Standard Enthalpy Change of Combustion Experiment

Definition

• Standard Enthalpy Change of Combustion: The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.

Example: Methanol Combustion

• Chemical Formula: Methanol (CH₃OH)
• Combustion Reaction:
  • Reactants: Methanol (liquid) + Oxygen (gas)
  • Products: Carbon Dioxide (gas) + Water (liquid)
  • Balanced Equation: [ \text{CH}_3\text{OH} + \frac{3}{2} \text{O}_2 \rightarrow \text{CO}_2 + 2 \text{H}_2\text{O} ]

Experiment Setup

• Equipment Used:
  • Spirit burner with methanol
  • Copper can (or beaker) filled with water
  • Thermometer to measure temperature

Initial Measurements

1. Volume of Water: Important for calculations
2. Initial Temperature of Water: Record before burning
3. Mass of Spirit Burner: Record before burning

Procedure

• Light the spirit burner and burn for 3-5 minutes.
• Stir water with thermometer to measure temperature during the experiment.
• After burning, extinguish flame and record:
  • Final Temperature of Water
  • Mass of Spirit Burner (after burning)

Data Collection and Calculation

1. Mass of Fuel Burned: Difference between initial and final mass of spirit burner.
2. Temperature Change (ΔT): Final Temperature - Initial Temperature.
3. Energy Change (q):
   • Formula: [ q = m imes c imes \Delta T ]
   • Parameters:
     • m = mass of water (150g)
     • c = specific heat capacity of water (4.18 J/g°C)
     • ΔT = temperature change (e.g., 41°C)
   • Example Calculation:
     • [ q = 150g \times 4.18 \frac{J}{g°C} \times 41°C = 25707 J ]
4. Convert q to kilojoules: [ 25707 J / 1000 = 25.707 kJ ]
5. Calculate Moles of Fuel Burned:
   • Mass of fuel = 1.6g
   • Molar Mass of Methanol (CH₃OH) = 32 g/mol
   • [ ext{Moles} = \frac{1.6g}{32g/mol} = 0.05 ext{ moles} ]
6. Calculate Enthalpy Change (ΔH):
   • [ ΔH = \frac{q}{\text{moles}} = \frac{25.707 kJ}{0.05} = 514.14 kJ/mol ]
   • Sign of ΔH: Since it is an exothermic reaction (temperature increased), the final answer is: [ ΔH = -514.14 kJ/mol ]

Accuracy of the Experiment

Sources of Error

1. Heat Loss to Surroundings: Not all heat transfers to water.
2. Evaporation of Fuel: Liquid fuels can evaporate, leading to inaccurate mass readings.
3. Assumption of Complete Combustion: Incomplete combustion can lead to different energy values.
4. Deviation from Standard Conditions: Room conditions may not be at 298 K or 100 kPa.

Importance of Accuracy

• Understanding these errors is crucial for improving experimental design and interpreting results correctly.