Learn how to measure and calculate energy transferred as heat during a chemical reaction using calorimetry.
Term | Definition |
---|---|
Calorimetry | Measurement of heat flow during physical or chemical processes. |
Calorimeter | Apparatus used to measure heat change. |
Constant-Pressure Calorimeter | Measures heat at constant pressure (e.g., coffee-cup calorimeter). |
Constant-Volume Calorimeter | Measures heat at constant volume (e.g., bomb calorimeter). |
q | Symbol for heat (energy transferred as heat). |
ΔH (Enthalpy change) | Heat change at constant pressure. |
ΔU (Internal energy change) | Heat change at constant volume. |
Specific heat capacity (c) | Amount of heat required to raise the temperature of 1 g of a substance by 1°C. |
Uses Styrofoam cups, a thermometer, and a lid.
Insulated system helps reduce heat exchange with surroundings.
Ideal for reactions in aqueous solution (e.g., acid-base neutralizations).
qreaction+qsolution=0⇒qr=−qsolutionq_{\text{reaction}} + q_{\text{solution}} = 0 \quad \Rightarrow \quad q_r = -q_{\text{solution}}qreaction+qsolution=0⇒qr=−qsolution
qsolution=mcΔTq_{\text{solution}} = mc\Delta Tqsolution=mcΔT
mmm: mass of the solution (usually ≈ mass of water in grams)
ccc: specific heat capacity (4.18 J/g·°C for water)
ΔT\Delta TΔT: change in temperature = Tfinal−TinitialT_{\text{final}} - T_{\text{initial}}Tfinal−Tinitial
No heat is lost to the calorimeter or surroundings (ideal but not perfect).
The entire heat exchange is within the solution.
Low precision due to imperfect insulation.
Used mostly for education or basic lab work, not precise research.
Sample is combusted in a rigid, sealed bomb filled with pure O₂.
The bomb is immersed in water inside an insulated container.
No volume change, so no pressure-volume work occurs.
qr+qbomb+qwater=0⇒qr=−(qbomb+qwater)q_r + q_{\text{bomb}} + q_{\text{water}} = 0 \quad \Rightarrow \quad q_r = -(q_{\text{bomb}} + q_{\text{water}})qr+qbomb+qwater=0⇒qr=−(qbomb+qwater)
qv=ΔUq_v = \Delta Uqv=ΔU
Energy measured is the change in internal energy, not enthalpy.
Measuring caloric values of food.
Evaluating fuel efficiency or combustion energy.
More accurate than coffee-cup calorimeters.
Requires knowledge of calorimeter’s heat capacity and mass of water.
Feature | Constant-Pressure (Coffee-Cup) | Constant-Volume (Bomb) |
---|---|---|
Measures | Enthalpy change (ΔH) | Internal energy change (ΔU) |
Apparatus | Styrofoam cup, lid, thermometer | Steel bomb in water container |
Common Use | Solution reactions | Combustion reactions |
Work (PΔV) | Can occur | None (volume fixed) |
Heat Equation | q=mcΔTq = mc\Delta Tq=mcΔT | q=−(qbomb+qwater)q = -(q_{\text{bomb}} + q_{\text{water}})q=−(qbomb+qwater) |
Main Limitation | Poor insulation, less precise | Expensive, more complex setup |
qreaction=−(mcΔT)q_{\text{reaction}} = - (mc\Delta T)qreaction=−(mcΔT)
qreaction=−(qwater+qbomb)q_{\text{reaction}} = - (q_{\text{water}} + q_{\text{bomb}})qreaction=−(qwater+qbomb)ΔU=qv\Delta U = q_vΔU=qv
Know which type of calorimetry measures ΔH vs. ΔU.
Remember to use negative signs to show direction of heat flow.
Know that specific heat of water = 4.18 J/g·°C.
Be prepared to calculate q using temperature change and mass.
Watch for unit conversions: grams to kilograms, J to kJ, etc.