A. Do you think the enthalpy of a methane combustion is more or less negative than...

A. Do you think the enthalpy of a methane combustion is more or less negative than the enthalpy of a reaction for methanol combustion? Why?
B. Do you think the enthalpy of an ethanol combustion is more or less negative than the enthalpy of a reaction for a methanol combution? Why?

Solutions

Expert Solution

A.The enthalpy of methane combustion is more negative than the enthalpy of a reaction for methanol combustion. Molecular formula for methane is CH₄ and for methanol is CH₃OH. Both have same carbon number but the values ΔH combustion for alcohols are slightly smaller than those for alkanes because the alcohols are already partially oxidized (the presence of a single oxygen atom in each alcohol molecule.)

2CH₄ +2O₂ ------> 2CO₂ + 2H₂O ΔH combustion –890KJ/mol

2CH₃OH +3 O₂ ------> 2CO₂ + 4H₂O ΔH combustion for methanol is -726 kJ/mol

B. Molecular formula of and ethanol is CH₃ CH₂OH. When the number of Carbon atoms in an alcohol increases; the enthalpy of combustion will become more negative. Ethanol has more carbon atom, so the enthalpy of ethanol combustion is more negative than the enthalpy of a reaction for methanol combustion. As we move down the homologous group, a CH₂ molecule is being added into the alcohol chain.

When ethanol is burnt, there will be one extra C-C bond and two extra C-H bonds to be broken. Therefore, much more energy is required for combustion. ΔH combustion for methanol is -726 kJ/mol and ΔH combustion for ethanol –1367 KJ/mol.

queen_honey_blossom answered 1 year ago

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