Two thermochemical equations and their molar enthalpy changes are shown below

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

Two thermochemical equations and their molar enthalpy changes are shown below. (1) C(graphite) + O2(g) → CO2(g)  ΔH1=−393.5 kJ\Delta H_1 = -393.5\,\text{kJ} (2) 2 CO(g) + O2(g) → 2 CO2(g)  ΔH2=−566.0 kJ\Delta H_2 = -566.0\,\text{kJ} A student seeks the molar enthalpy change for the reaction C(graphite) + 12\frac{1}{2} O2(g) → CO(g)  ΔHtarget=?\Delta H_{\text{target}} = ? Assume all reactions occur under identical conditions so that the enthalpy changes are directly comparable. (a) Describe, without performing any numerical calculation, how the two given equations could be combined algebraically to obtain the target reaction. (b) Explain—using the law of conservation of energy—why the algebraic combination of the individual enthalpy changes yields the enthalpy change for the overall reaction. (c) If equation (2) were to be written in the reverse direction, state how (qualitatively) the sign and magnitude of ΔH2\Delta H_2 would change, and explain the chemical reasoning behind this change.

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