Enthalpy Change of the Combustion of CH4

The enthalpy change of a reaction can be determined by measuring the heat evolved or absorbed during the reaction. In this case, the heat combustion of CH4 (methane) is measured at constant volume using a bomb calorimeter at a temperature of 298.2 K. The measured heat is -885389 J/mol.

Definition of Enthalpy Change

Enthalpy change (ΔH) is the difference in enthalpy between the products and reactants of a reaction. It represents the heat energy exchanged during the reaction at constant pressure.

Relationship between Enthalpy Change and Heat

The heat evolved or absorbed during a reaction at constant volume (qV) is related to the enthalpy change (ΔH) by the equation:

qV = ΔH + ΔnRT

Where:
qV is the heat evolved or absorbed at constant volume,
ΔH is the enthalpy change of the reaction,
Δn is the change in the number of moles of gas,
R is the ideal gas constant, and
T is the temperature in Kelvin.

Calculating Enthalpy Change

In this case, the combustion of CH4 produces CO2 and H2O:

CH4 + 2O2 → CO2 + 2H2O

The number of moles of gas changes from 1 mole of CH4 to 3 moles of gas (1 mole of CO2 and 2 moles of H2O).

Using the equation qV = ΔH + ΔnRT, we can rearrange it to solve for ΔH:

ΔH = qV - ΔnRT

Substituting the given values, we have:

ΔH = -885389 J/mol - (3 mol)(8.314 J/mol·K)(298.2 K)

Calculating the value, we find:

ΔH ≈ -885389 J/mol - 74193 J/mol

ΔH ≈ -959582 J/mol

Therefore, the enthalpy change of the combustion of CH4 at constant volume is approximately -959582 J/mol.

Conclusion

The enthalpy change of the combustion of CH4 at constant volume is determined by measuring the heat evolved or absorbed during the reaction. Using the equation qV = ΔH + ΔnRT, the enthalpy change can be calculated by substituting the known values. In this case, the enthalpy change is found to be -959582 J/mol.