Constant Pressure Calorimetry

Introduction

• Topic: Constant Pressure Calorimetry
• Different from previous calorimetry (energy transfer between substances of different temperatures)
• Used for measuring energy involved in reactions (enthalpy)

Calorimetry Overview

• Definition: Experimental technique to measure the amount of heat (q) transferred to/from a system
• Tool Used: Coffee cup calorimeter (Styrofoam cup)
  • Simple insulated system to minimize energy exchange with surroundings
  • Measures heat transferred by reactions in aqueous solutions

Coffee Cup Calorimeter Components

• Styrofoam Cup: Minimizes energy loss, simple insulation
• Thermometer: Measures change in temperature of the solution
• Aqueous Solution: Contains reactants dissolved in water (water is the surroundings)
• Atmospheric Pressure: Constant; inside and outside pressure are the same, hence constant pressure

Key Concepts

• Constant Pressure: Atmospheric pressure is constant, ΔH (enthalpy change) of reaction = q (heat transfer)
  • ΔH reaction = q at constant pressure
• Heat Exchange Equation:
  • q_solution = mass * specific heat * ΔT
  • q_solution = -q_reaction
  • ΔH_reaction = -mcΔT (solution)

System and Surroundings

• System: Reactants/products of the reaction
• Surroundings: Calorimeter, thermometer, water in the solution
• Energy Transfer:
  • Exothermic Reaction: Energy released to the solution, temperature increases, negative ΔH
  • Endothermic Reaction: Energy absorbed from the solution, temperature decreases, positive ΔH

Practice Problem

• Given:
  • 25 ml of strong acid + 25 ml of strong base
  • Temperature change: 20.5°C to 25.0°C
  • Specific heat of water: 4.184 J/g°C
  • Density of water: 1 g/ml
  • Acid concentration: 1.5 M
• Find:
  • Enthalpy change of the reaction (ΔH)
  • Enthalpy change per mole of acid
• Solution:
  1. Calculate mass of solution: 50 g (since 1 g/ml)
  2. Apply heat exchange equation:
     • ΔH_reaction = -mcΔT
     • = -50g * 4.184 J/g°C * 4.5°C = -941 J
     • Negative sign indicates exothermic reaction
  3. Convert to kJ: -941 J = -0.941 kJ
  4. Calculate moles of acid:
     • (25 ml * 1.5 mol/L) = 0.0375 moles
  5. Enthalpy change per mole:
     • ΔH/moles = -0.941 kJ / 0.0375 = -25 kJ/mol*

Conclusion

• Constant pressure calorimetry helps determine ΔH of reactions
• Key equation: ΔH_reaction = -mcΔT (solution)
• Next topic: Stoichiometry of thermal chemical reactions