5.6

📘 Chapter 5.6 Study Guide: Calorimetry

🎯 Goal of Section 5.6

• Learn how to measure and calculate energy transferred as heat during a chemical reaction using calorimetry.

🔑 Key Terms

☕ Constant-Pressure Calorimetry (Coffee-Cup Calorimeter)

🔬 Description

• 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).

🔁 Heat Flow in the System

qreaction+qsolution=0⇒qr=−qsolutionq_{\text{reaction}} + q_{\text{solution}} = 0 \quad \Rightarrow \quad q_r = -q_{\text{solution}}qreaction​+qsolution​=0⇒qr​=−qsolution​

🔢 To Calculate qsolutionq_{\text{solution}}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​

✅ Assumptions

• No heat is lost to the calorimeter or surroundings (ideal but not perfect).

• The entire heat exchange is within the solution.

⚠ Limitations

• Low precision due to imperfect insulation.

• Used mostly for education or basic lab work, not precise research.

💣 Constant-Volume Calorimetry (Bomb Calorimeter)

🔬 Description

• 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.

🔁 Heat Flow in the System

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​)

🔢 At Constant Volume:

qv=ΔUq_v = \Delta Uqv​=ΔU

• Energy measured is the change in internal energy, not enthalpy.

✅ Uses

• Measuring caloric values of food.

• Evaluating fuel efficiency or combustion energy.

📌 Notes:

• More accurate than coffee-cup calorimeters.

• Requires knowledge of calorimeter’s heat capacity and mass of water.

🧠 Concept Summary

✏ Key Equations to Know

Coffee-Cup (Constant Pressure):

qreaction=−(mcΔT)q_{\text{reaction}} = - (mc\Delta T)qreaction​=−(mcΔT)

Bomb Calorimeter (Constant Volume):

qreaction=−(qwater+qbomb)q_{\text{reaction}} = - (q_{\text{water}} + q_{\text{bomb}})qreaction​=−(qwater​+qbomb​)ΔU=qv\Delta U = q_vΔU=qv​

📝 Tips for Exam

• 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.