Enthalpy of Combustion in Chemical Reactions

Expert reviewed • 23 November 2024 • 4 minute read

Enthalpy of combustion measures the heat energy released when one mole of a substance completely combusts with excess oxygen under standard conditions. This fundamental concept is crucial for understanding energy changes in chemical reactions, particularly when studying alcohols and hydrocarbons.

Understanding Complete Combustion

Complete combustion occurs when a compound reacts with excess oxygen to form carbon dioxide and water. For alcohols, the general equation is:

CnH2n+1OH+3n+12O2→nCO2+(n+1)H2O\text{C}_n\text{H}_{2n+1}\text{OH} + \frac{3n+1}{2}\text{O}_2 \\\rightarrow n\text{CO}_2 + (n+1)\text{H}_2\text{O}

Calculating Enthalpy of Combustion

The enthalpy of combustion (ΔH_c) is calculated using the following equation:

ΔHc=mcΔTn\Delta H_c = \frac{mc\Delta T}{n}

Where:

  • m = mass of water heated (g)
  • c = specific heat capacity of water (4.18 J/g°C)
  • ΔT = temperature change (°C)
  • n = number of moles of fuel burned

Key Considerations for Accurate Calculations

  • Standard Conditions

    • Temperature: 25°C (298K)
    • Pressure: 1 atmosphere
    • All substances in their standard states

  • Units and Conversions

    • Express final answers in kJ/mol
    • Convert temperature changes to Kelvin when required
    • Use correct molar masses for calculations

  • Common Sources of Error

    • Heat loss to surroundings
    • Incomplete combustion
    • Incorrect measurement of mass or temperature
    • Errors in stoichiometric calculations

Practical Investigation Tips

When conducting combustion experiments:

  • Use a spirit burner or similar apparatus
  • Measure initial and final temperatures accurately
  • Record mass of fuel before and after burning
  • Ensure complete combustion
  • Account for heat losses where possible

Sample Calculation

For the combustion of ethanol:

C2H5OH+3O2→2CO2+3H2O\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}

If 0.5g of ethanol raises the temperature of 200g of water by 8.5°C:

ΔHc=(200 g)(4.18 J/g°C)(8.5°C)0.0109 mol=−1297 kJ/mol\Delta H_c = \frac{(200\text{ g})(4.18\text{ J/g°C})(8.5\text{°C})}{0.0109\text{ mol}} = -1297\text{ kJ/mol}

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