A student recorded the following two thermochemical equations in an attempt to…

CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics

Problem

A student recorded the following two thermochemical equations in an attempt to determine the enthalpy change for the reaction C(s)+12O2(g)→CO(g)C(s) + \tfrac{1}{2} O_{2}(g) \rightarrow CO(g) 1. C(s)+O2(g)→CO2(g)ΔH1=−394 kJC(s) + O_{2}(g) \rightarrow CO_{2}(g) \quad \Delta H_{1} = -394\,\text{kJ} 2. CO2(g)+12O2(g)→CO(g)+32O2(g)ΔH2=+283 kJCO_{2}(g) + \tfrac{1}{2} O_{2}(g) \rightarrow CO(g) + \tfrac{3}{2} O_{2}(g) \quad \Delta H_{2} = +283\,\text{kJ} The student summed the two equations exactly as written and concluded that ΔHrxn=−111 kJ\Delta H_{\text{rxn}} = -111\,\text{kJ}. Which of the following best identifies the fundamental error in the student’s procedure? (a) The student failed to reverse equation 2 before adding, so the sign of ΔH2\Delta H_{2} should be negative. (b) The student neglected to multiply equation 1 by 12\tfrac{1}{2}, giving an incorrect stoichiometric coefficient for O2\mathrm{O_{2}}. (c) The student attempted to cancel O2\mathrm{O_{2}} molecules that do not actually appear on both sides of the summed equation. (d) The student should have subtracted ΔH2\Delta H_{2} from ΔH1\Delta H_{1} rather than adding the two enthalpy values.

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