MCAT Question of the Day Solution! Thermodynamics
October 24, 2014
Our question Thursday was from Thermodynamics in General Chemistry. The question presents you with the combustion reaction of methane and asks you to calculate the energy of a carbon-hydrogen bond.
To begin this problem, you must use Hess’s Law, which says that the heat of reaction is equal to the sum of the heats of formation of the products minus the sum of the heats of formation for the reactants.
The heat of reaction is -802 kJ/mol, and the heats of formation of CO2, H2O, and O2 are -393 kJ/mol, -242 kJ/mol, and 0 kJ/mol (the heat of formation of a compound in its elemental state is always 0), respectively. Now you want to solve for the heat of formation for CH4:
-802 kJ/mol = -393kJ/mol + 2(-242 kJmol) – x
where x is the heat of formation of methane. Solving for x, we get -75 kJ/mol for the heat of formation of methane. Since there are four equal carbon-hydrogen bonds in methane, the energy of one bond is -75 kJ/mol divided by 4, or about -19 kJ/mol.
Match to answer choice B!
More question to come next week!