In an experiment, 30.0 g of ethane and 30.0 g of propanol are placed in separate reaction vessels. E — Thermodynamics Chemistry Question

STEPS:

1. Identify the starting mass of each reactant:
The experiment places exactly $30.0\text{ g}$ of ethane and $30.0\text{ g}$ of propanol in separate reaction vessels.
2. Retrieve the molar masses and standard enthalpies of combustion from the given data table:
* Ethane ($\text{C}_2\text{H}_6$): Molar mass = $30.\text{ g/mol}$; $\Delta H^\circ_{\text{comb}} = -1560\text{ kJ/mol}$.
* Propanol ($\text{C}_3\text{H}_8\text{O}$): Molar mass = $60.\text{ g/mol}$; $\Delta H^\circ_{\text{comb}} = -2020\text{ kJ/mol}$.
3. Convert the mass of each compound into moles ($n$):
Using the formula $n = \frac{m}{\mathcal{M}}$:
* $\text{Moles of ethane} = \frac{30.0\text{ g}}{30.\text{ g/mol}} = \mathbf{1.0\text{ mol of ethane}}$
* $\text{Moles of propanol} = \frac{30.0\text{ g}}{60.\text{ g/mol}} = \mathbf{0.50\text{ mol of propanol}}$
4. Calculate the quantity of heat released ($q$) by each combustion reaction:
Multiply the number of moles combusted by the magnitude of the molar heat of combustion ($q = n \times \Delta H^\circ_{\text{comb}}$):
* $q_{\text{ethane}} = 1.0\text{ mol} \times 1560\text{ kJ/mol} = \mathbf{1560\text{ kJ released}}$
* $q_{\text{propanol}} = 0.50\text{ mol} \times 2020\text{ kJ/mol} = \mathbf{1010\text{ kJ released}}$
5. Compare the quantities of heat energy:
Comparing the absolute quantities of heat released, $1560\text{ kJ}$ is greater than $1010\text{ kJ}$. Therefore, the combustion of ethane releases more heat energy than the combustion of propanol ($q_{\text{ethane}} > q_{\text{propanol}}$), identifying Option C as the correct answer.

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WHY_OTHERS_WRONG:

• Option A is incorrect ($q_{\text{ethane}} < q_{\text{propanol}}$): A student would select this option if they only compared the molar enthalpies of combustion from the table ($1560\text{ kJ/mol} < 2020\text{ kJ/mol}$) without converting the equal mass samples into moles. Since ethane is a smaller molecule with a lower molar mass, a $30.0\text{ g}$ sample contains twice as many moles of fuel as a $30.0\text{ g}$ sample of propanol, resulting in a larger total energy release.
• Option B is incorrect ($q_{\text{ethane}} = q_{\text{propanol}}$): Equal masses of different chemical fuels do not release equal amounts of heat. The total heat released depends on the molar mass of the compounds and their specific chemical structures (molar enthalpies of combustion).
• Option D is incorrect: The specific heat capacity ($c$) of a substance relates heat to temperature change ($q = mc\Delta T$). It is not required to calculate or compare the absolute enthalpy change ($\Delta H$) of a chemical reaction, which is determined purely by the quantities of reactants and their molar enthalpies of combustion.