Calorimetry Part 2

Week 6: Class 2 - Calorimetry Part Two

Overview of Constant Pressure Calorimetry

• Constant Pressure Calorimetry:

  • Used for reactions in aqueous solutions.

  • Economical and straightforward method for measuring heat changes.

Key Concepts

• Reaction Mixture:

  • Could involve a physical change or a chemical reaction.

  • Process generates or liberates heat (endothermic vs exothermic).

• Reactions in this context occur without expelling or consuming gases (pressure remains constant).

• Stirring Method:

  • In lab experiments, manual swirling of Styrofoam cups is sufficient for mixing.

  • Insulation is achieved using double or triple layers of Styrofoam.

Measurement of Enthalpy Changes

• Delta H Measurement:

  • In constant pressure calorimetry, direct measurement of ( \Delta H ) is performed.

  • Formulas involved:

    • Heat (Q) = MS( \Delta T ) or C( \Delta T )

  • Heat at constant pressure is defined as ( \Delta H ).

  • Definitions:

    • S = specific heat capacity

    • C = heat capacity

Master Equation and Calculations

• Understanding individual components:

  • Contributions to total heat change are based on those involved in the process:

    • Delta H of the reaction

    • Delta H of the solution

    • Delta H of the calorimeter

• Example Process:

  • Mixing of solutions in the calorimeter leads to observable temperature changes.

  • ( \Delta H ) for the reaction in an isolated system is represented as:

    • ( \Delta H_{ ext{total}} = 0 ) corresponds to the sum of components.

• Identifying the Components:

  • Reaction component: The process causing temperature changes.

  • Solution component: Includes solvent mass and any unreacted reactants and products.

  • Calorimeter component: Typically zero if insulated correctly.

Example Problem

• Chemical Reaction:

  • Adding 33 mL of 1.2 M HCl to excess NaOH.

  • Monitoring temperature changes provides insight into whether the reaction is exothermic (temperature rises) or endothermic (temperature falls).

• Calculations:

  • Determine the limiting reagent (HCl in this case) because it defines the reaction's theoretical yield.

  • Express the enthalpy change per mole of limiting reagent reacted.

Further Examples and Applications

• Additional Reaction:

  • For a different neutralization reaction (100 mL of 1.0 M NaOH with 100 mL of 1.0 M HCl), the process will also yield measurable delta H.

  • Follow the same principles outlined for calculating the change in enthalpy with relevant adjustments for solutions.

• Physical Process Example:

  • Dissolution of KBr in water observed in a calorimetry setup to illustrate the endothermic nature of this physical process.

  • Draw comparisons to chemical processes with similar calculations.

Conclusion

• Understanding calorimetry involves systematically breaking down reactions into measurable components.

• Familiarity with the process and careful recording of assumptions ensure accurate calculations.

• Practice with various problems enhances comprehension and ability to apply concepts effectively.