Chemistry

Chemistry

Problem

Hydrogenation of ethene is represented by C2_2H4_4(g) + H2_2(g) → C2_2H6_6(g) ΔHrxn=−136 kJ mol−1\Delta H_{\text{rxn}} = -136\,\text{kJ}\,\text{mol}^{-1}. A chemical plant needs this reaction to release 4.08×103 kJ4.08\times10^{3}\,\text{kJ} of heat. If the process operates at a 75%75\% yield with respect to C2_2H4_4, what mass of ethene (in grams) must be fed into the reactor? a) 8.40×102 g8.40\times10^{2}\,\text{g} b) 6.30×102 g6.30\times10^{2}\,\text{g} c) 1.12×103 g1.12\times10^{3}\,\text{g} d) 1.20×103 g1.20\times10^{3}\,\text{g}
Intuitive explanation

This problem links stoichiometry to real-world efficiency—you need to work backwards from the desired heat output to find how much reactant to start with. The key insight is that the enthalpy change tells you how much heat one mole of reaction produces, so you can convert your target heat into moles of ethene that must actually react. Then, because only 75% of your feedstock converts, you divide by the yield to find the initial mass you must feed in.

Intuitive explanation

This problem links stoichiometry to real-world efficiency—you need to work backwards from the desired heat output to find how much reactant to start with. The key insight is that the enthalpy change tells you how much heat one mole of reaction produces, so you can convert your target heat into moles of ethene that must actually react. Then, because only 75% of your feedstock converts, you divide by the yield to find the initial mass you must feed in.

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