the heat absorbed by the water.
- 2 In a coffee-cup calorimeter, 50.0 mL of acid and 50.0 mL of base react. Assume the final solution has a mass of 100.0 g and c = 4.184 J/(g·°C). If the temperature rises from 21.4°C to 27.9°C, calculate qsolution and qreaction.
- 3 A reaction in a coffee-cup calorimeter causes the solution temperature to drop. Explain whether the reaction is endothermic or exothermic, and describe the direction of heat flow.