AM

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

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.


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

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


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.