Calorimetry

Heat of Combustion of Methane

Concepts

The combustion reaction for methane is

CH₄ (g) + 2 O₂ (g)   →   CO₂ (g) + 2 H₂O (l)

The enthalpy change for this reaction is measured by pressurizing a strong metal reaction vessel (called a bomb) with a mixture of methane and oxygen gas. The bomb is immersed in a calorimeter filled with water. An electrical current is passed through ignition wire (a fine iron wire), which ignites the wire and the gas mixture.

The heat balance for this calorimetry experiment is:

0 = q_cal + q_wire + q_comb

The heat for the calorimeter, q_cal, is determined from the heat capacity of the calorimeter and the temperature change for the calorimetry experiment. Typically the amount of water in the calorimeter is always the same; therefore C_cal includes the heat capacities of the calorimeter, the water, and the bomb itself.

The burning of the ignition wire releases heat, q_wire, and this heat must be included in the calculations. (This heat is treated separately, because the amount of ignition wire used varies from one measurement to the next.)

The heat released by the combustion reaction is q_comb, which is related to the molar enthalpy of combustion by

ΔHcomb =

qcomb nmethane

Combustion experiments are general conducted with a large excess of oxygen, so that the fuel (methane in this case) is the limiting reactant.

Combustion reactions are often used to calculate the molar enthalpies of formation. For example, the standard molar enthalpy of combustion for methane can be expressed in terms of the standard molar enthalpies of formation of the reactants and products:

ΔH^o_comb = 2 ΔH^o_f,water + ΔH^o_{f,carbon dioxide} - ΔH^o_f,methane - 2 ΔH^o_f,oxygen

ΔH^o_comb is measured experimentally.

ΔH^o_f,oxygen = 0, because oxygen is a pure element.

The other molar enthalpies of formation are known from independent measurements. For example, one could determine the heat of combustion of hydrogen to obtain the molar enthalpy of formation for water.

For liquid water, ΔH^o_f = -285.8 kJ mole^-1

For gaseous carbon dioxide, ΔH^o_f = -393.5 kJ mole^-1

Experiment

Objective:

• Determine the molar enthalpy of combustion for methane.
• Determine the molar enthalpy of formation for methane.

Approach:

• Pressurize the bomb with a known amount of methane gas.
• Observe the temperature of the system before and after the combustion reaction occurs.
• Calculate the change in temperature for the system.
• Use the temperature change, the heat capacity for the calorimeter, and the heat released by burning the ignition wire to calculate the heat of combustion.
• Divide the heat of neutralization by the moles of methane to determine the molar enthalpy of combustion.
• Use the molar enthalpy of combustion of methane to calculate the molar enthalpy of formation of methane.

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