Understanding Enthalpy and Combustion Processes

Sep 3, 2024

Lecture Notes: Enthalpy and Combustion

Key Topics Covered

  • Energy Released in Combustion
  • Experimental Determination of Enthalpy
  • Thermochemical Calculations
  • Calorimetry and Measurement Techniques
  • Molar Enthalpy of Combustion
  • Errors in Measurement and Calculation

Enthalpy and Combustion

  • Enthalpy (ΔH): Heat or energy content of a system at constant pressure and volume.
    • Enthalpy change: Heat energy released or absorbed in a chemical reaction.
    • Formula: ( \Delta H = \text{enthalpy of products} - \text{enthalpy of reactants} ).

Calorimetry

  • Study and Measurement of Heat Changes: Introduced in early chemistry courses.
  • Combustion Reactions: Always exothermic (ΔH is negative).
  • Calorimeter: An insulated device used to measure heat changes.
    • Formula for heat change: ( Q = m \times C \times \Delta T ).
      • Q: Heat change (Joules).
      • m: Mass of water (grams).
      • C: Specific heat capacity of water (4.18 J/g°C).
      • ΔT: Temperature change (°C).

Example Calculations

  • Calculate Heat Energy:
    • Example: Calculate energy to heat 250 ml of water from 22°C to 98°C.
    • Mass = Volume = 250 g; ΔT = 76°C.
    • Result: 79,420 Joules or 79.4 kJ (3 significant figures).

Molar Enthalpy of Combustion

  • Definition: Heat released when one mole of fuel combusts completely.
  • Formula: ( \Delta H = \frac{Q}{n} ).
    • n: Number of moles.
  • Experimental Setup:
    • Use of a spirit burner and calorimeter.
    • Measure temperature change and mass of fuel burnt.

Errors and Assumptions

  • Random Errors:
    • Evaporation of fuel, measurement variations (volume, mass, temperature).
    • Wick height variations.
  • Systematic Errors:
    • Heat loss to can or surroundings.
    • Incomplete combustion.
    • Cooling before maximum temperature is reached.

Example Problems and Solutions

  • Experimental Molar Enthalpy for Methanol:
    • Calculate using formula and given data.
    • Result: Approx. (-599) kJ/mol (3 significant figures).
    • Compare to accepted value (-715 kJ/mol).
  • Theoretical Heat Release:
    • Calculation for 1g of methanol.
    • Result: 22.3 kJ of energy released.

Conclusion

  • Comparisons and Accuracy:
    • Future videos will address improving accuracy and comparing different fuels.